Doc/library/tokenize.rst

:mod:`tokenize` --- Tokenizer for Python source
===============================================

.. module:: tokenize
   :synopsis: Lexical scanner for Python source code.

.. moduleauthor:: Ka Ping Yee
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>

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

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

The :mod:`tokenize` module provides a lexical scanner for Python source code,
implemented in Python.  The scanner in this module returns comments as tokens
as well, making it useful for implementing "pretty-printers," including
colorizers for on-screen displays.

To simplify token stream handling, all :ref:`operators` and :ref:`delimiters`
tokens are returned using the generic :data:`~token.OP` token type.  The exact
type can be determined by checking the ``exact_type`` property on the
:term:`named tuple` returned from :func:`tokenize.tokenize`.

Tokenizing Input
----------------

The primary entry point is a :term:`generator`:

.. function:: tokenize(readline)

   The :func:`.tokenize` generator requires one argument, *readline*, which
   must be a callable object which provides the same interface as the
   :meth:`io.IOBase.readline` method of file objects.  Each call to the
   function should return one line of input as bytes.

   The generator produces 5-tuples with these members: the token type; the
   token string; a 2-tuple ``(srow, scol)`` of ints specifying the row and
   column where the token begins in the source; a 2-tuple ``(erow, ecol)`` of
   ints specifying the row and column where the token ends in the source; and
   the line on which the token was found. The line passed (the last tuple item)
   is the *logical* line; continuation lines are included.  The 5 tuple is
   returned as a :term:`named tuple` with the field names:
   ``type string start end line``.

   The returned :term:`named tuple` has an additional property named
   ``exact_type`` that contains the exact operator type for
   :data:`~token.OP` tokens.  For all other token types ``exact_type``
   equals the named tuple ``type`` field.

   .. versionchanged:: 3.1
      Added support for named tuples.

   .. versionchanged:: 3.3
      Added support for ``exact_type``.

   :func:`.tokenize` determines the source encoding of the file by looking for a
   UTF-8 BOM or encoding cookie, according to :pep:`263`.


All constants from the :mod:`token` module are also exported from
:mod:`tokenize`.

Another function is provided to reverse the tokenization process. This is
useful for creating tools that tokenize a script, modify the token stream, and
write back the modified script.


.. function:: untokenize(iterable)

    Converts tokens back into Python source code.  The *iterable* must return
    sequences with at least two elements, the token type and the token string.
    Any additional sequence elements are ignored.

    The reconstructed script is returned as a single string.  The result is
    guaranteed to tokenize back to match the input so that the conversion is
    lossless and round-trips are assured.  The guarantee applies only to the
    token type and token string as the spacing between tokens (column
    positions) may change.

    It returns bytes, encoded using the :data:`~token.ENCODING` token, which
    is the first token sequence output by :func:`.tokenize`.


:func:`.tokenize` needs to detect the encoding of source files it tokenizes. The
function it uses to do this is available:

.. function:: detect_encoding(readline)

    The :func:`detect_encoding` function is used to detect the encoding that
    should be used to decode a Python source file. It requires one argument,
    readline, in the same way as the :func:`.tokenize` generator.

    It will call readline a maximum of twice, and return the encoding used
    (as a string) and a list of any lines (not decoded from bytes) it has read
    in.

    It detects the encoding from the presence of a UTF-8 BOM or an encoding
    cookie as specified in :pep:`263`. If both a BOM and a cookie are present,
    but disagree, a :exc:`SyntaxError` will be raised. Note that if the BOM is found,
    ``'utf-8-sig'`` will be returned as an encoding.

    If no encoding is specified, then the default of ``'utf-8'`` will be
    returned.

    Use :func:`.open` to open Python source files: it uses
    :func:`detect_encoding` to detect the file encoding.


.. function:: open(filename)

   Open a file in read only mode using the encoding detected by
   :func:`detect_encoding`.

   .. versionadded:: 3.2

.. exception:: TokenError

   Raised when either a docstring or expression that may be split over several
   lines is not completed anywhere in the file, for example::

      """Beginning of
      docstring

   or::

      [1,
       2,
       3

Note that unclosed single-quoted strings do not cause an error to be
raised. They are tokenized as :data:`~token.ERRORTOKEN`, followed by the
tokenization of their contents.


.. _tokenize-cli:

Command-Line Usage
------------------

.. versionadded:: 3.3

The :mod:`tokenize` module can be executed as a script from the command line.
It is as simple as:

.. code-block:: sh

   python -m tokenize [-e] [filename.py]

The following options are accepted:

.. program:: tokenize

.. cmdoption:: -h, --help

   show this help message and exit

.. cmdoption:: -e, --exact

   display token names using the exact type

If :file:`filename.py` is specified its contents are tokenized to stdout.
Otherwise, tokenization is performed on stdin.

Examples
------------------

Example of a script rewriter that transforms float literals into Decimal
objects::

    from tokenize import tokenize, untokenize, NUMBER, STRING, NAME, OP
    from io import BytesIO

    def decistmt(s):
        """Substitute Decimals for floats in a string of statements.

        >>> from decimal import Decimal
        >>> s = 'print(+21.3e-5*-.1234/81.7)'
        >>> decistmt(s)
        "print (+Decimal ('21.3e-5')*-Decimal ('.1234')/Decimal ('81.7'))"

        The format of the exponent is inherited from the platform C library.
        Known cases are "e-007" (Windows) and "e-07" (not Windows).  Since
        we're only showing 12 digits, and the 13th isn't close to 5, the
        rest of the output should be platform-independent.

        >>> exec(s)  #doctest: +ELLIPSIS
        -3.21716034272e-0...7

        Output from calculations with Decimal should be identical across all
        platforms.

        >>> exec(decistmt(s))
        -3.217160342717258261933904529E-7
        """
        result = []
        g = tokenize(BytesIO(s.encode('utf-8')).readline)  # tokenize the string
        for toknum, tokval, _, _, _ in g:
            if toknum == NUMBER and '.' in tokval:  # replace NUMBER tokens
                result.extend([
                    (NAME, 'Decimal'),
                    (OP, '('),
                    (STRING, repr(tokval)),
                    (OP, ')')
                ])
            else:
                result.append((toknum, tokval))
        return untokenize(result).decode('utf-8')

Example of tokenizing from the command line.  The script::

    def say_hello():
        print("Hello, World!")

    say_hello()

will be tokenized to the following output where the first column is the range
of the line/column coordinates where the token is found, the second column is
the name of the token, and the final column is the value of the token (if any)

.. code-block:: sh

    $ python -m tokenize hello.py
    0,0-0,0:            ENCODING       'utf-8'
    1,0-1,3:            NAME           'def'
    1,4-1,13:           NAME           'say_hello'
    1,13-1,14:          OP             '('
    1,14-1,15:          OP             ')'
    1,15-1,16:          OP             ':'
    1,16-1,17:          NEWLINE        '\n'
    2,0-2,4:            INDENT         '    '
    2,4-2,9:            NAME           'print'
    2,9-2,10:           OP             '('
    2,10-2,25:          STRING         '"Hello, World!"'
    2,25-2,26:          OP             ')'
    2,26-2,27:          NEWLINE        '\n'
    3,0-3,1:            NL             '\n'
    4,0-4,0:            DEDENT         ''
    4,0-4,9:            NAME           'say_hello'
    4,9-4,10:           OP             '('
    4,10-4,11:          OP             ')'
    4,11-4,12:          NEWLINE        '\n'
    5,0-5,0:            ENDMARKER      ''

The exact token type names can be displayed using the :option:`-e` option:

.. code-block:: sh

    $ python -m tokenize -e hello.py
    0,0-0,0:            ENCODING       'utf-8'
    1,0-1,3:            NAME           'def'
    1,4-1,13:           NAME           'say_hello'
    1,13-1,14:          LPAR           '('
    1,14-1,15:          RPAR           ')'
    1,15-1,16:          COLON          ':'
    1,16-1,17:          NEWLINE        '\n'
    2,0-2,4:            INDENT         '    '
    2,4-2,9:            NAME           'print'
    2,9-2,10:           LPAR           '('
    2,10-2,25:          STRING         '"Hello, World!"'
    2,25-2,26:          RPAR           ')'
    2,26-2,27:          NEWLINE        '\n'
    3,0-3,1:            NL             '\n'
    4,0-4,0:            DEDENT         ''
    4,0-4,9:            NAME           'say_hello'
    4,9-4,10:           LPAR           '('
    4,10-4,11:          RPAR           ')'
    4,11-4,12:          NEWLINE        '\n'
    5,0-5,0:            ENDMARKER      ''