cpython/Doc/library/2to3.rst

.. _2to3-reference:

2to3 - Automated Python 2 to 3 code translation
===============================================

.. sectionauthor:: Benjamin Peterson

2to3 is a Python program that reads Python 2.x source code and applies a series
of *fixers* to transform it into valid Python 3.x code.  The standard library
contains a rich set of fixers that will handle almost all code.  2to3 supporting
library :mod:`lib2to3` is, however, a flexible and generic library, so it is
possible to write your own fixers for 2to3.  :mod:`lib2to3` could also be
adapted to custom applications in which Python code needs to be edited
automatically.


Using 2to3
----------

2to3 will usually be installed with the Python interpreter as a script.  It is
also located in the :file:`Tools/scripts` directory of the Python root.

2to3's basic arguments are a list of files or directories to transform.  The
directories are to recursively traversed for Python sources.

Here is a sample Python 2.x source file, :file:`example.py`::

   def greet(name):
       print "Hello, {0}!".format(name)
   print "What's your name?"
   name = raw_input()
   greet(name)

It can be converted to Python 3.x code via 2to3 on the command line::

   $ 2to3 example.py

A diff against the original source file is printed.  2to3 can also write the
needed modifications right back to the source file.  (Of course, a backup of the
original is also be made.)  Writing the changes back is enabled with the
:option:`-w` flag::

   $ 2to3 -w example.py

After transformation, :file:`example.py` looks like this::

   def greet(name):
       print("Hello, {0}!".format(name))
   print("What's your name?")
   name = input()
   greet(name)

Comments and and exact indentation are preserved throughout the translation
process.

By default, 2to3 runs a set of predefined fixers.  The :option:`-l` flag
lists all avaible fixers.  An explicit set of fixers to run can be given by use
of the :option:`-f` flag.  The following example runs only the ``imports`` and
``has_key`` fixers::

   $ 2to3 -f imports -f has_key example.py

Some fixers are *explicit*, meaning they aren't run be default and must be
listed on the command line to be run.  Here, in addition to the default fixers,
the ``idioms`` fixer is run::

   $ 2to3 -f all -f idioms example.py

Notice how passing ``all`` enables all default fixers.

Sometimes 2to3 will find will find a place in your source code that needs to be
changed, but 2to3 cannot fix automatically.  In this case, 2to3 will print a
warning beneath the diff for a file.  You should address the warning in order to
have compliant 3.x code.

2to3 can also refactor doctests.  To enable this mode, use the :option:`-d`
flag.  Note that *only* doctests will be refactored.  This also doesn't require
the module to be valid Python.  For example, doctest like examples in a reST
document could also be refactored with this option.

The :option:`-v` option enables the output of more information on the
translation process.

When the :option:`-p` is passed, 2to3 treats ``print`` as a function instead of
a statement.  This is useful when ``from __future__ import print_function`` is
being used.  If this option is not given, the print fixer will surround print
calls in an extra set of parentheses because it cannot differentiate between the
and print statement with parentheses (such as ``print ("a" + "b" + "c")``) and a
true function call.


:mod:`lib2to3` - 2to3's library
-------------------------------

.. module:: lib2to3
   :synopsis: the 2to3 library
.. moduleauthor:: Guido van Rossum
.. moduleauthor:: Collin Winter


.. warning::

   The :mod:`lib2to3` API should be considered unstable and may change
   drastically in the future.

.. XXX What is the public interface anyway?