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Quick-start guide

You can use future to help to port your code from Python 2 to Python 3 today -- and still have it run on Python 2.

If you already have Python 3 code, you can instead use future to offer Python 2 compatibility with almost no extra work.

Installation

To install the latest stable version, type:

pip install future

If you would prefer the latest development version, it is available here.

If you are writing code from scratch

The easiest way is to start each new module with these lines:

from __future__ import (absolute_import, division,
                        print_function, unicode_literals)
from future.builtins import *

Then write standard Python 3 code. The :mod:`future` package will provide support for running your code on Python 2.6 and 2.7 mostly unchanged.

See :ref:`what-else` for more details.

To convert existing Python 3 code

To offer backward compatibility with Python 2 from your Python 3 code, you can use the pasteurize script. This adds these lines at the top of each module:

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals
from future.builtins import open
from future.builtins import str
from past.utils import div
# etc., as needed

and converts a few Python 3-only constructs to a form compatible with both Py3 and Py2. Most remaining Python 3 code should simply work on Python 2.

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

To convert existing Python 2 code

Start with the :ref:`automatic-conversion` page.

Standard library reorganization

:mod:`future` supports the standard library reorganization (PEP 3108) via import hooks, allowing almost all moved standard library modules to be accessed under their Python 3 names and locations in Python 2:

from future import standard_library
with standard_library.hooks():
    import socketserver
    import queue
    import configparser
    import test.support
    import html.parser
    from collections import UserList
    from itertools import filterfalse, zip_longest
    from http.client import HttpConnection
    # and other moved modules and definitions

:mod:`future` also includes backports for these stdlib modules from Py3 that were heavily refactored versus Py2:

import html
import html.entities
import html.parser

import http
import http.client
import http.server

The following modules are currently not supported, but we aim to support them in the future:

import http.cookies
import http.cookiejar

import urllib
import urllib.parse
import urllib.request
import urllib.error

import xmlrpc.client
import xmlrpc.server

If you need one of these, please open an issue here.

For other forms of imports from the standard library, see :ref:`standard-library-imports`.

For more information on interfaces that have changed in the standard library between Python 2 and Python 3, see :ref:`stdlib-incompatibilities`.

Python 2-only dependencies

If you have dependencies that support only Python 2, you may be able to use the past module to automatically translate these Python 2 modules to Python 3 upon import. First, install the Python 2-only package into your Python 3 environment:

$ pip3 install mypackagename --no-compile   # to ignore SyntaxErrors

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

Then add the following code at the top of your (Py3 or Py2/3-compatible) code:

from past import autotranslate
autotranslate(['mypackagename'])
import mypackagename

This feature is experimental, and we would appreciate your feedback on how well this works or doesn't work for you. Please file an issue here or post to the python-porting mailing list.

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

Utilities

:mod:`future` also provides some useful functions and decorators to ease backward compatibility with Py2 in the :mod:`future.utils` and :mod:`past.utils` modules. These are a selection of the most useful functions from six and various home-grown Py2/3 compatibility modules from popular Python projects, such as Jinja2, Pandas, IPython, and Django. The goal is to consolidate these in one place, tested and documented, obviating the need for every project to repeat this work.

Examples:

# Functions like print() expect __str__ on Py2 to return a byte
# string. This decorator maps the __str__ to __unicode__ on Py2 and
# defines __str__ to encode it as utf-8:

from future.utils import python_2_unicode_compatible

@python_2_unicode_compatible
class MyClass(object):
    def __str__(self):
        return u'Unicode string: \u5b54\u5b50'
a = MyClass()

# This then prints the Chinese characters for Confucius:
print(a)


# Iterators on Py3 require a __next__() method, whereas on Py2 this
# is called next(). This decorator allows Py3-style iterators to work
# identically on Py2:

@implements_iterator
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

print(list(Upper('hello')))
# prints ['H', 'E', 'L', 'L', 'O']

On Python 3 these decorators are no-ops.

For more information, see :ref:`what-else`.
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