Six - Python 2 and 3 Compatibility Library

Six: Python 2 and 3 Compatibility Library

Six provides simple utilities for wrapping over differences between Python 2 and Python 3. It is intended to support codebases that work on both Python 2 and 3 without modification. six consists of only one Python file, so it is painless to copy into a project.

Six can be downloaded on PyPi. Its bug tracker and code hosting is on BitBucket.

The name, "six", comes from the fact that 2*3 equals 6. Why not addition? Multiplication is more powerful, and, anyway, "five" has already been snatched away by the (admittedly now moribund) Zope Five project.

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Package contents


Six provides constants that may differ between Python versions. Ones ending _types are mostly useful as the second argument to isinstance or issubclass.

Here's example usage of the module:

import six

def dispatch_types(value):
    if isinstance(value, six.integer_types):
    elif isinstance(value, six.class_types):
    elif isinstance(value, six.string_types):

Object model compatibility

Python 3 renamed the attributes of several intepreter data structures. The following accessors are available. Note that the recommended way to inspect functions and methods is the stdlib py3:inspect module.

A class for making portable iterators. The intention is that it be subclassed and subclasses provide a __next__ method. In Python 2, Iterator has one method: next. It simply delegates to __next__. An alternate way to do this would be to simply alias next to __next__. However, this interacts badly with subclasses that override __next__. Iterator is empty on Python 3. (In fact, it is just aliased to py3:object.)

Syntax compatibility

These functions smooth over operations which have different syntaxes between Python 2 and 3.

Binary and text data

Python 3 enforces the distinction between byte strings and text strings far more rigoriously than Python 2 does; binary data cannot be automatically coerced to or from text data. six provides several functions to assist in classifying string data in all Python versions.

unittest assertions

Six contains compatibility shims for unittest assertions that have been renamed. The parameters are the same as their aliases, but you must pass the test method as the first argument. For example:

import six
import unittest

class TestAssertCountEqual(unittest.TestCase):
    def test(self):
        six.assertCountEqual(self, (1, 2), [2, 1])

Note these functions are only available on Python 2.7 or later.

Renamed modules and attributes compatibility

Python 3 reorganized the standard library and moved several functions to different modules. Six provides a consistent interface to them through the fake six.moves module. For example, to load the module for parsing HTML on Python 2 or 3, write:

from six.moves import html_parser

Similarly, to get the function to reload modules, which was moved from the builtin module to the imp module, use:

from six.moves import reload_module

For the most part, six.moves aliases are the names of the modules in Python 3. When the new Python 3 name is a package, the components of the name are separated by underscores. For example, html.parser becomes html_parser. In some cases where several modules have been combined, the Python 2 name is retained. This is so the appropiate modules can be found when running on Python 2. For example, BaseHTTPServer which is in http.server in Python 3 is aliased as BaseHTTPServer.

Some modules which had two implementations have been merged in Python 3. For example, cPickle no longer exists in Python 3; it was merged with pickle. In these cases, fetching the fast version will load the fast one on Python 2 and the merged module in Python 3.

The py2:urllib, py2:urllib2, and py2:urlparse modules have been combined in the py3:urllib package in Python 3. The six.moves.urllib package is a version-independent location for this functionality; its structure mimics the structure of the Python 3 py3:urllib package.

In order to make imports of the form:

from six.moves.cPickle import loads

work, six places special proxy objects in in py3:sys.modules. These proxies lazily load the underlying module when an attribute is fetched. This will fail if the underlying module is not available in the Python interpreter. For example, sys.modules["six.moves.winreg"].LoadKey would fail on any non-Windows platform. Unfortunately, some applications try to load attributes on every module in py3:sys.modules. six mitigates this problem for some applications by pretending attributes on unimportable modules don't exist. This hack doesn't work in every case, though. If you are encountering problems with the lazy modules and don't use any from imports directly from six.moves modules, you can workaround the issue by removing the six proxy modules:

d = [name for name in sys.modules if name.startswith("six.moves.")]
for name in d:
    del sys.modules[name]

Supported renames:

Name Python 2 name Python 3 name
builtins py2:__builtin__ py3:builtins
configparser py2:ConfigParser py3:configparser
copyreg py2:copy_reg py3:copyreg
cPickle py2:cPickle py3:pickle
cStringIO py2:cStringIO.StringIO py3:io.StringIO
dbm_gnu py2:gdbm py3:dbm.gnu
_dummy_thread py2:dummy_thread py3:_dummy_thread
email_mime_multipart py2:email.MIMEMultipart py3:email.mime.multipart
email_mime_nonmultipart py2:email.MIMENonMultipart py3:email.mime.nonmultipart
email_mime_text py2:email.MIMEText py3:email.mime.text
email_mime_base py2:email.MIMEBase py3:email.mime.base
filter py2:itertools.ifilter py3:filter
filterfalse py2:itertools.ifilterfalse py3:itertools.filterfalse
getcwd py2:os.getcwdu py3:os.getcwd
getcwdb py2:os.getcwd py3:os.getcwdb
http_cookiejar py2:cookielib py3:http.cookiejar
http_cookies py2:Cookie py3:http.cookies
html_entities py2:htmlentitydefs py3:html.entities
html_parser py2:HTMLParser py3:html.parser
http_client py2:httplib py3:http.client
BaseHTTPServer py2:BaseHTTPServer py3:http.server
CGIHTTPServer py2:CGIHTTPServer py3:http.server
SimpleHTTPServer py2:SimpleHTTPServer py3:http.server
input py2:raw_input py3:input
intern py2:intern py3:sys.intern
map py2:itertools.imap py3:map
queue py2:Queue py3:queue
range py2:xrange py3:range
reduce py2:reduce py3:functools.reduce
reload_module py2:reload py3:imp.reload, py3:importlib.reload on Python 3.4+
reprlib py2:repr py3:reprlib
shlex_quote py2:pipes.quote py3:shlex.quote
socketserver py2:SocketServer py3:socketserver
_thread py2:thread py3:_thread
tkinter py2:Tkinter py3:tkinter
tkinter_dialog py2:Dialog py3:tkinter.dialog
tkinter_filedialog py2:FileDialog py3:tkinter.FileDialog
tkinter_scrolledtext py2:ScrolledText py3:tkinter.scrolledtext
tkinter_simpledialog py2:SimpleDialog py3:tkinter.simpledialog
tkinter_ttk py2:ttk py3:tkinter.ttk
tkinter_tix py2:Tix py3:tkinter.tix
tkinter_constants py2:Tkconstants py3:tkinter.constants
tkinter_dnd py2:Tkdnd py3:tkinter.dnd
tkinter_colorchooser py2:tkColorChooser py3:tkinter.colorchooser
tkinter_commondialog py2:tkCommonDialog py3:tkinter.commondialog
tkinter_tkfiledialog py2:tkFileDialog py3:tkinter.filedialog
tkinter_font py2:tkFont py3:tkinter.font
tkinter_messagebox py2:tkMessageBox py3:tkinter.messagebox
tkinter_tksimpledialog py2:tkSimpleDialog py3:tkinter.simpledialog
urllib.parse See six.moves.urllib.parse py3:urllib.parse
urllib.error See six.moves.urllib.error py3:urllib.error
urllib.request See six.moves.urllib.request py3:urllib.request
urllib.response See six.moves.urllib.response py3:urllib.response
urllib.robotparser py2:robotparser py3:urllib.robotparser
urllib_robotparser py2:robotparser py3:urllib.robotparser
UserDict py2:UserDict.UserDict py3:collections.UserDict
UserList py2:UserList.UserList py3:collections.UserList
UserString py2:UserString.UserString py3:collections.UserString
winreg py2:_winreg py3:winreg
xmlrpc_client py2:xmlrpclib py3:xmlrpc.client
xmlrpc_server py2:SimpleXMLRPCServer py3:xmlrpc.server
xrange py2:xrange py3:range
zip py2:itertools.izip py3:zip
zip_longest py2:itertools.izip_longest py3:itertools.zip_longest

urllib parse

Contains functions from Python 3's py3:urllib.parse and Python 2's:


  • py2:urlparse.ParseResult
  • py2:urlparse.SplitResult
  • py2:urlparse.urlparse
  • py2:urlparse.urlunparse
  • py2:urlparse.parse_qs
  • py2:urlparse.parse_qsl
  • py2:urlparse.urljoin
  • py2:urlparse.urldefrag
  • py2:urlparse.urlsplit
  • py2:urlparse.urlunsplit
  • py2:urlparse.splitquery
  • py2:urlparse.uses_fragment
  • py2:urlparse.uses_netloc
  • py2:urlparse.uses_params
  • py2:urlparse.uses_query
  • py2:urlparse.uses_relative

and py2:urllib:

  • py2:urllib.quote
  • py2:urllib.quote_plus
  • py2:urllib.splittag
  • py2:urllib.splituser
  • py2:urllib.unquote
  • py2:urllib.unquote_plus
  • py2:urllib.urlencode

urllib error

Contains exceptions from Python 3's py3:urllib.error and Python 2's:


  • py2:urllib.ContentTooShortError

and py2:urllib2:

  • py2:urllib2.URLError
  • py2:urllib2.HTTPError

urllib request

Contains items from Python 3's py3:urllib.request and Python 2's:


  • py2:urllib.pathname2url
  • py2:urllib.url2pathname
  • py2:urllib.getproxies
  • py2:urllib.urlretrieve
  • py2:urllib.urlcleanup
  • py2:urllib.URLopener
  • py2:urllib.FancyURLopener
  • py2:urllib.proxy_bypass

and py2:urllib2:

  • py2:urllib2.urlopen
  • py2:urllib2.install_opener
  • py2:urllib2.build_opener
  • py2:urllib2.Request
  • py2:urllib2.OpenerDirector
  • py2:urllib2.HTTPDefaultErrorHandler
  • py2:urllib2.HTTPRedirectHandler
  • py2:urllib2.HTTPCookieProcessor
  • py2:urllib2.ProxyHandler
  • py2:urllib2.BaseHandler
  • py2:urllib2.HTTPPasswordMgr
  • py2:urllib2.HTTPPasswordMgrWithDefaultRealm
  • py2:urllib2.AbstractBasicAuthHandler
  • py2:urllib2.HTTPBasicAuthHandler
  • py2:urllib2.ProxyBasicAuthHandler
  • py2:urllib2.AbstractDigestAuthHandler
  • py2:urllib2.HTTPDigestAuthHandler
  • py2:urllib2.ProxyDigestAuthHandler
  • py2:urllib2.HTTPHandler
  • py2:urllib2.HTTPSHandler
  • py2:urllib2.FileHandler
  • py2:urllib2.FTPHandler
  • py2:urllib2.CacheFTPHandler
  • py2:urllib2.UnknownHandler
  • py2:urllib2.HTTPErrorProcessor

urllib response

Contains classes from Python 3's py3:urllib.response and Python 2's:


  • py2:urllib.addbase
  • py2:urllib.addclosehook
  • py2:urllib.addinfo
  • py2:urllib.addinfourl

Advanced - Customizing renames

It is possible to add additional names to the six.moves namespace.

Instances of the following classes can be passed to add_move. Neither have any public members.

Create a mapping for six.moves called name that references different modules in Python 2 and 3. old_mod is the name of the Python 2 module. new_mod is the name of the Python 3 module.

Create a mapping for six.moves called name that references different attributes in Python 2 and 3. old_mod is the name of the Python 2 module. new_mod is the name of the Python 3 module. If new_attr is not given, it defaults to old_attr. If neither is given, they both default to name.