Doc/library/py_compile.rst

:mod:`py_compile` --- Compile Python source files
=================================================

.. module:: py_compile
   :synopsis: Generate byte-code files from Python source files.

.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
.. documentation based on module docstrings

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

.. index:: pair: file; byte-code

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

The :mod:`py_compile` module provides a function to generate a byte-code file
from a source file, and another function used when the module source file is
invoked as a script.

Though not often needed, this function can be useful when installing modules for
shared use, especially if some of the users may not have permission to write the
byte-code cache files in the directory containing the source code.


.. exception:: PyCompileError

   Exception raised when an error occurs while attempting to compile the file.


.. function:: compile(file, cfile=None, dfile=None, doraise=False, optimize=-1, invalidation_mode=PycInvalidationMode.TIMESTAMP)

   Compile a source file to byte-code and write out the byte-code cache file.
   The source code is loaded from the file named *file*.  The byte-code is
   written to *cfile*, which defaults to the :pep:`3147`/:pep:`488` path, ending
   in ``.pyc``.
   For example, if *file* is ``/foo/bar/baz.py`` *cfile* will default to
   ``/foo/bar/__pycache__/baz.cpython-32.pyc`` for Python 3.2.  If *dfile* is
   specified, it is used as the name of the source file in error messages when
   instead of *file*.  If *doraise* is true, a :exc:`PyCompileError` is raised
   when an error is encountered while compiling *file*. If *doraise* is false
   (the default), an error string is written to ``sys.stderr``, but no exception
   is raised.  This function returns the path to byte-compiled file, i.e.
   whatever *cfile* value was used.

   If the path that *cfile* becomes (either explicitly specified or computed)
   is a symlink or non-regular file, :exc:`FileExistsError` will be raised.
   This is to act as a warning that import will turn those paths into regular
   files if it is allowed to write byte-compiled files to those paths. This is
   a side-effect of import using file renaming to place the final byte-compiled
   file into place to prevent concurrent file writing issues.

   *optimize* controls the optimization level and is passed to the built-in
   :func:`compile` function.  The default of ``-1`` selects the optimization
   level of the current interpreter.

   *invalidation_mode* should be a member of the :class:`PycInvalidationMode`
   enum and controls how the generated bytecode cache is invalidated at
   runtime.  The default is :attr:`PycInvalidationMode.CHECKED_HASH` if
   the :envvar:`SOURCE_DATE_EPOCH` environment variable is set, otherwise
   the default is :attr:`PycInvalidationMode.TIMESTAMP`.

   .. versionchanged:: 3.2
      Changed default value of *cfile* to be :PEP:`3147`-compliant.  Previous
      default was *file* + ``'c'`` (``'o'`` if optimization was enabled).
      Also added the *optimize* parameter.

   .. versionchanged:: 3.4
      Changed code to use :mod:`importlib` for the byte-code cache file writing.
      This means file creation/writing semantics now match what :mod:`importlib`
      does, e.g. permissions, write-and-move semantics, etc. Also added the
      caveat that :exc:`FileExistsError` is raised if *cfile* is a symlink or
      non-regular file.

   .. versionchanged:: 3.7
      The *invalidation_mode* parameter was added as specified in :pep:`552`.
      If the :envvar:`SOURCE_DATE_EPOCH` environment variable is set,
      *invalidation_mode* will be forced to
      :attr:`PycInvalidationMode.CHECKED_HASH`.

   .. versionchanged:: 3.7.2
      The :envvar:`SOURCE_DATE_EPOCH` environment variable no longer
      overrides the value of the *invalidation_mode* argument, and determines
      its default value instead.


.. class:: PycInvalidationMode

   A enumeration of possible methods the interpreter can use to determine
   whether a bytecode file is up to date with a source file. The ``.pyc`` file
   indicates the desired invalidation mode in its header. See
   :ref:`pyc-invalidation` for more information on how Python invalidates
   ``.pyc`` files at runtime.

   .. versionadded:: 3.7

   .. attribute:: TIMESTAMP

      The ``.pyc`` file includes the timestamp and size of the source file,
      which Python will compare against the metadata of the source file at
      runtime to determine if the ``.pyc`` file needs to be regenerated.

   .. attribute:: CHECKED_HASH

      The ``.pyc`` file includes a hash of the source file content, which Python
      will compare against the source at runtime to determine if the ``.pyc``
      file needs to be regenerated.

   .. attribute:: UNCHECKED_HASH

      Like :attr:`CHECKED_HASH`, the ``.pyc`` file includes a hash of the source
      file content. However, Python will at runtime assume the ``.pyc`` file is
      up to date and not validate the ``.pyc`` against the source file at all.

      This option is useful when the ``.pycs`` are kept up to date by some
      system external to Python like a build system.


.. function:: main(args=None)

   Compile several source files.  The files named in *args* (or on the command
   line, if *args* is ``None``) are compiled and the resulting byte-code is
   cached in the normal manner.  This function does not search a directory
   structure to locate source files; it only compiles files named explicitly.
   If ``'-'`` is the only parameter in args, the list of files is taken from
   standard input.

   .. versionchanged:: 3.2
      Added support for ``'-'``.

When this module is run as a script, the :func:`main` is used to compile all the
files named on the command line.  The exit status is nonzero if one of the files
could not be compiled.


.. seealso::

   Module :mod:`compileall`
      Utilities to compile all Python source files in a directory tree.