Doc/library/doctest.rst

:keepdoctest:

:mod:`doctest` --- Test interactive Python examples
===================================================

.. module:: doctest
   :synopsis: Test pieces of code within docstrings.

.. moduleauthor:: Tim Peters <tim@python.org>
.. sectionauthor:: Tim Peters <tim@python.org>
.. sectionauthor:: Moshe Zadka <moshez@debian.org>
.. sectionauthor:: Edward Loper <edloper@users.sourceforge.net>

**Source code:** :source:`Lib/doctest.py`

--------------

The :mod:`doctest` module searches for pieces of text that look like interactive
Python sessions, and then executes those sessions to verify that they work
exactly as shown.  There are several common ways to use doctest:

* To check that a module's docstrings are up-to-date by verifying that all
  interactive examples still work as documented.

* To perform regression testing by verifying that interactive examples from a
  test file or a test object work as expected.

* To write tutorial documentation for a package, liberally illustrated with
  input-output examples.  Depending on whether the examples or the expository text
  are emphasized, this has the flavor of "literate testing" or "executable
  documentation".

Here's a complete but small example module::

   """
   This is the "example" module.

   The example module supplies one function, factorial().  For example,

   >>> factorial(5)
   120
   """

   def factorial(n):
       """Return the factorial of n, an exact integer >= 0.

       >>> [factorial(n) for n in range(6)]
       [1, 1, 2, 6, 24, 120]
       >>> factorial(30)
       265252859812191058636308480000000
       >>> factorial(-1)
       Traceback (most recent call last):
           ...
       ValueError: n must be >= 0

       Factorials of floats are OK, but the float must be an exact integer:
       >>> factorial(30.1)
       Traceback (most recent call last):
           ...
       ValueError: n must be exact integer
       >>> factorial(30.0)
       265252859812191058636308480000000

       It must also not be ridiculously large:
       >>> factorial(1e100)
       Traceback (most recent call last):
           ...
       OverflowError: n too large
       """

       import math
       if not n >= 0:
           raise ValueError("n must be >= 0")
       if math.floor(n) != n:
           raise ValueError("n must be exact integer")
       if n+1 == n:  # catch a value like 1e300
           raise OverflowError("n too large")
       result = 1
       factor = 2
       while factor <= n:
           result *= factor
           factor += 1
       return result


   if __name__ == "__main__":
       import doctest
       doctest.testmod()

If you run :file:`example.py` directly from the command line, :mod:`doctest`
works its magic::

   $ python example.py
   $

There's no output!  That's normal, and it means all the examples worked.  Pass
``-v`` to the script, and :mod:`doctest` prints a detailed log of what
it's trying, and prints a summary at the end::

   $ python example.py -v
   Trying:
       factorial(5)
   Expecting:
       120
   ok
   Trying:
       [factorial(n) for n in range(6)]
   Expecting:
       [1, 1, 2, 6, 24, 120]
   ok

And so on, eventually ending with::

   Trying:
       factorial(1e100)
   Expecting:
       Traceback (most recent call last):
           ...
       OverflowError: n too large
   ok
   2 items passed all tests:
      1 tests in __main__
      8 tests in __main__.factorial
   9 tests in 2 items.
   9 passed and 0 failed.
   Test passed.
   $

That's all you need to know to start making productive use of :mod:`doctest`!
Jump in.  The following sections provide full details.  Note that there are many
examples of doctests in the standard Python test suite and libraries.
Especially useful examples can be found in the standard test file
:file:`Lib/test/test_doctest.py`.


.. _doctest-simple-testmod:

Simple Usage: Checking Examples in Docstrings
---------------------------------------------

The simplest way to start using doctest (but not necessarily the way you'll
continue to do it) is to end each module :mod:`M` with::

   if __name__ == "__main__":
       import doctest
       doctest.testmod()

:mod:`doctest` then examines docstrings in module :mod:`M`.

Running the module as a script causes the examples in the docstrings to get
executed and verified::

   python M.py

This won't display anything unless an example fails, in which case the failing
example(s) and the cause(s) of the failure(s) are printed to stdout, and the
final line of output is ``***Test Failed*** N failures.``, where *N* is the
number of examples that failed.

Run it with the ``-v`` switch instead::

   python M.py -v

and a detailed report of all examples tried is printed to standard output, along
with assorted summaries at the end.

You can force verbose mode by passing ``verbose=True`` to :func:`testmod`, or
prohibit it by passing ``verbose=False``.  In either of those cases,
``sys.argv`` is not examined by :func:`testmod` (so passing ``-v`` or not
has no effect).

There is also a command line shortcut for running :func:`testmod`.  You can
instruct the Python interpreter to run the doctest module directly from the
standard library and pass the module name(s) on the command line::

   python -m doctest -v example.py

This will import :file:`example.py` as a standalone module and run
:func:`testmod` on it.  Note that this may not work correctly if the file is
part of a package and imports other submodules from that package.

For more information on :func:`testmod`, see section :ref:`doctest-basic-api`.


.. _doctest-simple-testfile:

Simple Usage: Checking Examples in a Text File
----------------------------------------------

Another simple application of doctest is testing interactive examples in a text
file.  This can be done with the :func:`testfile` function::

   import doctest
   doctest.testfile("example.txt")

That short script executes and verifies any interactive Python examples
contained in the file :file:`example.txt`.  The file content is treated as if it
were a single giant docstring; the file doesn't need to contain a Python
program!   For example, perhaps :file:`example.txt` contains this::

   The ``example`` module
   ======================

   Using ``factorial``
   -------------------

   This is an example text file in reStructuredText format.  First import
   ``factorial`` from the ``example`` module:

       >>> from example import factorial

   Now use it:

       >>> factorial(6)
       120

Running ``doctest.testfile("example.txt")`` then finds the error in this
documentation::

   File "./example.txt", line 14, in example.txt
   Failed example:
       factorial(6)
   Expected:
       120
   Got:
       720

As with :func:`testmod`, :func:`testfile` won't display anything unless an
example fails.  If an example does fail, then the failing example(s) and the
cause(s) of the failure(s) are printed to stdout, using the same format as
:func:`testmod`.

By default, :func:`testfile` looks for files in the calling module's directory.
See section :ref:`doctest-basic-api` for a description of the optional arguments
that can be used to tell it to look for files in other locations.

Like :func:`testmod`, :func:`testfile`'s verbosity can be set with the
``-v`` command-line switch or with the optional keyword argument
*verbose*.

There is also a command line shortcut for running :func:`testfile`.  You can
instruct the Python interpreter to run the doctest module directly from the
standard library and pass the file name(s) on the command line::

   python -m doctest -v example.txt

Because the file name does not end with :file:`.py`, :mod:`doctest` infers that
it must be run with :func:`testfile`, not :func:`testmod`.

For more information on :func:`testfile`, see section :ref:`doctest-basic-api`.


.. _doctest-how-it-works:

How It Works
------------

This section examines in detail how doctest works: which docstrings it looks at,
how it finds interactive examples, what execution context it uses, how it
handles exceptions, and how option flags can be used to control its behavior.
This is the information that you need to know to write doctest examples; for
information about actually running doctest on these examples, see the following
sections.


.. _doctest-which-docstrings:

Which Docstrings Are Examined?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The module docstring, and all function, class and method docstrings are
searched.  Objects imported into the module are not searched.

In addition, if ``M.__test__`` exists and "is true", it must be a dict, and each
entry maps a (string) name to a function object, class object, or string.
Function and class object docstrings found from ``M.__test__`` are searched, and
strings are treated as if they were docstrings.  In output, a key ``K`` in
``M.__test__`` appears with name ::

   <name of M>.__test__.K

Any classes found are recursively searched similarly, to test docstrings in
their contained methods and nested classes.

.. impl-detail::
   Prior to version 3.4, extension modules written in C were not fully
   searched by doctest.


.. _doctest-finding-examples:

How are Docstring Examples Recognized?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In most cases a copy-and-paste of an interactive console session works fine,
but doctest isn't trying to do an exact emulation of any specific Python shell.

::

   >>> # comments are ignored
   >>> x = 12
   >>> x
   12
   >>> if x == 13:
   ...     print("yes")
   ... else:
   ...     print("no")
   ...     print("NO")
   ...     print("NO!!!")
   ...
   no
   NO
   NO!!!
   >>>

Any expected output must immediately follow the final ``'>>> '`` or ``'... '``
line containing the code, and the expected output (if any) extends to the next
``'>>> '`` or all-whitespace line.

The fine print:

* Expected output cannot contain an all-whitespace line, since such a line is
  taken to signal the end of expected output.  If expected output does contain a
  blank line, put ``<BLANKLINE>`` in your doctest example each place a blank line
  is expected.

* All hard tab characters are expanded to spaces, using 8-column tab stops.
  Tabs in output generated by the tested code are not modified.  Because any
  hard tabs in the sample output *are* expanded, this means that if the code
  output includes hard tabs, the only way the doctest can pass is if the
  :const:`NORMALIZE_WHITESPACE` option or :ref:`directive <doctest-directives>`
  is in effect.
  Alternatively, the test can be rewritten to capture the output and compare it
  to an expected value as part of the test.  This handling of tabs in the
  source was arrived at through trial and error, and has proven to be the least
  error prone way of handling them.  It is possible to use a different
  algorithm for handling tabs by writing a custom :class:`DocTestParser` class.

* Output to stdout is captured, but not output to stderr (exception tracebacks
  are captured via a different means).

* If you continue a line via backslashing in an interactive session, or for any
  other reason use a backslash, you should use a raw docstring, which will
  preserve your backslashes exactly as you type them::

     >>> def f(x):
     ...     r'''Backslashes in a raw docstring: m\n'''
     >>> print(f.__doc__)
     Backslashes in a raw docstring: m\n

  Otherwise, the backslash will be interpreted as part of the string. For example,
  the ``\n`` above would be interpreted as a newline character.  Alternatively, you
  can double each backslash in the doctest version (and not use a raw string)::

     >>> def f(x):
     ...     '''Backslashes in a raw docstring: m\\n'''
     >>> print(f.__doc__)
     Backslashes in a raw docstring: m\n

* The starting column doesn't matter::

     >>> assert "Easy!"
           >>> import math
               >>> math.floor(1.9)
               1

  and as many leading whitespace characters are stripped from the expected output
  as appeared in the initial ``'>>> '`` line that started the example.


.. _doctest-execution-context:

What's the Execution Context?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By default, each time :mod:`doctest` finds a docstring to test, it uses a
*shallow copy* of :mod:`M`'s globals, so that running tests doesn't change the
module's real globals, and so that one test in :mod:`M` can't leave behind
crumbs that accidentally allow another test to work.  This means examples can
freely use any names defined at top-level in :mod:`M`, and names defined earlier
in the docstring being run. Examples cannot see names defined in other
docstrings.

You can force use of your own dict as the execution context by passing
``globs=your_dict`` to :func:`testmod` or :func:`testfile` instead.


.. _doctest-exceptions:

What About Exceptions?
^^^^^^^^^^^^^^^^^^^^^^

No problem, provided that the traceback is the only output produced by the
example:  just paste in the traceback. [#]_ Since tracebacks contain details
that are likely to change rapidly (for example, exact file paths and line
numbers), this is one case where doctest works hard to be flexible in what it
accepts.

Simple example::

   >>> [1, 2, 3].remove(42)
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ValueError: list.remove(x): x not in list

That doctest succeeds if :exc:`ValueError` is raised, with the ``list.remove(x):
x not in list`` detail as shown.

The expected output for an exception must start with a traceback header, which
may be either of the following two lines, indented the same as the first line of
the example::

   Traceback (most recent call last):
   Traceback (innermost last):

The traceback header is followed by an optional traceback stack, whose contents
are ignored by doctest.  The traceback stack is typically omitted, or copied
verbatim from an interactive session.

The traceback stack is followed by the most interesting part: the line(s)
containing the exception type and detail.  This is usually the last line of a
traceback, but can extend across multiple lines if the exception has a
multi-line detail::

   >>> raise ValueError('multi\n    line\ndetail')
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ValueError: multi
       line
   detail

The last three lines (starting with :exc:`ValueError`) are compared against the
exception's type and detail, and the rest are ignored.

Best practice is to omit the traceback stack, unless it adds significant
documentation value to the example.  So the last example is probably better as::

   >>> raise ValueError('multi\n    line\ndetail')
   Traceback (most recent call last):
       ...
   ValueError: multi
       line
   detail

Note that tracebacks are treated very specially.  In particular, in the
rewritten example, the use of ``...`` is independent of doctest's
:const:`ELLIPSIS` option.  The ellipsis in that example could be left out, or
could just as well be three (or three hundred) commas or digits, or an indented
transcript of a Monty Python skit.

Some details you should read once, but won't need to remember:

* Doctest can't guess whether your expected output came from an exception
  traceback or from ordinary printing.  So, e.g., an example that expects
  ``ValueError: 42 is prime`` will pass whether :exc:`ValueError` is actually
  raised or if the example merely prints that traceback text.  In practice,
  ordinary output rarely begins with a traceback header line, so this doesn't
  create real problems.

* Each line of the traceback stack (if present) must be indented further than
  the first line of the example, *or* start with a non-alphanumeric character.
  The first line following the traceback header indented the same and starting
  with an alphanumeric is taken to be the start of the exception detail.  Of
  course this does the right thing for genuine tracebacks.

* When the :const:`IGNORE_EXCEPTION_DETAIL` doctest option is specified,
  everything following the leftmost colon and any module information in the
  exception name is ignored.

* The interactive shell omits the traceback header line for some
  :exc:`SyntaxError`\ s.  But doctest uses the traceback header line to
  distinguish exceptions from non-exceptions.  So in the rare case where you need
  to test a :exc:`SyntaxError` that omits the traceback header, you will need to
  manually add the traceback header line to your test example.

* For some :exc:`SyntaxError`\ s, Python displays the character position of the
  syntax error, using a ``^`` marker::

     >>> 1 1
       File "<stdin>", line 1
         1 1
           ^
     SyntaxError: invalid syntax

  Since the lines showing the position of the error come before the exception type
  and detail, they are not checked by doctest.  For example, the following test
  would pass, even though it puts the ``^`` marker in the wrong location::

     >>> 1 1
     Traceback (most recent call last):
       File "<stdin>", line 1
         1 1
         ^
     SyntaxError: invalid syntax


.. _option-flags-and-directives:
.. _doctest-options:

Option Flags
^^^^^^^^^^^^

A number of option flags control various aspects of doctest's behavior.
Symbolic names for the flags are supplied as module constants, which can be
or'ed together and passed to various functions.  The names can also be used in
:ref:`doctest directives <doctest-directives>`, and may be passed to the
doctest command line interface via the ``-o`` option.

.. versionadded:: 3.4
   The ``-o`` command line option.

The first group of options define test semantics, controlling aspects of how
doctest decides whether actual output matches an example's expected output:


.. data:: DONT_ACCEPT_TRUE_FOR_1

   By default, if an expected output block contains just ``1``, an actual output
   block containing just ``1`` or just ``True`` is considered to be a match, and
   similarly for ``0`` versus ``False``.  When :const:`DONT_ACCEPT_TRUE_FOR_1` is
   specified, neither substitution is allowed.  The default behavior caters to that
   Python changed the return type of many functions from integer to boolean;
   doctests expecting "little integer" output still work in these cases.  This
   option will probably go away, but not for several years.


.. data:: DONT_ACCEPT_BLANKLINE

   By default, if an expected output block contains a line containing only the
   string ``<BLANKLINE>``, then that line will match a blank line in the actual
   output.  Because a genuinely blank line delimits the expected output, this is
   the only way to communicate that a blank line is expected.  When
   :const:`DONT_ACCEPT_BLANKLINE` is specified, this substitution is not allowed.


.. data:: NORMALIZE_WHITESPACE

   When specified, all sequences of whitespace (blanks and newlines) are treated as
   equal.  Any sequence of whitespace within the expected output will match any
   sequence of whitespace within the actual output. By default, whitespace must
   match exactly. :const:`NORMALIZE_WHITESPACE` is especially useful when a line of
   expected output is very long, and you want to wrap it across multiple lines in
   your source.


.. data:: ELLIPSIS

   When specified, an ellipsis marker (``...``) in the expected output can match
   any substring in the actual output.  This includes substrings that span line
   boundaries, and empty substrings, so it's best to keep usage of this simple.
   Complicated uses can lead to the same kinds of "oops, it matched too much!"
   surprises that ``.*`` is prone to in regular expressions.


.. data:: IGNORE_EXCEPTION_DETAIL

   When specified, an example that expects an exception passes if an exception of
   the expected type is raised, even if the exception detail does not match.  For
   example, an example expecting ``ValueError: 42`` will pass if the actual
   exception raised is ``ValueError: 3*14``, but will fail, e.g., if
   :exc:`TypeError` is raised.

   It will also ignore the module name used in Python 3 doctest reports. Hence
   both of these variations will work with the flag specified, regardless of
   whether the test is run under Python 2.7 or Python 3.2 (or later versions)::

      >>> raise CustomError('message')
      Traceback (most recent call last):
      CustomError: message

      >>> raise CustomError('message')
      Traceback (most recent call last):
      my_module.CustomError: message

   Note that :const:`ELLIPSIS` can also be used to ignore the
   details of the exception message, but such a test may still fail based
   on whether or not the module details are printed as part of the
   exception name. Using :const:`IGNORE_EXCEPTION_DETAIL` and the details
   from Python 2.3 is also the only clear way to write a doctest that doesn't
   care about the exception detail yet continues to pass under Python 2.3 or
   earlier (those releases do not support :ref:`doctest directives
   <doctest-directives>` and ignore them as irrelevant comments). For example::

      >>> (1, 2)[3] = 'moo'
      Traceback (most recent call last):
        File "<stdin>", line 1, in ?
      TypeError: object doesn't support item assignment

   passes under Python 2.3 and later Python versions with the flag specified,
   even though the detail
   changed in Python 2.4 to say "does not" instead of "doesn't".

   .. versionchanged:: 3.2
      :const:`IGNORE_EXCEPTION_DETAIL` now also ignores any information relating
      to the module containing the exception under test.


.. data:: SKIP

   When specified, do not run the example at all.  This can be useful in contexts
   where doctest examples serve as both documentation and test cases, and an
   example should be included for documentation purposes, but should not be
   checked.  E.g., the example's output might be random; or the example might
   depend on resources which would be unavailable to the test driver.

   The SKIP flag can also be used for temporarily "commenting out" examples.


.. data:: COMPARISON_FLAGS

   A bitmask or'ing together all the comparison flags above.

The second group of options controls how test failures are reported:


.. data:: REPORT_UDIFF

   When specified, failures that involve multi-line expected and actual outputs are
   displayed using a unified diff.


.. data:: REPORT_CDIFF

   When specified, failures that involve multi-line expected and actual outputs
   will be displayed using a context diff.


.. data:: REPORT_NDIFF

   When specified, differences are computed by ``difflib.Differ``, using the same
   algorithm as the popular :file:`ndiff.py` utility. This is the only method that
   marks differences within lines as well as across lines.  For example, if a line
   of expected output contains digit ``1`` where actual output contains letter
   ``l``, a line is inserted with a caret marking the mismatching column positions.


.. data:: REPORT_ONLY_FIRST_FAILURE

   When specified, display the first failing example in each doctest, but suppress
   output for all remaining examples.  This will prevent doctest from reporting
   correct examples that break because of earlier failures; but it might also hide
   incorrect examples that fail independently of the first failure.  When
   :const:`REPORT_ONLY_FIRST_FAILURE` is specified, the remaining examples are
   still run, and still count towards the total number of failures reported; only
   the output is suppressed.


.. data:: FAIL_FAST

   When specified, exit after the first failing example and don't attempt to run
   the remaining examples. Thus, the number of failures reported will be at most
   1.  This flag may be useful during debugging, since examples after the first
   failure won't even produce debugging output.

   The doctest command line accepts the option ``-f`` as a shorthand for ``-o
   FAIL_FAST``.

   .. versionadded:: 3.4


.. data:: REPORTING_FLAGS

   A bitmask or'ing together all the reporting flags above.


There is also a way to register new option flag names, though this isn't
useful unless you intend to extend :mod:`doctest` internals via subclassing:


.. function:: register_optionflag(name)

   Create a new option flag with a given name, and return the new flag's integer
   value.  :func:`register_optionflag` can be used when subclassing
   :class:`OutputChecker` or :class:`DocTestRunner` to create new options that are
   supported by your subclasses.  :func:`register_optionflag` should always be
   called using the following idiom::

      MY_FLAG = register_optionflag('MY_FLAG')


.. _doctest-directives:

Directives
^^^^^^^^^^

Doctest directives may be used to modify the :ref:`option flags
<doctest-options>` for an individual example.  Doctest directives are
special Python comments following an example's source code:

.. productionlist:: doctest
   directive: "#" "doctest:" `directive_options`
   directive_options: `directive_option` ("," `directive_option`)\*
   directive_option: `on_or_off` `directive_option_name`
   on_or_off: "+" \| "-"
   directive_option_name: "DONT_ACCEPT_BLANKLINE" \| "NORMALIZE_WHITESPACE" \| ...

Whitespace is not allowed between the ``+`` or ``-`` and the directive option
name.  The directive option name can be any of the option flag names explained
above.

An example's doctest directives modify doctest's behavior for that single
example.  Use ``+`` to enable the named behavior, or ``-`` to disable it.

For example, this test passes::

   >>> print(list(range(20))) # doctest: +NORMALIZE_WHITESPACE
   [0,   1,  2,  3,  4,  5,  6,  7,  8,  9,
   10,  11, 12, 13, 14, 15, 16, 17, 18, 19]

Without the directive it would fail, both because the actual output doesn't have
two blanks before the single-digit list elements, and because the actual output
is on a single line.  This test also passes, and also requires a directive to do
so::

   >>> print(list(range(20))) # doctest: +ELLIPSIS
   [0, 1, ..., 18, 19]

Multiple directives can be used on a single physical line, separated by
commas::

   >>> print(list(range(20))) # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
   [0,    1, ...,   18,    19]

If multiple directive comments are used for a single example, then they are
combined::

   >>> print(list(range(20))) # doctest: +ELLIPSIS
   ...                        # doctest: +NORMALIZE_WHITESPACE
   [0,    1, ...,   18,    19]

As the previous example shows, you can add ``...`` lines to your example
containing only directives.  This can be useful when an example is too long for
a directive to comfortably fit on the same line::

   >>> print(list(range(5)) + list(range(10, 20)) + list(range(30, 40)))
   ... # doctest: +ELLIPSIS
   [0, ..., 4, 10, ..., 19, 30, ..., 39]

Note that since all options are disabled by default, and directives apply only
to the example they appear in, enabling options (via ``+`` in a directive) is
usually the only meaningful choice.  However, option flags can also be passed to
functions that run doctests, establishing different defaults.  In such cases,
disabling an option via ``-`` in a directive can be useful.


.. _doctest-warnings:

Warnings
^^^^^^^^

:mod:`doctest` is serious about requiring exact matches in expected output.  If
even a single character doesn't match, the test fails.  This will probably
surprise you a few times, as you learn exactly what Python does and doesn't
guarantee about output.  For example, when printing a dict, Python doesn't
guarantee that the key-value pairs will be printed in any particular order, so a
test like ::

   >>> foo()
   {"Hermione": "hippogryph", "Harry": "broomstick"}

is vulnerable!  One workaround is to do ::

   >>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
   True

instead.  Another is to do ::

   >>> d = sorted(foo().items())
   >>> d
   [('Harry', 'broomstick'), ('Hermione', 'hippogryph')]

There are others, but you get the idea.

Another bad idea is to print things that embed an object address, like ::

   >>> id(1.0) # certain to fail some of the time
   7948648
   >>> class C: pass
   >>> C()   # the default repr() for instances embeds an address
   <__main__.C instance at 0x00AC18F0>

The :const:`ELLIPSIS` directive gives a nice approach for the last example::

   >>> C() #doctest: +ELLIPSIS
   <__main__.C instance at 0x...>

Floating-point numbers are also subject to small output variations across
platforms, because Python defers to the platform C library for float formatting,
and C libraries vary widely in quality here. ::

   >>> 1./7  # risky
   0.14285714285714285
   >>> print(1./7) # safer
   0.142857142857
   >>> print(round(1./7, 6)) # much safer
   0.142857

Numbers of the form ``I/2.**J`` are safe across all platforms, and I often
contrive doctest examples to produce numbers of that form::

   >>> 3./4  # utterly safe
   0.75

Simple fractions are also easier for people to understand, and that makes for
better documentation.


.. _doctest-basic-api:

Basic API
---------

The functions :func:`testmod` and :func:`testfile` provide a simple interface to
doctest that should be sufficient for most basic uses.  For a less formal
introduction to these two functions, see sections :ref:`doctest-simple-testmod`
and :ref:`doctest-simple-testfile`.


.. function:: testfile(filename, module_relative=True, name=None, package=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, parser=DocTestParser(), encoding=None)

   All arguments except *filename* are optional, and should be specified in keyword
   form.

   Test examples in the file named *filename*.  Return ``(failure_count,
   test_count)``.

   Optional argument *module_relative* specifies how the filename should be
   interpreted:

   * If *module_relative* is ``True`` (the default), then *filename* specifies an
     OS-independent module-relative path.  By default, this path is relative to the
     calling module's directory; but if the *package* argument is specified, then it
     is relative to that package.  To ensure OS-independence, *filename* should use
     ``/`` characters to separate path segments, and may not be an absolute path
     (i.e., it may not begin with ``/``).

   * If *module_relative* is ``False``, then *filename* specifies an OS-specific
     path.  The path may be absolute or relative; relative paths are resolved with
     respect to the current working directory.

   Optional argument *name* gives the name of the test; by default, or if ``None``,
   ``os.path.basename(filename)`` is used.

   Optional argument *package* is a Python package or the name of a Python package
   whose directory should be used as the base directory for a module-relative
   filename.  If no package is specified, then the calling module's directory is
   used as the base directory for module-relative filenames.  It is an error to
   specify *package* if *module_relative* is ``False``.

   Optional argument *globs* gives a dict to be used as the globals when executing
   examples.  A new shallow copy of this dict is created for the doctest, so its
   examples start with a clean slate. By default, or if ``None``, a new empty dict
   is used.

   Optional argument *extraglobs* gives a dict merged into the globals used to
   execute examples.  This works like :meth:`dict.update`:  if *globs* and
   *extraglobs* have a common key, the associated value in *extraglobs* appears in
   the combined dict.  By default, or if ``None``, no extra globals are used.  This
   is an advanced feature that allows parameterization of doctests.  For example, a
   doctest can be written for a base class, using a generic name for the class,
   then reused to test any number of subclasses by passing an *extraglobs* dict
   mapping the generic name to the subclass to be tested.

   Optional argument *verbose* prints lots of stuff if true, and prints only
   failures if false; by default, or if ``None``, it's true if and only if ``'-v'``
   is in ``sys.argv``.

   Optional argument *report* prints a summary at the end when true, else prints
   nothing at the end.  In verbose mode, the summary is detailed, else the summary
   is very brief (in fact, empty if all tests passed).

   Optional argument *optionflags* (default value 0) takes the bitwise-or of
   option flags.  See section :ref:`doctest-options`.

   Optional argument *raise_on_error* defaults to false.  If true, an exception is
   raised upon the first failure or unexpected exception in an example.  This
   allows failures to be post-mortem debugged. Default behavior is to continue
   running examples.

   Optional argument *parser* specifies a :class:`DocTestParser` (or subclass) that
   should be used to extract tests from the files.  It defaults to a normal parser
   (i.e., ``DocTestParser()``).

   Optional argument *encoding* specifies an encoding that should be used to
   convert the file to unicode.


.. function:: testmod(m=None, name=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, exclude_empty=False)

   All arguments are optional, and all except for *m* should be specified in
   keyword form.

   Test examples in docstrings in functions and classes reachable from module *m*
   (or module :mod:`__main__` if *m* is not supplied or is ``None``), starting with
   ``m.__doc__``.

   Also test examples reachable from dict ``m.__test__``, if it exists and is not
   ``None``.  ``m.__test__`` maps names (strings) to functions, classes and
   strings; function and class docstrings are searched for examples; strings are
   searched directly, as if they were docstrings.

   Only docstrings attached to objects belonging to module *m* are searched.

   Return ``(failure_count, test_count)``.

   Optional argument *name* gives the name of the module; by default, or if
   ``None``, ``m.__name__`` is used.

   Optional argument *exclude_empty* defaults to false.  If true, objects for which
   no doctests are found are excluded from consideration. The default is a backward
   compatibility hack, so that code still using :meth:`doctest.master.summarize` in
   conjunction with :func:`testmod` continues to get output for objects with no
   tests. The *exclude_empty* argument to the newer :class:`DocTestFinder`
   constructor defaults to true.

   Optional arguments *extraglobs*, *verbose*, *report*, *optionflags*,
   *raise_on_error*, and *globs* are the same as for function :func:`testfile`
   above, except that *globs* defaults to ``m.__dict__``.


.. function:: run_docstring_examples(f, globs, verbose=False, name="NoName", compileflags=None, optionflags=0)

   Test examples associated with object *f*; for example, *f* may be a string,
   a module, a function, or a class object.

   A shallow copy of dictionary argument *globs* is used for the execution context.

   Optional argument *name* is used in failure messages, and defaults to
   ``"NoName"``.

   If optional argument *verbose* is true, output is generated even if there are no
   failures.  By default, output is generated only in case of an example failure.

   Optional argument *compileflags* gives the set of flags that should be used by
   the Python compiler when running the examples.  By default, or if ``None``,
   flags are deduced corresponding to the set of future features found in *globs*.

   Optional argument *optionflags* works as for function :func:`testfile` above.


.. _doctest-unittest-api:

Unittest API
------------

As your collection of doctest'ed modules grows, you'll want a way to run all
their doctests systematically.  :mod:`doctest` provides two functions that can
be used to create :mod:`unittest` test suites from modules and text files
containing doctests.  To integrate with :mod:`unittest` test discovery, include
a :func:`load_tests` function in your test module::

   import unittest
   import doctest
   import my_module_with_doctests

   def load_tests(loader, tests, ignore):
       tests.addTests(doctest.DocTestSuite(my_module_with_doctests))
       return tests

There are two main functions for creating :class:`unittest.TestSuite` instances
from text files and modules with doctests:


.. function:: DocFileSuite(*paths, module_relative=True, package=None, setUp=None, tearDown=None, globs=None, optionflags=0, parser=DocTestParser(), encoding=None)

   Convert doctest tests from one or more text files to a
   :class:`unittest.TestSuite`.

   The returned :class:`unittest.TestSuite` is to be run by the unittest framework
   and runs the interactive examples in each file.  If an example in any file
   fails, then the synthesized unit test fails, and a :exc:`failureException`
   exception is raised showing the name of the file containing the test and a
   (sometimes approximate) line number.

   Pass one or more paths (as strings) to text files to be examined.

   Options may be provided as keyword arguments:

   Optional argument *module_relative* specifies how the filenames in *paths*
   should be interpreted:

   * If *module_relative* is ``True`` (the default), then each filename in
     *paths* specifies an OS-independent module-relative path.  By default, this
     path is relative to the calling module's directory; but if the *package*
     argument is specified, then it is relative to that package.  To ensure
     OS-independence, each filename should use ``/`` characters to separate path
     segments, and may not be an absolute path (i.e., it may not begin with
     ``/``).

   * If *module_relative* is ``False``, then each filename in *paths* specifies
     an OS-specific path.  The path may be absolute or relative; relative paths
     are resolved with respect to the current working directory.

   Optional argument *package* is a Python package or the name of a Python
   package whose directory should be used as the base directory for
   module-relative filenames in *paths*.  If no package is specified, then the
   calling module's directory is used as the base directory for module-relative
   filenames.  It is an error to specify *package* if *module_relative* is
   ``False``.

   Optional argument *setUp* specifies a set-up function for the test suite.
   This is called before running the tests in each file.  The *setUp* function
   will be passed a :class:`DocTest` object.  The setUp function can access the
   test globals as the *globs* attribute of the test passed.

   Optional argument *tearDown* specifies a tear-down function for the test
   suite.  This is called after running the tests in each file.  The *tearDown*
   function will be passed a :class:`DocTest` object.  The setUp function can
   access the test globals as the *globs* attribute of the test passed.

   Optional argument *globs* is a dictionary containing the initial global
   variables for the tests.  A new copy of this dictionary is created for each
   test.  By default, *globs* is a new empty dictionary.

   Optional argument *optionflags* specifies the default doctest options for the
   tests, created by or-ing together individual option flags.  See section
   :ref:`doctest-options`. See function :func:`set_unittest_reportflags` below
   for a better way to set reporting options.

   Optional argument *parser* specifies a :class:`DocTestParser` (or subclass)
   that should be used to extract tests from the files.  It defaults to a normal
   parser (i.e., ``DocTestParser()``).

   Optional argument *encoding* specifies an encoding that should be used to
   convert the file to unicode.

   The global ``__file__`` is added to the globals provided to doctests loaded
   from a text file using :func:`DocFileSuite`.


.. function:: DocTestSuite(module=None, globs=None, extraglobs=None, test_finder=None, setUp=None, tearDown=None, checker=None)

   Convert doctest tests for a module to a :class:`unittest.TestSuite`.

   The returned :class:`unittest.TestSuite` is to be run by the unittest framework
   and runs each doctest in the module.  If any of the doctests fail, then the
   synthesized unit test fails, and a :exc:`failureException` exception is raised
   showing the name of the file containing the test and a (sometimes approximate)
   line number.

   Optional argument *module* provides the module to be tested.  It can be a module
   object or a (possibly dotted) module name.  If not specified, the module calling
   this function is used.

   Optional argument *globs* is a dictionary containing the initial global
   variables for the tests.  A new copy of this dictionary is created for each
   test.  By default, *globs* is a new empty dictionary.

   Optional argument *extraglobs* specifies an extra set of global variables, which
   is merged into *globs*.  By default, no extra globals are used.

   Optional argument *test_finder* is the :class:`DocTestFinder` object (or a
   drop-in replacement) that is used to extract doctests from the module.

   Optional arguments *setUp*, *tearDown*, and *optionflags* are the same as for
   function :func:`DocFileSuite` above.

   This function uses the same search technique as :func:`testmod`.

   .. versionchanged:: 3.5
      :func:`DocTestSuite` returns an empty :class:`unittest.TestSuite` if *module*
      contains no docstrings instead of raising :exc:`ValueError`.


Under the covers, :func:`DocTestSuite` creates a :class:`unittest.TestSuite` out
of :class:`doctest.DocTestCase` instances, and :class:`DocTestCase` is a
subclass of :class:`unittest.TestCase`. :class:`DocTestCase` isn't documented
here (it's an internal detail), but studying its code can answer questions about
the exact details of :mod:`unittest` integration.

Similarly, :func:`DocFileSuite` creates a :class:`unittest.TestSuite` out of
:class:`doctest.DocFileCase` instances, and :class:`DocFileCase` is a subclass
of :class:`DocTestCase`.

So both ways of creating a :class:`unittest.TestSuite` run instances of
:class:`DocTestCase`.  This is important for a subtle reason: when you run
:mod:`doctest` functions yourself, you can control the :mod:`doctest` options in
use directly, by passing option flags to :mod:`doctest` functions.  However, if
you're writing a :mod:`unittest` framework, :mod:`unittest` ultimately controls
when and how tests get run.  The framework author typically wants to control
:mod:`doctest` reporting options (perhaps, e.g., specified by command line
options), but there's no way to pass options through :mod:`unittest` to
:mod:`doctest` test runners.

For this reason, :mod:`doctest` also supports a notion of :mod:`doctest`
reporting flags specific to :mod:`unittest` support, via this function:


.. function:: set_unittest_reportflags(flags)

   Set the :mod:`doctest` reporting flags to use.

   Argument *flags* takes the bitwise-or of option flags.  See section
   :ref:`doctest-options`.  Only "reporting flags" can be used.

   This is a module-global setting, and affects all future doctests run by module
   :mod:`unittest`:  the :meth:`runTest` method of :class:`DocTestCase` looks at
   the option flags specified for the test case when the :class:`DocTestCase`
   instance was constructed.  If no reporting flags were specified (which is the
   typical and expected case), :mod:`doctest`'s :mod:`unittest` reporting flags are
   or'ed into the option flags, and the option flags so augmented are passed to the
   :class:`DocTestRunner` instance created to run the doctest.  If any reporting
   flags were specified when the :class:`DocTestCase` instance was constructed,
   :mod:`doctest`'s :mod:`unittest` reporting flags are ignored.

   The value of the :mod:`unittest` reporting flags in effect before the function
   was called is returned by the function.


.. _doctest-advanced-api:

Advanced API
------------

The basic API is a simple wrapper that's intended to make doctest easy to use.
It is fairly flexible, and should meet most users' needs; however, if you
require more fine-grained control over testing, or wish to extend doctest's
capabilities, then you should use the advanced API.

The advanced API revolves around two container classes, which are used to store
the interactive examples extracted from doctest cases:

* :class:`Example`: A single Python :term:`statement`, paired with its expected
  output.

* :class:`DocTest`: A collection of :class:`Example`\ s, typically extracted
  from a single docstring or text file.

Additional processing classes are defined to find, parse, and run, and check
doctest examples:

* :class:`DocTestFinder`: Finds all docstrings in a given module, and uses a
  :class:`DocTestParser` to create a :class:`DocTest` from every docstring that
  contains interactive examples.

* :class:`DocTestParser`: Creates a :class:`DocTest` object from a string (such
  as an object's docstring).

* :class:`DocTestRunner`: Executes the examples in a :class:`DocTest`, and uses
  an :class:`OutputChecker` to verify their output.

* :class:`OutputChecker`: Compares the actual output from a doctest example with
  the expected output, and decides whether they match.

The relationships among these processing classes are summarized in the following
diagram::

                               list of:
   +------+                   +---------+
   |module| --DocTestFinder-> | DocTest | --DocTestRunner-> results
   +------+    |        ^     +---------+     |       ^    (printed)
               |        |     | Example |     |       |
               v        |     |   ...   |     v       |
              DocTestParser   | Example |   OutputChecker
                              +---------+


.. _doctest-doctest:

DocTest Objects
^^^^^^^^^^^^^^^


.. class:: DocTest(examples, globs, name, filename, lineno, docstring)

   A collection of doctest examples that should be run in a single namespace.  The
   constructor arguments are used to initialize the attributes of the same names.


   :class:`DocTest` defines the following attributes.  They are initialized by
   the constructor, and should not be modified directly.


   .. attribute:: examples

      A list of :class:`Example` objects encoding the individual interactive Python
      examples that should be run by this test.


   .. attribute:: globs

      The namespace (aka globals) that the examples should be run in. This is a
      dictionary mapping names to values.  Any changes to the namespace made by the
      examples (such as binding new variables) will be reflected in :attr:`globs`
      after the test is run.


   .. attribute:: name

      A string name identifying the :class:`DocTest`.  Typically, this is the name
      of the object or file that the test was extracted from.


   .. attribute:: filename

      The name of the file that this :class:`DocTest` was extracted from; or
      ``None`` if the filename is unknown, or if the :class:`DocTest` was not
      extracted from a file.


   .. attribute:: lineno

      The line number within :attr:`filename` where this :class:`DocTest` begins, or
      ``None`` if the line number is unavailable.  This line number is zero-based
      with respect to the beginning of the file.


   .. attribute:: docstring

      The string that the test was extracted from, or 'None' if the string is
      unavailable, or if the test was not extracted from a string.


.. _doctest-example:

Example Objects
^^^^^^^^^^^^^^^


.. class:: Example(source, want, exc_msg=None, lineno=0, indent=0, options=None)

   A single interactive example, consisting of a Python statement and its expected
   output.  The constructor arguments are used to initialize the attributes of
   the same names.


   :class:`Example` defines the following attributes.  They are initialized by
   the constructor, and should not be modified directly.


   .. attribute:: source

      A string containing the example's source code.  This source code consists of a
      single Python statement, and always ends with a newline; the constructor adds
      a newline when necessary.


   .. attribute:: want

      The expected output from running the example's source code (either from
      stdout, or a traceback in case of exception).  :attr:`want` ends with a
      newline unless no output is expected, in which case it's an empty string.  The
      constructor adds a newline when necessary.


   .. attribute:: exc_msg

      The exception message generated by the example, if the example is expected to
      generate an exception; or ``None`` if it is not expected to generate an
      exception.  This exception message is compared against the return value of
      :func:`traceback.format_exception_only`.  :attr:`exc_msg` ends with a newline
      unless it's ``None``.  The constructor adds a newline if needed.


   .. attribute:: lineno

      The line number within the string containing this example where the example
      begins.  This line number is zero-based with respect to the beginning of the
      containing string.


   .. attribute:: indent

      The example's indentation in the containing string, i.e., the number of space
      characters that precede the example's first prompt.


   .. attribute:: options

      A dictionary mapping from option flags to ``True`` or ``False``, which is used
      to override default options for this example.  Any option flags not contained
      in this dictionary are left at their default value (as specified by the
      :class:`DocTestRunner`'s :attr:`optionflags`). By default, no options are set.


.. _doctest-doctestfinder:

DocTestFinder objects
^^^^^^^^^^^^^^^^^^^^^


.. class:: DocTestFinder(verbose=False, parser=DocTestParser(), recurse=True, exclude_empty=True)

   A processing class used to extract the :class:`DocTest`\ s that are relevant to
   a given object, from its docstring and the docstrings of its contained objects.
   :class:`DocTest`\ s can be extracted from modules, classes, functions,
   methods, staticmethods, classmethods, and properties.

   The optional argument *verbose* can be used to display the objects searched by
   the finder.  It defaults to ``False`` (no output).

   The optional argument *parser* specifies the :class:`DocTestParser` object (or a
   drop-in replacement) that is used to extract doctests from docstrings.

   If the optional argument *recurse* is false, then :meth:`DocTestFinder.find`
   will only examine the given object, and not any contained objects.

   If the optional argument *exclude_empty* is false, then
   :meth:`DocTestFinder.find` will include tests for objects with empty docstrings.


   :class:`DocTestFinder` defines the following method:


   .. method:: find(obj[, name][, module][, globs][, extraglobs])

      Return a list of the :class:`DocTest`\ s that are defined by *obj*'s
      docstring, or by any of its contained objects' docstrings.

      The optional argument *name* specifies the object's name; this name will be
      used to construct names for the returned :class:`DocTest`\ s.  If *name* is
      not specified, then ``obj.__name__`` is used.

      The optional parameter *module* is the module that contains the given object.
      If the module is not specified or is None, then the test finder will attempt
      to automatically determine the correct module.  The object's module is used:

      * As a default namespace, if *globs* is not specified.

      * To prevent the DocTestFinder from extracting DocTests from objects that are
        imported from other modules.  (Contained objects with modules other than
        *module* are ignored.)

      * To find the name of the file containing the object.

      * To help find the line number of the object within its file.

      If *module* is ``False``, no attempt to find the module will be made.  This is
      obscure, of use mostly in testing doctest itself: if *module* is ``False``, or
      is ``None`` but cannot be found automatically, then all objects are considered
      to belong to the (non-existent) module, so all contained objects will
      (recursively) be searched for doctests.

      The globals for each :class:`DocTest` is formed by combining *globs* and
      *extraglobs* (bindings in *extraglobs* override bindings in *globs*).  A new
      shallow copy of the globals dictionary is created for each :class:`DocTest`.
      If *globs* is not specified, then it defaults to the module's *__dict__*, if
      specified, or ``{}`` otherwise.  If *extraglobs* is not specified, then it
      defaults to ``{}``.


.. _doctest-doctestparser:

DocTestParser objects
^^^^^^^^^^^^^^^^^^^^^


.. class:: DocTestParser()

   A processing class used to extract interactive examples from a string, and use
   them to create a :class:`DocTest` object.


   :class:`DocTestParser` defines the following methods:


   .. method:: get_doctest(string, globs, name, filename, lineno)

      Extract all doctest examples from the given string, and collect them into a
      :class:`DocTest` object.

      *globs*, *name*, *filename*, and *lineno* are attributes for the new
      :class:`DocTest` object.  See the documentation for :class:`DocTest` for more
      information.


   .. method:: get_examples(string, name='<string>')

      Extract all doctest examples from the given string, and return them as a list
      of :class:`Example` objects.  Line numbers are 0-based.  The optional argument
      *name* is a name identifying this string, and is only used for error messages.


   .. method:: parse(string, name='<string>')

      Divide the given string into examples and intervening text, and return them as
      a list of alternating :class:`Example`\ s and strings. Line numbers for the
      :class:`Example`\ s are 0-based.  The optional argument *name* is a name
      identifying this string, and is only used for error messages.


.. _doctest-doctestrunner:

DocTestRunner objects
^^^^^^^^^^^^^^^^^^^^^


.. class:: DocTestRunner(checker=None, verbose=None, optionflags=0)

   A processing class used to execute and verify the interactive examples in a
   :class:`DocTest`.

   The comparison between expected outputs and actual outputs is done by an
   :class:`OutputChecker`.  This comparison may be customized with a number of
   option flags; see section :ref:`doctest-options` for more information.  If the
   option flags are insufficient, then the comparison may also be customized by
   passing a subclass of :class:`OutputChecker` to the constructor.

   The test runner's display output can be controlled in two ways. First, an output
   function can be passed to :meth:`TestRunner.run`; this function will be called
   with strings that should be displayed.  It defaults to ``sys.stdout.write``.  If
   capturing the output is not sufficient, then the display output can be also
   customized by subclassing DocTestRunner, and overriding the methods
   :meth:`report_start`, :meth:`report_success`,
   :meth:`report_unexpected_exception`, and :meth:`report_failure`.

   The optional keyword argument *checker* specifies the :class:`OutputChecker`
   object (or drop-in replacement) that should be used to compare the expected
   outputs to the actual outputs of doctest examples.

   The optional keyword argument *verbose* controls the :class:`DocTestRunner`'s
   verbosity.  If *verbose* is ``True``, then information is printed about each
   example, as it is run.  If *verbose* is ``False``, then only failures are
   printed.  If *verbose* is unspecified, or ``None``, then verbose output is used
   iff the command-line switch ``-v`` is used.

   The optional keyword argument *optionflags* can be used to control how the test
   runner compares expected output to actual output, and how it displays failures.
   For more information, see section :ref:`doctest-options`.


   :class:`DocTestParser` defines the following methods:


   .. method:: report_start(out, test, example)

      Report that the test runner is about to process the given example. This method
      is provided to allow subclasses of :class:`DocTestRunner` to customize their
      output; it should not be called directly.

      *example* is the example about to be processed.  *test* is the test
      *containing example*.  *out* is the output function that was passed to
      :meth:`DocTestRunner.run`.


   .. method:: report_success(out, test, example, got)

      Report that the given example ran successfully.  This method is provided to
      allow subclasses of :class:`DocTestRunner` to customize their output; it
      should not be called directly.

      *example* is the example about to be processed.  *got* is the actual output
      from the example.  *test* is the test containing *example*.  *out* is the
      output function that was passed to :meth:`DocTestRunner.run`.


   .. method:: report_failure(out, test, example, got)

      Report that the given example failed.  This method is provided to allow
      subclasses of :class:`DocTestRunner` to customize their output; it should not
      be called directly.

      *example* is the example about to be processed.  *got* is the actual output
      from the example.  *test* is the test containing *example*.  *out* is the
      output function that was passed to :meth:`DocTestRunner.run`.


   .. method:: report_unexpected_exception(out, test, example, exc_info)

      Report that the given example raised an unexpected exception. This method is
      provided to allow subclasses of :class:`DocTestRunner` to customize their
      output; it should not be called directly.

      *example* is the example about to be processed. *exc_info* is a tuple
      containing information about the unexpected exception (as returned by
      :func:`sys.exc_info`). *test* is the test containing *example*.  *out* is the
      output function that was passed to :meth:`DocTestRunner.run`.


   .. method:: run(test, compileflags=None, out=None, clear_globs=True)

      Run the examples in *test* (a :class:`DocTest` object), and display the
      results using the writer function *out*.

      The examples are run in the namespace ``test.globs``.  If *clear_globs* is
      true (the default), then this namespace will be cleared after the test runs,
      to help with garbage collection. If you would like to examine the namespace
      after the test completes, then use *clear_globs=False*.

      *compileflags* gives the set of flags that should be used by the Python
      compiler when running the examples.  If not specified, then it will default to
      the set of future-import flags that apply to *globs*.

      The output of each example is checked using the :class:`DocTestRunner`'s
      output checker, and the results are formatted by the
      :meth:`DocTestRunner.report_\*` methods.


   .. method:: summarize(verbose=None)

      Print a summary of all the test cases that have been run by this DocTestRunner,
      and return a :term:`named tuple` ``TestResults(failed, attempted)``.

      The optional *verbose* argument controls how detailed the summary is.  If the
      verbosity is not specified, then the :class:`DocTestRunner`'s verbosity is
      used.

.. _doctest-outputchecker:

OutputChecker objects
^^^^^^^^^^^^^^^^^^^^^


.. class:: OutputChecker()

   A class used to check the whether the actual output from a doctest example
   matches the expected output.  :class:`OutputChecker` defines two methods:
   :meth:`check_output`, which compares a given pair of outputs, and returns true
   if they match; and :meth:`output_difference`, which returns a string describing
   the differences between two outputs.


   :class:`OutputChecker` defines the following methods:

   .. method:: check_output(want, got, optionflags)

      Return ``True`` iff the actual output from an example (*got*) matches the
      expected output (*want*).  These strings are always considered to match if
      they are identical; but depending on what option flags the test runner is
      using, several non-exact match types are also possible.  See section
      :ref:`doctest-options` for more information about option flags.


   .. method:: output_difference(example, got, optionflags)

      Return a string describing the differences between the expected output for a
      given example (*example*) and the actual output (*got*).  *optionflags* is the
      set of option flags used to compare *want* and *got*.


.. _doctest-debugging:

Debugging
---------

Doctest provides several mechanisms for debugging doctest examples:

* Several functions convert doctests to executable Python programs, which can be
  run under the Python debugger, :mod:`pdb`.

* The :class:`DebugRunner` class is a subclass of :class:`DocTestRunner` that
  raises an exception for the first failing example, containing information about
  that example. This information can be used to perform post-mortem debugging on
  the example.

* The :mod:`unittest` cases generated by :func:`DocTestSuite` support the
  :meth:`debug` method defined by :class:`unittest.TestCase`.

* You can add a call to :func:`pdb.set_trace` in a doctest example, and you'll
  drop into the Python debugger when that line is executed.  Then you can inspect
  current values of variables, and so on.  For example, suppose :file:`a.py`
  contains just this module docstring::

     """
     >>> def f(x):
     ...     g(x*2)
     >>> def g(x):
     ...     print(x+3)
     ...     import pdb; pdb.set_trace()
     >>> f(3)
     9
     """

  Then an interactive Python session may look like this::

     >>> import a, doctest
     >>> doctest.testmod(a)
     --Return--
     > <doctest a[1]>(3)g()->None
     -> import pdb; pdb.set_trace()
     (Pdb) list
       1     def g(x):
       2         print(x+3)
       3  ->     import pdb; pdb.set_trace()
     [EOF]
     (Pdb) p x
     6
     (Pdb) step
     --Return--
     > <doctest a[0]>(2)f()->None
     -> g(x*2)
     (Pdb) list
       1     def f(x):
       2  ->     g(x*2)
     [EOF]
     (Pdb) p x
     3
     (Pdb) step
     --Return--
     > <doctest a[2]>(1)?()->None
     -> f(3)
     (Pdb) cont
     (0, 3)
     >>>


Functions that convert doctests to Python code, and possibly run the synthesized
code under the debugger:


.. function:: script_from_examples(s)

   Convert text with examples to a script.

   Argument *s* is a string containing doctest examples.  The string is converted
   to a Python script, where doctest examples in *s* are converted to regular code,
   and everything else is converted to Python comments.  The generated script is
   returned as a string. For example, ::

      import doctest
      print(doctest.script_from_examples(r"""
          Set x and y to 1 and 2.
          >>> x, y = 1, 2

          Print their sum:
          >>> print(x+y)
          3
      """))

   displays::

      # Set x and y to 1 and 2.
      x, y = 1, 2
      #
      # Print their sum:
      print(x+y)
      # Expected:
      ## 3

   This function is used internally by other functions (see below), but can also be
   useful when you want to transform an interactive Python session into a Python
   script.


.. function:: testsource(module, name)

   Convert the doctest for an object to a script.

   Argument *module* is a module object, or dotted name of a module, containing the
   object whose doctests are of interest.  Argument *name* is the name (within the
   module) of the object with the doctests of interest.  The result is a string,
   containing the object's docstring converted to a Python script, as described for
   :func:`script_from_examples` above.  For example, if module :file:`a.py`
   contains a top-level function :func:`f`, then ::

      import a, doctest
      print(doctest.testsource(a, "a.f"))

   prints a script version of function :func:`f`'s docstring, with doctests
   converted to code, and the rest placed in comments.


.. function:: debug(module, name, pm=False)

   Debug the doctests for an object.

   The *module* and *name* arguments are the same as for function
   :func:`testsource` above.  The synthesized Python script for the named object's
   docstring is written to a temporary file, and then that file is run under the
   control of the Python debugger, :mod:`pdb`.

   A shallow copy of ``module.__dict__`` is used for both local and global
   execution context.

   Optional argument *pm* controls whether post-mortem debugging is used.  If *pm*
   has a true value, the script file is run directly, and the debugger gets
   involved only if the script terminates via raising an unhandled exception.  If
   it does, then post-mortem debugging is invoked, via :func:`pdb.post_mortem`,
   passing the traceback object from the unhandled exception.  If *pm* is not
   specified, or is false, the script is run under the debugger from the start, via
   passing an appropriate :func:`exec` call to :func:`pdb.run`.


.. function:: debug_src(src, pm=False, globs=None)

   Debug the doctests in a string.

   This is like function :func:`debug` above, except that a string containing
   doctest examples is specified directly, via the *src* argument.

   Optional argument *pm* has the same meaning as in function :func:`debug` above.

   Optional argument *globs* gives a dictionary to use as both local and global
   execution context.  If not specified, or ``None``, an empty dictionary is used.
   If specified, a shallow copy of the dictionary is used.


The :class:`DebugRunner` class, and the special exceptions it may raise, are of
most interest to testing framework authors, and will only be sketched here.  See
the source code, and especially :class:`DebugRunner`'s docstring (which is a
doctest!) for more details:


.. class:: DebugRunner(checker=None, verbose=None, optionflags=0)

   A subclass of :class:`DocTestRunner` that raises an exception as soon as a
   failure is encountered.  If an unexpected exception occurs, an
   :exc:`UnexpectedException` exception is raised, containing the test, the
   example, and the original exception.  If the output doesn't match, then a
   :exc:`DocTestFailure` exception is raised, containing the test, the example, and
   the actual output.

   For information about the constructor parameters and methods, see the
   documentation for :class:`DocTestRunner` in section :ref:`doctest-advanced-api`.

There are two exceptions that may be raised by :class:`DebugRunner` instances:


.. exception:: DocTestFailure(test, example, got)

   An exception raised by :class:`DocTestRunner` to signal that a doctest example's
   actual output did not match its expected output. The constructor arguments are
   used to initialize the attributes of the same names.

:exc:`DocTestFailure` defines the following attributes:


.. attribute:: DocTestFailure.test

   The :class:`DocTest` object that was being run when the example failed.


.. attribute:: DocTestFailure.example

   The :class:`Example` that failed.


.. attribute:: DocTestFailure.got

   The example's actual output.


.. exception:: UnexpectedException(test, example, exc_info)

   An exception raised by :class:`DocTestRunner` to signal that a doctest
   example raised an unexpected exception.  The constructor arguments are used
   to initialize the attributes of the same names.

:exc:`UnexpectedException` defines the following attributes:


.. attribute:: UnexpectedException.test

   The :class:`DocTest` object that was being run when the example failed.


.. attribute:: UnexpectedException.example

   The :class:`Example` that failed.


.. attribute:: UnexpectedException.exc_info

   A tuple containing information about the unexpected exception, as returned by
   :func:`sys.exc_info`.


.. _doctest-soapbox:

Soapbox
-------

As mentioned in the introduction, :mod:`doctest` has grown to have three primary
uses:

#. Checking examples in docstrings.

#. Regression testing.

#. Executable documentation / literate testing.

These uses have different requirements, and it is important to distinguish them.
In particular, filling your docstrings with obscure test cases makes for bad
documentation.

When writing a docstring, choose docstring examples with care. There's an art to
this that needs to be learned---it may not be natural at first.  Examples should
add genuine value to the documentation.  A good example can often be worth many
words. If done with care, the examples will be invaluable for your users, and
will pay back the time it takes to collect them many times over as the years go
by and things change.  I'm still amazed at how often one of my :mod:`doctest`
examples stops working after a "harmless" change.

Doctest also makes an excellent tool for regression testing, especially if you
don't skimp on explanatory text.  By interleaving prose and examples, it becomes
much easier to keep track of what's actually being tested, and why.  When a test
fails, good prose can make it much easier to figure out what the problem is, and
how it should be fixed.  It's true that you could write extensive comments in
code-based testing, but few programmers do. Many have found that using doctest
approaches instead leads to much clearer tests.  Perhaps this is simply because
doctest makes writing prose a little easier than writing code, while writing
comments in code is a little harder.  I think it goes deeper than just that:
the natural attitude when writing a doctest-based test is that you want to
explain the fine points of your software, and illustrate them with examples.
This in turn naturally leads to test files that start with the simplest
features, and logically progress to complications and edge cases.  A coherent
narrative is the result, instead of a collection of isolated functions that test
isolated bits of functionality seemingly at random.  It's a different attitude,
and produces different results, blurring the distinction between testing and
explaining.

Regression testing is best confined to dedicated objects or files.  There are
several options for organizing tests:

* Write text files containing test cases as interactive examples, and test the
  files using :func:`testfile` or :func:`DocFileSuite`.  This is recommended,
  although is easiest to do for new projects, designed from the start to use
  doctest.

* Define functions named ``_regrtest_topic`` that consist of single docstrings,
  containing test cases for the named topics.  These functions can be included in
  the same file as the module, or separated out into a separate test file.

* Define a ``__test__`` dictionary mapping from regression test topics to
  docstrings containing test cases.

When you have placed your tests in a module, the module can itself be the test
runner.  When a test fails, you can arrange for your test runner to re-run only
the failing doctest while you debug the problem.  Here is a minimal example of
such a test runner::

    if __name__ == '__main__':
        import doctest
        flags = doctest.REPORT_NDIFF|doctest.FAIL_FAST
        if len(sys.argv) > 1:
            name = sys.argv[1]
            if name in globals():
                obj = globals()[name]
            else:
                obj = __test__[name]
            doctest.run_docstring_examples(obj, globals(), name=name,
                                           optionflags=flags)
        else:
            fail, total = doctest.testmod(optionflags=flags)
            print("{} failures out of {} tests".format(fail, total))


.. rubric:: Footnotes

.. [#] Examples containing both expected output and an exception are not supported.
   Trying to guess where one ends and the other begins is too error-prone, and that
   also makes for a confusing test.