cpython/Doc/library/2to3.rst

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.. _2to3-reference:
2to3 - Automated Python 2 to 3 code translation
===============================================
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.. sectionauthor:: Benjamin Peterson <benjamin@python.org>
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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.
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.. _2to3-using:
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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.
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Here is a sample Python 2.x source file, :file:`example.py`::
def greet(name):
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print "Hello, {0}!".format(name)
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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
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needed modifications right back to the source file. (A backup of the original
file is made unless :option:`-n` is also given.) Writing the changes back is
enabled with the :option:`-w` flag::
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$ 2to3 -w example.py
After transformation, :file:`example.py` looks like this::
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def greet(name):
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print("Hello, {0}!".format(name))
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print("What's your name?")
name = input()
greet(name)
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Comments and exact indentation are preserved throughout the translation process.
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By default, 2to3 runs a set of :ref:`predefined fixers <2to3-fixers>`. The
:option:`-l` flag lists all available fixers. An explicit set of fixers to run
can be given with :option:`-f`. Likewise the :option:`-x` explicitly disables a
fixer. The following example runs only the ``imports`` and ``has_key`` fixers::
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$ 2to3 -f imports -f has_key example.py
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This command runs every fixer except the ``apply`` fixer::
$ 2to3 -x apply example.py
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Some fixers are *explicit*, meaning they aren't run by default and must be
listed on the command line to be run. Here, in addition to the default fixers,
the ``idioms`` fixer is run::
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$ 2to3 -f all -f idioms example.py
Notice how passing ``all`` enables all default fixers.
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Sometimes 2to3 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.
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The :option:`-v` option enables output of more information on the translation
process.
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Since some print statements can be parsed as function calls or statements, 2to3
cannot always read files containing the print function. When 2to3 detects the
presence of the ``from __future__ import print_function`` compiler directive, it
modifies its internal grammar to interpret :func:`print` as a function. This
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change can also be enabled manually with the :option:`-p` flag. Use
:option:`-p` to run fixers on code that already has had its print statements
converted.
The :option:`-o` or :option:`--output-dir` option allows specification of an
alternate directory for processed output files to be written to. The
:option:`-n` flag is required when using this as backup files do not make sense
when not overwriting the input files.
.. versionadded:: 2.7.3
The :option:`-o` option was added.
The :option:`-W` or :option:`--write-unchanged-files` flag tells 2to3 to always
write output files even if no changes were required to the file. This is most
useful with :option:`-o` so that an entire Python source tree is copied with
translation from one directory to another.
This option implies the :option:`-w` flag as it would not make sense otherwise.
.. versionadded:: 2.7.3
The :option:`-W` flag was added.
The :option:`--add-suffix` option specifies a string to append to all output
filenames. The :option:`-n` flag is required when specifying this as backups
are not necessary when writing to different filenames. Example::
$ 2to3 -n -W --add-suffix=3 example.py
Will cause a converted file named ``example.py3`` to be written.
.. versionadded:: 2.7.3
The :option:`--add-suffix` option was added.
To translate an entire project from one directory tree to another use::
$ 2to3 --output-dir=python3-version/mycode -W -n python2-version/mycode
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.. _2to3-fixers:
Fixers
------
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Each step of transforming code is encapsulated in a fixer. The command ``2to3
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-l`` lists them. As :ref:`documented above <2to3-using>`, each can be turned on
and off individually. They are described here in more detail.
.. 2to3fixer:: apply
Removes usage of :func:`apply`. For example ``apply(function, *args,
**kwargs)`` is converted to ``function(*args, **kwargs)``.
.. 2to3fixer:: basestring
Converts :class:`basestring` to :class:`str`.
.. 2to3fixer:: buffer
Converts :class:`buffer` to :class:`memoryview`. This fixer is optional
because the :class:`memoryview` API is similar but not exactly the same as
that of :class:`buffer`.
.. 2to3fixer:: callable
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Converts ``callable(x)`` to ``isinstance(x, collections.Callable)``, adding
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an import to :mod:`collections` if needed. Note ``callable(x)`` has returned
in Python 3.2, so if you do not intend to support Python 3.1, you can disable
this fixer.
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.. 2to3fixer:: dict
Fixes dictionary iteration methods. :meth:`dict.iteritems` is converted to
:meth:`dict.items`, :meth:`dict.iterkeys` to :meth:`dict.keys`, and
:meth:`dict.itervalues` to :meth:`dict.values`. Similarly,
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:meth:`dict.viewitems`, :meth:`dict.viewkeys` and :meth:`dict.viewvalues` are
converted respectively to :meth:`dict.items`, :meth:`dict.keys` and
:meth:`dict.values`. It also wraps existing usages of :meth:`dict.items`,
:meth:`dict.keys`, and :meth:`dict.values` in a call to :class:`list`.
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.. 2to3fixer:: except
Converts ``except X, T`` to ``except X as T``.
.. 2to3fixer:: exec
Converts the :keyword:`exec` statement to the :func:`exec` function.
.. 2to3fixer:: execfile
Removes usage of :func:`execfile`. The argument to :func:`execfile` is
wrapped in calls to :func:`open`, :func:`compile`, and :func:`exec`.
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.. 2to3fixer:: exitfunc
Changes assignment of :attr:`sys.exitfunc` to use of the :mod:`atexit`
module.
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.. 2to3fixer:: filter
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Wraps :func:`filter` usage in a :class:`list` call.
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.. 2to3fixer:: funcattrs
Fixes function attributes that have been renamed. For example,
``my_function.func_closure`` is converted to ``my_function.__closure__``.
.. 2to3fixer:: future
Removes ``from __future__ import new_feature`` statements.
.. 2to3fixer:: getcwdu
Renames :func:`os.getcwdu` to :func:`os.getcwd`.
.. 2to3fixer:: has_key
Changes ``dict.has_key(key)`` to ``key in dict``.
.. 2to3fixer:: idioms
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This optional fixer performs several transformations that make Python code
more idiomatic. Type comparisons like ``type(x) is SomeClass`` and
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``type(x) == SomeClass`` are converted to ``isinstance(x, SomeClass)``.
``while 1`` becomes ``while True``. This fixer also tries to make use of
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:func:`sorted` in appropriate places. For example, this block ::
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L = list(some_iterable)
L.sort()
is changed to ::
L = sorted(some_iterable)
.. 2to3fixer:: import
Detects sibling imports and converts them to relative imports.
.. 2to3fixer:: imports
Handles module renames in the standard library.
.. 2to3fixer:: imports2
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Handles other modules renames in the standard library. It is separate from
the :2to3fixer:`imports` fixer only because of technical limitations.
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.. 2to3fixer:: input
Converts ``input(prompt)`` to ``eval(input(prompt))``
.. 2to3fixer:: intern
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Converts :func:`intern` to :func:`sys.intern`.
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.. 2to3fixer:: isinstance
Fixes duplicate types in the second argument of :func:`isinstance`. For
example, ``isinstance(x, (int, int))`` is converted to ``isinstance(x,
(int))``.
.. 2to3fixer:: itertools_imports
Removes imports of :func:`itertools.ifilter`, :func:`itertools.izip`, and
:func:`itertools.imap`. Imports of :func:`itertools.ifilterfalse` are also
changed to :func:`itertools.filterfalse`.
.. 2to3fixer:: itertools
Changes usage of :func:`itertools.ifilter`, :func:`itertools.izip`, and
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:func:`itertools.imap` to their built-in equivalents.
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:func:`itertools.ifilterfalse` is changed to :func:`itertools.filterfalse`.
.. 2to3fixer:: long
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Strips the ``L`` suffix on long literals and renames :class:`long` to
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:class:`int`.
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.. 2to3fixer:: map
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Wraps :func:`map` in a :class:`list` call. It also changes ``map(None, x)``
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to ``list(x)``. Using ``from future_builtins import map`` disables this
fixer.
.. 2to3fixer:: metaclass
Converts the old metaclass syntax (``__metaclass__ = Meta`` in the class
body) to the new (``class X(metaclass=Meta)``).
.. 2to3fixer:: methodattrs
Fixes old method attribute names. For example, ``meth.im_func`` is converted
to ``meth.__func__``.
.. 2to3fixer:: ne
Converts the old not-equal syntax, ``<>``, to ``!=``.
.. 2to3fixer:: next
Converts the use of iterator's :meth:`~iterator.next` methods to the
:func:`next` function. It also renames :meth:`next` methods to
:meth:`~object.__next__`.
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.. 2to3fixer:: nonzero
Renames :meth:`~object.__nonzero__` to :meth:`~object.__bool__`.
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.. 2to3fixer:: numliterals
Converts octal literals into the new syntax.
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.. 2to3fixer:: paren
Add extra parenthesis where they are required in list comprehensions. For
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example, ``[x for x in 1, 2]`` becomes ``[x for x in (1, 2)]``.
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.. 2to3fixer:: print
Converts the :keyword:`print` statement to the :func:`print` function.
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.. 2to3fixer:: raise
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Converts ``raise E, V`` to ``raise E(V)``, and ``raise E, V, T`` to ``raise
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E(V).with_traceback(T)``. If ``E`` is a tuple, the translation will be
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incorrect because substituting tuples for exceptions has been removed in Python 3.
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.. 2to3fixer:: raw_input
Converts :func:`raw_input` to :func:`input`.
.. 2to3fixer:: reduce
Handles the move of :func:`reduce` to :func:`functools.reduce`.
.. 2to3fixer:: renames
Changes :data:`sys.maxint` to :data:`sys.maxsize`.
.. 2to3fixer:: repr
Replaces backtick repr with the :func:`repr` function.
.. 2to3fixer:: set_literal
Replaces use of the :class:`set` constructor with set literals. This fixer
is optional.
.. 2to3fixer:: standard_error
Renames :exc:`StandardError` to :exc:`Exception`.
.. 2to3fixer:: sys_exc
Changes the deprecated :data:`sys.exc_value`, :data:`sys.exc_type`,
:data:`sys.exc_traceback` to use :func:`sys.exc_info`.
.. 2to3fixer:: throw
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Fixes the API change in generator's :meth:`throw` method.
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.. 2to3fixer:: tuple_params
Removes implicit tuple parameter unpacking. This fixer inserts temporary
variables.
.. 2to3fixer:: types
Fixes code broken from the removal of some members in the :mod:`types`
module.
.. 2to3fixer:: unicode
Renames :class:`unicode` to :class:`str`.
.. 2to3fixer:: urllib
Handles the rename of :mod:`urllib` and :mod:`urllib2` to the :mod:`urllib`
package.
.. 2to3fixer:: ws_comma
Removes excess whitespace from comma separated items. This fixer is
optional.
.. 2to3fixer:: xrange
Renames :func:`xrange` to :func:`range` and wraps existing :func:`range`
calls with :class:`list`.
.. 2to3fixer:: xreadlines
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Changes ``for x in file.xreadlines()`` to ``for x in file``.
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.. 2to3fixer:: zip
Wraps :func:`zip` usage in a :class:`list` call. This is disabled when
``from future_builtins import zip`` appears.
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:mod:`lib2to3` - 2to3's library
-------------------------------
.. module:: lib2to3
:synopsis: the 2to3 library
.. moduleauthor:: Guido van Rossum
.. moduleauthor:: Collin Winter
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.. moduleauthor:: Benjamin Peterson <benjamin@python.org>
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.. note::
The :mod:`lib2to3` API should be considered unstable and may change
drastically in the future.
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.. XXX What is the public interface anyway?