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Cheat Sheet: Writing Python 2-3 compatible code

  • Copyright (c): 2013-2016 Python Charmers Pty Ltd, Australia.
  • Author: Ed Schofield.
  • Licence: Creative Commons Attribution.

A PDF version is here: http://python-future.org/compatible_idioms.pdf

This notebook shows you idioms for writing future-proof code that is compatible with both versions of Python: 2 and 3. It accompanies Ed Schofield’s talk at PyCon AU 2014, “Writing 2/3 compatible code”. (The video is here: http://www.youtube.com/watch?v=KOqk8j11aAI&t=10m14s.)

Minimum versions:

  • Python 2: 2.6+
  • Python 3: 3.3+

Setup

The imports below refer to these pip-installable packages on PyPI:

import future        # pip install future
import builtins      # pip install future
import past          # pip install future
import six           # pip install six

The following scripts are also pip-installable:

futurize             # pip install future
pasteurize           # pip install future

See http://python-future.org and https://pythonhosted.org/six/ for more information.

Essential syntax differences

print

# Python 2 only:
print 'Hello'
# Python 2 and 3:
print('Hello')

To print multiple strings, import print_function to prevent Py2 from interpreting it as a tuple:

# Python 2 only:
print 'Hello', 'Guido'
# Python 2 and 3:
from __future__ import print_function    # (at top of module)

print('Hello', 'Guido')
# Python 2 only:
print >> sys.stderr, 'Hello'
# Python 2 and 3:
from __future__ import print_function

print('Hello', file=sys.stderr)
# Python 2 only:
print 'Hello',
# Python 2 and 3:
from __future__ import print_function

print('Hello', end='')

Raising exceptions

# Python 2 only:
raise ValueError, "dodgy value"
# Python 2 and 3:
raise ValueError("dodgy value")

Raising exceptions with a traceback:

# Python 2 only:
traceback = sys.exc_info()[2]
raise ValueError, "dodgy value", traceback
# Python 3 only:
raise ValueError("dodgy value").with_traceback()
# Python 2 and 3: option 1
from six import reraise as raise_
# or
from future.utils import raise_

traceback = sys.exc_info()[2]
raise_(ValueError, "dodgy value", traceback)
# Python 2 and 3: option 2
from future.utils import raise_with_traceback

raise_with_traceback(ValueError("dodgy value"))

Exception chaining (PEP 3134):

# Setup:
class DatabaseError(Exception):
    pass
# Python 3 only
class FileDatabase:
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError as exc:
            raise DatabaseError('failed to open') from exc
# Python 2 and 3:
from future.utils import raise_from

class FileDatabase:
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError as exc:
            raise_from(DatabaseError('failed to open'), exc)
# Testing the above:
try:
    fd = FileDatabase('non_existent_file.txt')
except Exception as e:
    assert isinstance(e.__cause__, IOError)    # FileNotFoundError on Py3.3+ inherits from IOError

Catching exceptions

# Python 2 only:
try:
    ...
except ValueError, e:
    ...
# Python 2 and 3:
try:
    ...
except ValueError as e:
    ...

Division

Integer division (rounding down):

# Python 2 only:
assert 2 / 3 == 0
# Python 2 and 3:
assert 2 // 3 == 0

“True division” (float division):

# Python 3 only:
assert 3 / 2 == 1.5
# Python 2 and 3:
from __future__ import division    # (at top of module)

assert 3 / 2 == 1.5

“Old division” (i.e. compatible with Py2 behaviour):

# Python 2 only:
a = b / c            # with any types
# Python 2 and 3:
from past.utils import old_div

a = old_div(b, c)    # always same as / on Py2

Long integers

Short integers are gone in Python 3 and long has become int (without the trailing L in the repr).

# Python 2 only
k = 9223372036854775808L

# Python 2 and 3:
k = 9223372036854775808
# Python 2 only
bigint = 1L

# Python 2 and 3
from builtins import int
bigint = int(1)

To test whether a value is an integer (of any kind):

# Python 2 only:
if isinstance(x, (int, long)):
    ...

# Python 3 only:
if isinstance(x, int):
    ...

# Python 2 and 3: option 1
from builtins import int    # subclass of long on Py2

if isinstance(x, int):             # matches both int and long on Py2
    ...

# Python 2 and 3: option 2
from past.builtins import long

if isinstance(x, (int, long)):
    ...

Octal constants

0644     # Python 2 only
0o644    # Python 2 and 3

Backtick repr

`x`      # Python 2 only
repr(x)  # Python 2 and 3

Metaclasses

class BaseForm(object):
    pass

class FormType(type):
    pass
# Python 2 only:
class Form(BaseForm):
    __metaclass__ = FormType
    pass
# Python 3 only:
class Form(BaseForm, metaclass=FormType):
    pass
# Python 2 and 3:
from six import with_metaclass
# or
from future.utils import with_metaclass

class Form(with_metaclass(FormType, BaseForm)):
    pass

Strings and bytes

Unicode (text) string literals

If you are upgrading an existing Python 2 codebase, it may be preferable to mark up all string literals as unicode explicitly with u prefixes:

# Python 2 only
s1 = 'The Zen of Python'
s2 = u'きたないのよりきれいな方がいい\n'

# Python 2 and 3
s1 = u'The Zen of Python'
s2 = u'きたないのよりきれいな方がいい\n'

The futurize and python-modernize tools do not currently offer an option to do this automatically.

If you are writing code for a new project or new codebase, you can use this idiom to make all string literals in a module unicode strings:

# Python 2 and 3
from __future__ import unicode_literals    # at top of module

s1 = 'The Zen of Python'
s2 = 'きたないのよりきれいな方がいい\n'

See http://python-future.org/unicode_literals.html for more discussion on which style to use.

Byte-string literals

# Python 2 only
s = 'This must be a byte-string'

# Python 2 and 3
s = b'This must be a byte-string'

To loop over a byte-string with possible high-bit characters, obtaining each character as a byte-string of length 1:

# Python 2 only:
for bytechar in 'byte-string with high-bit chars like \xf9':
    ...

# Python 3 only:
for myint in b'byte-string with high-bit chars like \xf9':
    bytechar = bytes([myint])

# Python 2 and 3:
from builtins import bytes
for myint in bytes(b'byte-string with high-bit chars like \xf9'):
    bytechar = bytes([myint])

As an alternative, chr() and .encode('latin-1') can be used to convert an int into a 1-char byte string:

# Python 3 only:
for myint in b'byte-string with high-bit chars like \xf9':
    char = chr(myint)    # returns a unicode string
    bytechar = char.encode('latin-1')

# Python 2 and 3:
from builtins import bytes, chr
for myint in bytes(b'byte-string with high-bit chars like \xf9'):
    char = chr(myint)    # returns a unicode string
    bytechar = char.encode('latin-1')    # forces returning a byte str

basestring

# Python 2 only:
a = u'abc'
b = 'def'
assert (isinstance(a, basestring) and isinstance(b, basestring))

# Python 2 and 3: alternative 1
from past.builtins import basestring    # pip install future

a = u'abc'
b = b'def'
assert (isinstance(a, basestring) and isinstance(b, basestring))
# Python 2 and 3: alternative 2: refactor the code to avoid considering
# byte-strings as strings.

from builtins import str
a = u'abc'
b = b'def'
c = b.decode()
assert isinstance(a, str) and isinstance(c, str)
# ...

unicode

# Python 2 only:
templates = [u"blog/blog_post_detail_%s.html" % unicode(slug)]
# Python 2 and 3: alternative 1
from builtins import str
templates = [u"blog/blog_post_detail_%s.html" % str(slug)]
# Python 2 and 3: alternative 2
from builtins import str as text
templates = [u"blog/blog_post_detail_%s.html" % text(slug)]

StringIO

# Python 2 only:
from StringIO import StringIO
# or:
from cStringIO import StringIO

# Python 2 and 3:
from io import BytesIO     # for handling byte strings
from io import StringIO    # for handling unicode strings

Imports relative to a package

Suppose the package is:

mypackage/
    __init__.py
    submodule1.py
    submodule2.py

and the code below is in submodule1.py:

# Python 2 only:
import submodule2
# Python 2 and 3:
from . import submodule2
# Python 2 and 3:
# To make Py2 code safer (more like Py3) by preventing
# implicit relative imports, you can also add this to the top:
from __future__ import absolute_import

Dictionaries

heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}

Iterating through dict keys/values/items

Iterable dict keys:

# Python 2 only:
for key in heights.iterkeys():
    ...
# Python 2 and 3:
for key in heights:
    ...

Iterable dict values:

# Python 2 only:
for value in heights.itervalues():
    ...
# Idiomatic Python 3
for value in heights.values():    # extra memory overhead on Py2
    ...
# Python 2 and 3: option 1
from builtins import dict

heights = dict(Fred=175, Anne=166, Joe=192)
for key in heights.values():    # efficient on Py2 and Py3
    ...
# Python 2 and 3: option 2
from builtins import itervalues
# or
from six import itervalues

for key in itervalues(heights):
    ...

Iterable dict items:

# Python 2 only:
for (key, value) in heights.iteritems():
    ...
# Python 2 and 3: option 1
for (key, value) in heights.items():    # inefficient on Py2
    ...
# Python 2 and 3: option 2
from future.utils import viewitems

for (key, value) in viewitems(heights):   # also behaves like a set
    ...
# Python 2 and 3: option 3
from future.utils import iteritems
# or
from six import iteritems

for (key, value) in iteritems(heights):
    ...

dict keys/values/items as a list

dict keys as a list:

# Python 2 only:
keylist = heights.keys()
assert isinstance(keylist, list)
# Python 2 and 3:
keylist = list(heights)
assert isinstance(keylist, list)

dict values as a list:

# Python 2 only:
heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}
valuelist = heights.values()
assert isinstance(valuelist, list)
# Python 2 and 3: option 1
valuelist = list(heights.values())    # inefficient on Py2
# Python 2 and 3: option 2
from builtins import dict

heights = dict(Fred=175, Anne=166, Joe=192)
valuelist = list(heights.values())
# Python 2 and 3: option 3
from future.utils import listvalues

valuelist = listvalues(heights)
# Python 2 and 3: option 4
from future.utils import itervalues
# or
from six import itervalues

valuelist = list(itervalues(heights))

dict items as a list:

# Python 2 and 3: option 1
itemlist = list(heights.items())    # inefficient on Py2
# Python 2 and 3: option 2
from future.utils import listitems

itemlist = listitems(heights)
# Python 2 and 3: option 3
from future.utils import iteritems
# or
from six import iteritems

itemlist = list(iteritems(heights))

Custom class behaviour

Custom iterators

# Python 2 only
class Upper(object):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def next(self):          # Py2-style
        return self._iter.next().upper()
    def __iter__(self):
        return self

itr = Upper('hello')
assert itr.next() == 'H'     # Py2-style
assert list(itr) == list('ELLO')
# Python 2 and 3: option 1
from builtins import object

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

itr = Upper('hello')
assert next(itr) == 'H'      # compatible style
assert list(itr) == list('ELLO')
# Python 2 and 3: option 2
from future.utils import implements_iterator

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

itr = Upper('hello')
assert next(itr) == 'H'
assert list(itr) == list('ELLO')

Custom __str__ methods

# Python 2 only:
class MyClass(object):
    def __unicode__(self):
        return 'Unicode string: \u5b54\u5b50'
    def __str__(self):
        return unicode(self).encode('utf-8')

a = MyClass()
print(a)    # prints encoded string
# Python 2 and 3:
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()
print(a)    # prints string encoded as utf-8 on Py2
Unicode string: 孔子

Custom __nonzero__ vs __bool__ method:

# Python 2 only:
class AllOrNothing(object):
    def __init__(self, l):
        self.l = l
    def __nonzero__(self):
        return all(self.l)

container = AllOrNothing([0, 100, 200])
assert not bool(container)
# Python 2 and 3:
from builtins import object

class AllOrNothing(object):
    def __init__(self, l):
        self.l = l
    def __bool__(self):
        return all(self.l)

container = AllOrNothing([0, 100, 200])
assert not bool(container)

Lists versus iterators

xrange

# Python 2 only:
for i in xrange(10**8):
    ...
# Python 2 and 3: forward-compatible
from builtins import range
for i in range(10**8):
    ...
# Python 2 and 3: backward-compatible
from past.builtins import xrange
for i in xrange(10**8):
    ...

range

# Python 2 only
mylist = range(5)
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 1
mylist = list(range(5))            # copies memory on Py2
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 2
from builtins import range

mylist = list(range(5))
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: option 3
from future.utils import lrange

mylist = lrange(5)
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: backward compatible
from past.builtins import range

mylist = range(5)
assert mylist == [0, 1, 2, 3, 4]

map

# Python 2 only:
mynewlist = map(f, myoldlist)
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 1
# Idiomatic Py3, but inefficient on Py2
mynewlist = list(map(f, myoldlist))
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 2
from builtins import map

mynewlist = list(map(f, myoldlist))
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 3
try:
    import itertools.imap as map
except ImportError:
    pass

mynewlist = list(map(f, myoldlist))    # inefficient on Py2
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 4
from future.utils import lmap

mynewlist = lmap(f, myoldlist)
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 5
from past.builtins import map

mynewlist = map(f, myoldlist)
assert mynewlist == [f(x) for x in myoldlist]

imap

# Python 2 only:
from itertools import imap

myiter = imap(func, myoldlist)
assert isinstance(myiter, iter)
# Python 3 only:
myiter = map(func, myoldlist)
assert isinstance(myiter, iter)
# Python 2 and 3: option 1
from builtins import map

myiter = map(func, myoldlist)
assert isinstance(myiter, iter)
# Python 2 and 3: option 2
try:
    import itertools.imap as map
except ImportError:
    pass

myiter = map(func, myoldlist)
assert isinstance(myiter, iter)

zip, izip

As above with zip and itertools.izip.

filter, ifilter

As above with filter and itertools.ifilter too.

Other builtins

File IO with open()

# Python 2 only
f = open('myfile.txt')
data = f.read()              # as a byte string
text = data.decode('utf-8')

# Python 2 and 3: alternative 1
from io import open
f = open('myfile.txt', 'rb')
data = f.read()              # as bytes
text = data.decode('utf-8')  # unicode, not bytes

# Python 2 and 3: alternative 2
from io import open
f = open('myfile.txt', encoding='utf-8')
text = f.read()    # unicode, not bytes

reduce()

# Python 2 only:
assert reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) == 1+2+3+4+5
# Python 2 and 3:
from functools import reduce

assert reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) == 1+2+3+4+5

raw_input()

# Python 2 only:
name = raw_input('What is your name? ')
assert isinstance(name, str)    # native str
# Python 2 and 3:
from builtins import input

name = input('What is your name? ')
assert isinstance(name, str)    # native str on Py2 and Py3

input()

# Python 2 only:
input("Type something safe please: ")
# Python 2 and 3
from builtins import input
eval(input("Type something safe please: "))

Warning: using either of these is unsafe with untrusted input.

file()

# Python 2 only:
f = file(pathname)
# Python 2 and 3:
f = open(pathname)

# But preferably, use this:
from io import open
f = open(pathname, 'rb')   # if f.read() should return bytes
# or
f = open(pathname, 'rt')   # if f.read() should return unicode text

exec

# Python 2 only:
exec 'x = 10'

# Python 2 and 3:
exec('x = 10')
# Python 2 only:
g = globals()
exec 'x = 10' in g

# Python 2 and 3:
g = globals()
exec('x = 10', g)
# Python 2 only:
l = locals()
exec 'x = 10' in g, l

# Python 2 and 3:
exec('x = 10', g, l)

execfile()

# Python 2 only:
execfile('myfile.py')
# Python 2 and 3: alternative 1
from past.builtins import execfile

execfile('myfile.py')
# Python 2 and 3: alternative 2
exec(compile(open('myfile.py').read()))

# This can sometimes cause this:
#     SyntaxError: function ... uses import * and bare exec ...
# See https://github.com/PythonCharmers/python-future/issues/37

unichr()

# Python 2 only:
assert unichr(8364) == '€'
# Python 3 only:
assert chr(8364) == '€'
# Python 2 and 3:
from builtins import chr
assert chr(8364) == '€'

intern()

# Python 2 only:
intern('mystring')
# Python 3 only:
from sys import intern
intern('mystring')
# Python 2 and 3: alternative 1
from past.builtins import intern
intern('mystring')
# Python 2 and 3: alternative 2
from six.moves import intern
intern('mystring')
# Python 2 and 3: alternative 3
from future.standard_library import install_aliases
install_aliases()
from sys import intern
intern('mystring')
# Python 2 and 3: alternative 2
try:
    from sys import intern
except ImportError:
    pass
intern('mystring')

apply()

args = ('a', 'b')
kwargs = {'kwarg1': True}
# Python 2 only:
apply(f, args, kwargs)
# Python 2 and 3: alternative 1
f(*args, **kwargs)
# Python 2 and 3: alternative 2
from past.builtins import apply
apply(f, args, kwargs)

chr()

# Python 2 only:
assert chr(64) == b'@'
assert chr(200) == b'\xc8'
# Python 3 only: option 1
assert chr(64).encode('latin-1') == b'@'
assert chr(0xc8).encode('latin-1') == b'\xc8'
# Python 2 and 3: option 1
from builtins import chr

assert chr(64).encode('latin-1') == b'@'
assert chr(0xc8).encode('latin-1') == b'\xc8'
# Python 3 only: option 2
assert bytes([64]) == b'@'
assert bytes([0xc8]) == b'\xc8'
# Python 2 and 3: option 2
from builtins import bytes

assert bytes([64]) == b'@'
assert bytes([0xc8]) == b'\xc8'

cmp()

# Python 2 only:
assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0
# Python 2 and 3: alternative 1
from past.builtins import cmp
assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0
# Python 2 and 3: alternative 2
cmp = lambda(x, y): (x > y) - (x < y)
assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0

reload()

# Python 2 only:
reload(mymodule)
# Python 2 and 3
from imp import reload
reload(mymodule)

Standard library

dbm modules

# Python 2 only
import anydbm
import whichdb
import dbm
import dumbdbm
import gdbm

# Python 2 and 3: alternative 1
from future import standard_library
standard_library.install_aliases()

import dbm
import dbm.ndbm
import dbm.dumb
import dbm.gnu

# Python 2 and 3: alternative 2
from future.moves import dbm
from future.moves.dbm import dumb
from future.moves.dbm import ndbm
from future.moves.dbm import gnu

# Python 2 and 3: alternative 3
from six.moves import dbm_gnu
# (others not supported)

commands / subprocess modules

# Python 2 only
from commands import getoutput, getstatusoutput

# Python 2 and 3
from future import standard_library
standard_library.install_aliases()

from subprocess import getoutput, getstatusoutput

subprocess.check_output()

# Python 2.7 and above
from subprocess import check_output

# Python 2.6 and above: alternative 1
from future.moves.subprocess import check_output

# Python 2.6 and above: alternative 2
from future import standard_library
standard_library.install_aliases()

from subprocess import check_output

collections: Counter and OrderedDict

# Python 2.7 and above
from collections import Counter, OrderedDict

# Python 2.6 and above: alternative 1
from future.moves.collections import Counter, OrderedDict

# Python 2.6 and above: alternative 2
from future import standard_library
standard_library.install_aliases()

from collections import Counter, OrderedDict

StringIO module

# Python 2 only
from StringIO import StringIO
from cStringIO import StringIO
# Python 2 and 3
from io import BytesIO
# and refactor StringIO() calls to BytesIO() if passing byte-strings

http module

# Python 2 only:
import httplib
import Cookie
import cookielib
import BaseHTTPServer
import SimpleHTTPServer
import CGIHttpServer

# Python 2 and 3 (after ``pip install future``):
import http.client
import http.cookies
import http.cookiejar
import http.server

xmlrpc module

# Python 2 only:
import DocXMLRPCServer
import SimpleXMLRPCServer

# Python 2 and 3 (after ``pip install future``):
import xmlrpc.server
# Python 2 only:
import xmlrpclib

# Python 2 and 3 (after ``pip install future``):
import xmlrpc.client

html escaping and entities

# Python 2 and 3:
from cgi import escape

# Safer (Python 2 and 3, after ``pip install future``):
from html import escape

# Python 2 only:
from htmlentitydefs import codepoint2name, entitydefs, name2codepoint

# Python 2 and 3 (after ``pip install future``):
from html.entities import codepoint2name, entitydefs, name2codepoint

html parsing

# Python 2 only:
from HTMLParser import HTMLParser

# Python 2 and 3 (after ``pip install future``)
from html.parser import HTMLParser

# Python 2 and 3 (alternative 2):
from future.moves.html.parser import HTMLParser

urllib module

urllib is the hardest module to use from Python 2/3 compatible code. You may like to use Requests (http://python-requests.org) instead.

# Python 2 only:
from urlparse import urlparse
from urllib import urlencode
from urllib2 import urlopen, Request, HTTPError
# Python 3 only:
from urllib.parse import urlparse, urlencode
from urllib.request import urlopen, Request
from urllib.error import HTTPError
# Python 2 and 3: easiest option
from future.standard_library import install_aliases
install_aliases()

from urllib.parse import urlparse, urlencode
from urllib.request import urlopen, Request
from urllib.error import HTTPError
# Python 2 and 3: alternative 2
from future.standard_library import hooks

with hooks():
    from urllib.parse import urlparse, urlencode
    from urllib.request import urlopen, Request
    from urllib.error import HTTPError
# Python 2 and 3: alternative 3
from future.moves.urllib.parse import urlparse, urlencode
from future.moves.urllib.request import urlopen, Request
from future.moves.urllib.error import HTTPError
# or
from six.moves.urllib.parse import urlparse, urlencode
from six.moves.urllib.request import urlopen
from six.moves.urllib.error import HTTPError
# Python 2 and 3: alternative 4
try:
    from urllib.parse import urlparse, urlencode
    from urllib.request import urlopen, Request
    from urllib.error import HTTPError
except ImportError:
    from urlparse import urlparse
    from urllib import urlencode
    from urllib2 import urlopen, Request, HTTPError

Tkinter

# Python 2 only:
import Tkinter
import Dialog
import FileDialog
import ScrolledText
import SimpleDialog
import Tix
import Tkconstants
import Tkdnd
import tkColorChooser
import tkCommonDialog
import tkFileDialog
import tkFont
import tkMessageBox
import tkSimpleDialog
import ttk

# Python 2 and 3 (after ``pip install future``):
import tkinter
import tkinter.dialog
import tkinter.filedialog
import tkinter.scrolledtext
import tkinter.simpledialog
import tkinter.tix
import tkinter.constants
import tkinter.dnd
import tkinter.colorchooser
import tkinter.commondialog
import tkinter.filedialog
import tkinter.font
import tkinter.messagebox
import tkinter.simpledialog
import tkinter.ttk

socketserver

# Python 2 only:
import SocketServer

# Python 2 and 3 (after ``pip install future``):
import socketserver

copy_reg, copyreg

# Python 2 only:
import copy_reg

# Python 2 and 3 (after ``pip install future``):
import copyreg

configparser

# Python 2 only:
from ConfigParser import ConfigParser

# Python 2 and 3 (after ``pip install configparser``):
from configparser import ConfigParser

queue

# Python 2 only:
from Queue import Queue, heapq, deque

# Python 2 and 3 (after ``pip install future``):
from queue import Queue, heapq, deque

repr, reprlib

# Python 2 only:
from repr import aRepr, repr

# Python 2 and 3 (after ``pip install future``):
from reprlib import aRepr, repr

UserDict, UserList, UserString

# Python 2 only:
from UserDict import UserDict
from UserList import UserList
from UserString import UserString

# Python 3 only:
from collections import UserDict, UserList, UserString

# Python 2 and 3: alternative 1
from future.moves.collections import UserDict, UserList, UserString

# Python 2 and 3: alternative 2
from six.moves import UserDict, UserList, UserString

# Python 2 and 3: alternative 3
from future.standard_library import install_aliases
install_aliases()
from collections import UserDict, UserList, UserString

itertools: filterfalse, zip_longest

# Python 2 only:
from itertools import ifilterfalse, izip_longest

# Python 3 only:
from itertools import filterfalse, zip_longest

# Python 2 and 3: alternative 1
from future.moves.itertools import filterfalse, zip_longest

# Python 2 and 3: alternative 2
from six.moves import filterfalse, zip_longest

# Python 2 and 3: alternative 3
from future.standard_library import install_aliases
install_aliases()
from itertools import filterfalse, zip_longest