future 0.4.0

Clean single-source support for Python 3 and 2

future: clean single-source support for Python 3 and 2

The future module helps run Python 3.x-compatible code under Python 2 with minimal code cruft.

The goal is to allow you to write clean, modern, forward-compatible Python 3 code today and to run it with minimal effort under Python 2 alongside a Python 2 stack that may contain dependencies that have not yet been ported to Python 3.

It is designed to be used as follows:

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

followed by clean Python 3 code (with a few restrictions) that can run unchanged on Python 2.7.

On Python 3, from future import standard_library has no effect. On Python 2, it module installs import hooks to allow renamed and moved standard library modules to be imported from their new Py3 locations.

Likewise, on Python 3, the from future.builtins import * line has no effect (i.e. zero namespace pollution.) On Python 2 it shadows builtins to provide their Python 3 semantics. (See below for the explicit import form.)

After the imports, this code runs identically on Python 3 and 2:

# Support for renamed standard library modules (see below)
from http.client import HttpConnection
from itertools import filterfalse
from test import support

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

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

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

# These raise NameErrors:
# apply(), cmp(), coerce(), reduce(), xrange(), etc.

# This identity is restored. This is normally valid on Py3 and Py2, but
# 'from __future__ import unicode_literals' breaks it on Py2:
assert isinstance('happy', str)

# 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() is now safe (no eval()):
name = input('What is your name? ')
print('Hello ' + name)

Standard library reorganization

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:

from future import standard_library

import socketserver
import queue
import configparser
import test.support
from collections import UserList
from itertools import filterfalse, zip_longest
# and other moved modules and definitions

It also includes backports for these stdlib packages from Py3 that were heavily refactored versus Py2:

import html, html.entities, html.parser
import http, http.client

These currently are not supported, but we may support them in the future:

import http.server, http.cookies, http.cookiejar
import urllib, urllib.parse, urllib.request, urllib.error

Explicit imports

If you prefer explicit imports, the explicit equivalent of the from future.builtins import * line above is:

from future.builtins.iterators import zip, map, filter
from future.builtins.misc import ascii, oct, hex, chr, input
from future.builtins.backports import range, super, round
from future.builtins.disabled import (apply, cmp, coerce,
        execfile, file, long, raw_input, reduce, reload, unicode,
        xrange, StandardError)
from future.builtins.str_is_unicode import str

But please note that the API is still evolving rapidly.

See the docstrings for each of these modules for more info:

- future.standard_library
- future.builtins
- future.utils

Automatic conversion

A script called futurize is included to aid in making either Python 2 code or Python 3 code compatible with both platforms using the future module. See https://github.com/edschofield/python-future#automatic-conversion.

Credits

Author:

Ed Schofield

Sponsor:

Python Charmers Pty Ltd, Australia, and Python Charmers Pte Ltd, Singapore. http://pythoncharmers.com

Others:

• future incorporates the six module by Benjamin Peterson.

• The futurize script uses lib2to3, lib3to2, and parts of Armin Ronacher’s python-modernize code.

• The backported super() and range() functions are derived from Ryan Kelly’s magicsuper module and Dan Crosta’s xrange module.

• The python_2_unicode_compatible decorator is from django.utils.encoding.

Licensing

Copyright 2013 Python Charmers Pty Ltd, Australia. The software is distributed under an MIT licence. See LICENSE.txt.

FAQ

See https://github.com/edschofield/python-future#faq.