Overview: Easy, clean, reliable Python 2/3 compatibility

python-future is the missing compatibility layer between Python 2 and Python 3. It allows you to use a single, clean Python 3.x-compatible codebase to support both Python 2 and Python 3 with minimal overhead.

It provides future and past packages with backports and forward ports of features from Python 3 and 2. It also comes with futurize and pasteurize, customized 2to3-based scripts that helps you to convert either Py2 or Py3 code easily to support both Python 2 and 3 in a single clean Py3-style codebase, module by module.

Notable projects that use python-future for Python 2/3 compatibility are Mezzanine and ObsPy.

Features

• future.builtins package (also available as builtins on Py2) provides backports and remappings for 20 builtins with different semantics on Py3 versus Py2
• support for directly importing 30 standard library modules under their Python 3 names on Py2
• support for importing the other 14 refactored standard library modules under their Py3 names relatively cleanly via future.standard_library and future.moves
• past.builtins package provides forward-ports of 19 Python 2 types and builtin functions. These can aid with per-module code migrations.
• past.translation package supports transparent translation of Python 2 modules to Python 3 upon import. [This feature is currently in alpha.]
• 1000+ unit tests, including many from the Py3.3 source tree.
• futurize and pasteurize scripts based on 2to3 and parts of 3to2 and python-modernize, for automatic conversion from either Py2 or Py3 to a clean single-source codebase compatible with Python 2.6+ and Python 3.3+.
• a curated set of utility functions and decorators in future.utils and past.utils selected from Py2/3 compatibility interfaces from projects like six, IPython, Jinja2, Django, and Pandas.
• support for the surrogateescape error handler when encoding and decoding the backported str and bytes objects. [This feature is currently in alpha.]

Code examples

Replacements for Py2's built-in functions and types are designed to be imported at the top of each Python module together with Python's built-in __future__ statements. For example, this code behaves identically on Python 2.6/2.7 after these imports as it does on Python 3.3+:

from __future__ import absolute_import, division, print_function
from builtins import (bytes, str, open, super, range,
                      zip, round, input, int, pow, object)

# Backported Py3 bytes object
b = bytes(b'ABCD')
assert list(b) == [65, 66, 67, 68]
assert repr(b) == "b'ABCD'"
# These raise TypeErrors:
# b + u'EFGH'
# bytes(b',').join([u'Fred', u'Bill'])

# Backported Py3 str object
s = str(u'ABCD')
assert s != bytes(b'ABCD')
assert isinstance(s.encode('utf-8'), bytes)
assert isinstance(b.decode('utf-8'), str)
assert repr(s) == "'ABCD'"      # consistent repr with Py3 (no u prefix)
# These raise TypeErrors:
# bytes(b'B') in s
# s.find(bytes(b'A'))

# Extra arguments for the open() function
f = open('japanese.txt', encoding='utf-8', errors='replace')

# New zero-argument super() function:
class VerboseList(list):
    def append(self, item):
        print('Adding an item')
        super().append(item)

# New iterable range object with slicing support
for i in range(10**15)[:10]:
    pass

# Other iterators: map, zip, filter
my_iter = zip(range(3), ['a', 'b', 'c'])
assert my_iter != list(my_iter)

# The round() function behaves as it does in Python 3, using
# "Banker's Rounding" to the nearest even last digit:
assert round(0.1250, 2) == 0.12

# input() replaces Py2's raw_input() (with no eval()):
name = input('What is your name? ')
print('Hello ' + name)

# pow() supports fractional exponents of negative numbers like in Py3:
z = pow(-1, 0.5)

# Compatible output from isinstance() across Py2/3:
assert isinstance(2**64, int)        # long integers
assert isinstance(u'blah', str)
assert isinstance('blah', str)       # only if unicode_literals is in effect

# Py3-style iterators written as new-style classes (subclasses of
# future.types.newobject) are automatically backward compatible with Py2:
class Upper(object):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def __next__(self):                 # note the Py3 interface
        return next(self._iter).upper()
    def __iter__(self):
        return self
assert list(Upper('hello')) == list('HELLO')

There is also support for renamed standard library modules. The recommended interface works like this:

# Many Py3 module names are supported directly on both Py2.x and 3.x:
from http.client import HttpConnection
import html.parser
import queue
import xmlrpc.client

# Refactored modules with clashing names on Py2 and Py3 are supported
# as follows:
from future import standard_library
standard_library.install_aliases()

# Then, for example:
from itertools import filterfalse, zip_longest
from urllib.request import urlopen
from collections import ChainMap
from collections import UserDict, UserList, UserString
from subprocess import getoutput, getstatusoutput
from collections import Counter, OrderedDict   # backported to Py2.6

Automatic conversion to Py2/3-compatible code

python-future comes with two scripts called futurize and pasteurize to aid in making Python 2 code or Python 3 code compatible with both platforms (Py2/3). It is based on 2to3 and uses fixers from lib2to3, lib3to2, and python-modernize, as well as custom fixers.

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 so that it also runs under Python 2.

For conversions from Python 3 code to Py2/3, use the pasteurize script instead. This converts Py3-only constructs (e.g. new metaclass syntax) to Py2/3 compatible constructs 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.

Futurize: 2 to both

For example, running futurize -w mymodule.py turns this Python 2 code:

import Queue
from urllib2 import urlopen

def greet(name):
    print 'Hello',
    print name

print "What's your name?",
name = raw_input()
greet(name)

into this code which runs on both Py2 and Py3:

from __future__ import print_function
from future import standard_library
standard_library.install_aliases()
from builtins import input
import queue
from urllib.request import urlopen

def greet(name):
    print('Hello', end=' ')
    print(name)

print("What's your name?", end=' ')
name = input()
greet(name)

See :ref:`forwards-conversion` and :ref:`backwards-conversion` for more details.

Automatic translation

The past package can automatically translate some simple Python 2 modules to Python 3 upon import. The goal is to support the "long tail" of real-world Python 2 modules (e.g. on PyPI) that have not been ported yet. For example, here is how to use a Python 2-only package called plotrique on Python 3. First install it:

$ pip3 install plotrique==0.2.5-7 --no-compile   # to ignore SyntaxErrors

(or use pip if this points to your Py3 environment.)

Then pass a whitelist of module name prefixes to the autotranslate() function. Example:

$ python3

>>> from past import autotranslate
>>> autotranslate(['plotrique'])
>>> import plotrique

This transparently translates and runs the plotrique module and any submodules in the plotrique package that plotrique imports.

This is intended to help you migrate to Python 3 without the need for all your code's dependencies to support Python 3 yet. It should be used as a last resort; ideally Python 2-only dependencies should be ported properly to a Python 2/3 compatible codebase using a tool like futurize and the changes should be pushed to the upstream project.

Note: the auto-translation feature is still in alpha; it needs more testing and development, and will likely never be perfect.

For more info, see :ref:`translation`.

Licensing

Next steps

If you are new to Python-Future, check out the Quickstart Guide.

For an update on changes in the latest version, see the What's New page.