The future source tree includes scripts called futurize and pasteurize to aid in making Python 2 code or Python 3 code compatible with both platforms (Py2&3) using the :mod:`future` module. It is based on 2to3 and uses fixers from lib2to3, lib3to2, and python-modernize.

futurize passes Python 2 code through all the appropriate fixers to turn it into valid Python 3 code, and then adds __future__ and future package imports.

For conversions from Python 3 code to Py2/3, use the pasteurize script instead. This converts Py3-only constructs (e.g. new metaclass syntax) and adds __future__ and future imports to the top of each module.

In both cases, the result should be relatively clean Py3-style code that runs mostly unchanged on both Python 2 and Python 3.

For example, running futurize turns this Python 2 code:

import ConfigParser

class Blah(object):
    pass
print 'Hello',

into this code which runs on both Py2 and Py3:

from __future__ import print_function
from future import standard_library

import configparser

class Blah(object):
    pass
print('Hello', end=' ')

To write out all the changes to your Python files that futurize suggests, use the -w flag.

For complex projects, it may be better to divide the porting into two stages. Stage 1 is for "safe" changes that modernize the code but do not break Python 2.6 compatibility or introduce a depdendency on the future package. Stage 2 is to complete the process.

Run with:

futurize --stage1

This applies fixes that modernize Python 2 code without changing the effect of the code. With luck, this will not introduce any bugs into the code, or will at least be trivial to fix. The changes are those that bring the Python code up-to-date without breaking Py2 compatibility. The resulting code will be modern Python 2.6-compatible code plus __future__ imports from the following set:

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

Only those __future__ imports deemed necessary will be added unless the --all-imports command-line option is passed to futurize, in which case they are all added.

The from __future__ import unicode_literals declaration is not added during stage 1.

The changes include:

- except MyException, e:
+ except MyException as e:

- print >>stderr, "Blah"
+ from __future__ import print_function
+ print("Blah", stderr)

Implicit relative imports fixed, e.g.:

- import mymodule
+ from __future__ import absolute_import
+ from . import mymodule

Stage 1 does not add any imports from the future package. The output of stage 1 will probably not (yet) run on Python 3.

The goal for this stage is to create most of the diff for the entire porting process, but without introducing any bugs. It should be uncontroversial and safe to apply to every Python 2 package. The subsequent patches introducing Python 3 compatibility should then be shorter and easier to review.

Run with:

futurize —-stage2 myfolder/*.py

This stage adds a dependency on the future package. The goal for stage 2 is to make further mostly safe changes to the Python 2 code to use Python 3-style code that then still runs on Python 2 with the help of the appropriate builtins and utilities in future.

For example:

name = raw_input('What is your name?\n')

for k, v in d.iteritems():
    assert isinstance(v, basestring)

class MyClass(object):
    def __unicode__(self):
        return u'My object'
    def __str__(self):
        return unicode(self).encode('utf-8')

would be converted by Stage 2 to this code:

from future.builtins import input
from future.builtins import str
from future.utils import iteritems, python_2_unicode_compatible

name = input('What is your name?\n')

for k, v in iteritems(d):
    assert isinstance(v, (str, bytes))

@python_2_unicode_compatible
class MyClass(object):
    def __str__(self):
        return u'My object'

Stage 2 also renames standard-library imports to their Py3 names and adds this import:

from future import standard_library

For example:

import ConfigParser
import

All strings are then unicode (on Py2 as on Py3) unless explicitly marked with a b'' prefix.

Ideally the output of this stage should not be a SyntaxError on either Python 3 or Python 2.

After this, you can run your tests on Python 3 and make further code changes until they pass on Python 3.

The next step would be manually adding wrappers from future to re-enable Python 2 compatibility. See :ref:`what-else` for more info.

After applying stage 2, the recommended step is to decide which of your Python 2 strings represent binary data and to prefix all byte-string literals for binary data with b like b'\x00ABCD'.

After stage 2 conversion, all string literals for textual data without b prefixes will use Python 3's str type (or the backported str object from future on Python 2).

The goal for this stage is to get the tests passing first on Py3 and then on Py2 again with the help of the future package.

Run with:

futurize —-stage2 myfolder/*.py

This adds these further imports to each module:

from __future__ import unicode_literals
from future import standard_library
from future.builtins import open
from future.builtins import str   # etc. for the builtins used in the module

and makes other changes needed to support Python 3, such as renaming standard-library imports to their Py3 names.

All strings are then implicitly unicode (on Py2 as on Py3) unless they were previously explicitly marked with a b'' prefix.

Ideally the output of this stage should not be a SyntaxError on either Python 3 or Python 2.

After this, you can run your tests on Python 3 and make further code changes until they pass on Python 3.

The next step would be manually adding wrappers from future to re-enable Python 2 compatibility. See :ref:`what-else` for more info.

Running pasteurize -w mypy3module.py turns this Python 3 code:

import configparser

class Blah:
    pass
print('Hello', end=None)

into this code which runs on both Py2 and Py3:

from __future__ import print_function
from future import standard_library

import configparser

class Blah(object):
    pass
print('Hello', end=None)

Notice that both futurize and pasteurize create explicit new-style classes that inherit from object on both Python versions, and both refer to stdlib modules (as well as builtins) under their Py3 names.

pasteurize also handles the following Python 3 features:

• keyword-only arguments
• metaclasses (using :func:`~future.utils.with_metaclass`)
• extended tuple unpacking (PEP 3132)

To handle function annotations (PEP 3107), see :ref:`func_annotations`.

They are still incomplete and make some mistakes, like 2to3, on which it is based.

Nevertheless, futurize and pasteurize are useful to automate much of the work of porting, particularly the boring repetitive text substitutions. They also help to flag which parts of the code require attention.

Please report bugs on GitHub.

Contributions to the lib2to3-based fixers for futurize and pasteurize are particularly welcome! Please see :ref:`contributing`.

futurize doesn't currently make any of these changes automatically:

1. A source encoding declaration line like:

   # -*- coding:utf-8 -*-
   

   is not kept at the top of a file. It must be moved manually back to line 1 to take effect.

2. Strings containing \U produce a SyntaxError on Python 3. An example is:

   s = 'C:\Users'.
   

   Python 2 expands this to s = 'C:\\Users', but Python 3 requires a raw prefix (r'...'). This also applies to multi-line strings (including multi-line docstrings).