Doc/library/codeop.rst

:mod:`codeop` --- Compile Python code
=====================================

.. module:: codeop
   :synopsis: Compile (possibly incomplete) Python code.
.. sectionauthor:: Moshe Zadka <moshez@zadka.site.co.il>
.. sectionauthor:: Michael Hudson <mwh@python.net>

The :mod:`codeop` module provides utilities upon which the Python
read-eval-print loop can be emulated, as is done in the :mod:`code` module.  As
a result, you probably don't want to use the module directly; if you want to
include such a loop in your program you probably want to use the :mod:`code`
module instead.

There are two parts to this job:

#. Being able to tell if a line of input completes a Python  statement: in
   short, telling whether to print '``>>>``' or '``...``' next.

#. Remembering which future statements the user has entered, so  subsequent
   input can be compiled with these in effect.

The :mod:`codeop` module provides a way of doing each of these things, and a way
of doing them both.

To do just the former:

.. function:: compile_command(source[, filename[, symbol]])

   Tries to compile *source*, which should be a string of Python code and return a
   code object if *source* is valid Python code. In that case, the filename
   attribute of the code object will be *filename*, which defaults to
   ``'<input>'``. Returns ``None`` if *source* is *not* valid Python code, but is a
   prefix of valid Python code.

   If there is a problem with *source*, an exception will be raised.
   :exc:`SyntaxError` is raised if there is invalid Python syntax, and
   :exc:`OverflowError` or :exc:`ValueError` if there is an invalid literal.

   The *symbol* argument determines whether *source* is compiled as a statement
   (``'single'``, the default) or as an :term:`expression` (``'eval'``).  Any
   other value will cause :exc:`ValueError` to  be raised.

   .. warning::

      It is possible (but not likely) that the parser stops parsing with a
      successful outcome before reaching the end of the source; in this case,
      trailing symbols may be ignored instead of causing an error.  For example,
      a backslash followed by two newlines may be followed by arbitrary garbage.
      This will be fixed once the API for the parser is better.


.. class:: Compile()

   Instances of this class have :meth:`__call__` methods identical in signature to
   the built-in function :func:`compile`, but with the difference that if the
   instance compiles program text containing a :mod:`__future__` statement, the
   instance 'remembers' and compiles all subsequent program texts with the
   statement in force.


.. class:: CommandCompiler()

   Instances of this class have :meth:`__call__` methods identical in signature to
   :func:`compile_command`; the difference is that if the instance compiles program
   text containing a ``__future__`` statement, the instance 'remembers' and
   compiles all subsequent program texts with the statement in force.

A note on version compatibility: the :class:`Compile` and
:class:`CommandCompiler` are new in Python 2.2.  If you want to enable the
future-tracking features of 2.2 but also retain compatibility with 2.1 and
earlier versions of Python you can either write ::

   try:
       from codeop import CommandCompiler
       compile_command = CommandCompiler()
       del CommandCompiler
   except ImportError:
       from codeop import compile_command

which is a low-impact change, but introduces possibly unwanted global state into
your program, or you can write::

   try:
       from codeop import CommandCompiler
   except ImportError:
       def CommandCompiler():
           from codeop import compile_command
           return compile_command

and then call ``CommandCompiler`` every time you need a fresh compiler object.