Quick-start guide — Python-Future documentation

# 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 builtins import *

Then write standard Python 3 code. The `future` package will provide support for running your code on Python 2.7, and 3.4+ mostly unchanged.

- For explicit import forms, see Explicit imports.

- For more details, see What else you need to know.

- For a cheat sheet, see Cheat Sheet: Writing Python 2-3 compatible code.

## 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 builtins import open
    from builtins import str
    # etc., as needed

    from future import standard_library
    standard_library.install_aliases()

and converts several Python 3-only constructs (like keyword-only arguments) to a form compatible with both Py3 and Py2. Most remaining Python 3 code should simply work on Python 2.

See pasteurize: Py3 to Py2/3 for more details.

## To convert existing Python 2 code

The `futurize` script 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 to re-enable compatibility with Python 2.

For example, running `futurize` turns this Python 2 code:

    import ConfigParser                 # Py2 module name

    class Upper(object):
        def __init__(self, iterable):
            self._iter = iter(iterable)
        def next(self):                 # Py2-style iterator interface
            return next(self._iter).upper()
        def __iter__(self):
            return self

    itr = Upper('hello')
    print next(itr),
    for letter in itr:
        print letter,                   # Py2-style print statement

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 future.builtins import next
    from future.builtins import object
    import configparser                 # Py3-style import

    class Upper(object):
        def __init__(self, iterable):
            self._iter = iter(iterable)
        def __next__(self):             # Py3-style iterator interface
            return next(self._iter).upper()
        def __iter__(self):
            return self

    itr = Upper('hello')
    print(next(itr), end=' ')           # Py3-style print function
    for letter in itr:
        print(letter, end=' ')

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

For complex projects, it is probably best to divide the porting into two stages. Stage 1 is for “safe” changes that modernize the code but do not break Python 2.7 compatibility or introduce a dependency on the `future` package. Stage 2 is to complete the process.

See Stage 1: “safe” fixes and Stage 2: Py3-style code with wrappers for Py2 for more details.

## Standard library reorganization

`future` supports the standard library reorganization (PEP 3108) via one of several mechanisms, allowing most moved standard library modules to be accessed under their Python 3 names and locations in Python 2:

    from future import standard_library
    standard_library.install_aliases()

    # Then these Py3-style imports work on both Python 2 and Python 3:
    import socketserver
    import queue
    from collections import UserDict, UserList, UserString
    from collections import ChainMap  # even on Py2.7
    from itertools import filterfalse, zip_longest

    import html
    import html.entities
    import html.parser

    import http
    import http.client
    import http.server
    import http.cookies
    import http.cookiejar

    import urllib.request
    import urllib.parse
    import urllib.response
    import urllib.error
    import urllib.robotparser

    import xmlrpc.client
    import xmlrpc.server

and others. For a complete list, see Direct imports.

## 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.translation 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.

For more information on the automatic translation feature, see Using Python 2-only dependencies on Python 3.

## Next steps

For more information about writing Py2/3-compatible code, see:

- Cheat Sheet: Writing Python 2-3 compatible code

- What else you need to know.