1 .. _2to3-reference:

     2 

     3 2to3 - Automated Python 2 to 3 code translation

     4 ===============================================

     5 

     6 .. sectionauthor:: Benjamin Peterson

     7 

     8 2to3 is a Python program that reads Python 2.x source code and applies a series

     9 of *fixers* to transform it into valid Python 3.x code.  The standard library

    10 contains a rich set of fixers that will handle almost all code.  2to3 supporting

    11 library :mod:`lib2to3` is, however, a flexible and generic library, so it is

    12 possible to write your own fixers for 2to3.  :mod:`lib2to3` could also be

    13 adapted to custom applications in which Python code needs to be edited

    14 automatically.

    15 

    16 

    17 Using 2to3

    18 ----------

    19 

    20 2to3 will usually be installed with the Python interpreter as a script.  It is

    21 also located in the :file:`Tools/scripts` directory of the Python root.

    22 

    23 2to3's basic arguments are a list of files or directories to transform.  The

    24 directories are to recursively traversed for Python sources.

    25 

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

    27 

    28    def greet(name):

    29        print "Hello, {0}!".format(name)

    30    print "What's your name?"

    31    name = raw_input()

    32    greet(name)

    33 

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

    35 

    36    $ 2to3 example.py

    37 

    38 A diff against the original source file is printed.  2to3 can also write the

    39 needed modifications right back to the source file.  (Of course, a backup of the

    40 original is also be made unless :option:`-n` is also given.)  Writing the

    41 changes back is enabled with the :option:`-w` flag::

    42 

    43    $ 2to3 -w example.py

    44 

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

    46 

    47    def greet(name):

    48        print("Hello, {0}!".format(name))

    49    print("What's your name?")

    50    name = input()

    51    greet(name)

    52 

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

    54 

    55 By default, 2to3 runs a set of predefined fixers.  The :option:`-l` flag lists

    56 all available fixers.  An explicit set of fixers to run can be given with

    57 :option:`-f`.  Likewise the :option:`-x` explicitly disables a fixer.  The

    58 following example runs only the ``imports`` and ``has_key`` fixers::

    59 

    60    $ 2to3 -f imports -f has_key example.py

    61 

    62 This command runs every fixer except the ``apply`` fixer::

    63 

    64    $ 2to3 -x apply example.py

    65 

    66 Some fixers are *explicit*, meaning they aren't run by default and must be

    67 listed on the command line to be run.  Here, in addition to the default fixers,

    68 the ``idioms`` fixer is run::

    69 

    70    $ 2to3 -f all -f idioms example.py

    71 

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

    73 

    74 Sometimes 2to3 will find a place in your source code that needs to be changed,

    75 but 2to3 cannot fix automatically.  In this case, 2to3 will print a warning

    76 beneath the diff for a file.  You should address the warning in order to have

    77 compliant 3.x code.

    78 

    79 2to3 can also refactor doctests.  To enable this mode, use the :option:`-d`

    80 flag.  Note that *only* doctests will be refactored.  This also doesn't require

    81 the module to be valid Python.  For example, doctest like examples in a reST

    82 document could also be refactored with this option.

    83 

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

    85 process.

    86 

    87 When the :option:`-p` is passed, 2to3 treats ``print`` as a function instead of

    88 a statement.  This is useful when ``from __future__ import print_function`` is

    89 being used.  If this option is not given, the print fixer will surround print

    90 calls in an extra set of parentheses because it cannot differentiate between the

    91 print statement with parentheses (such as ``print ("a" + "b" + "c")``) and a

    92 true function call.

    93 

    94 

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

    96 -------------------------------

    97 

    98 .. module:: lib2to3

    99    :synopsis: the 2to3 library

   100 .. moduleauthor:: Guido van Rossum

   101 .. moduleauthor:: Collin Winter

   102 

   103 

   104 .. warning::

   105 

   106    The :mod:`lib2to3` API should be considered unstable and may change

   107    drastically in the future.

   108 

   109 .. XXX What is the public interface anyway?