six

Common errors

Warnings

Install compatibility verified last tested: 2026-05-12

Imports

• six

  import six

  Top-level import; all constants, helpers, and the moves virtual module are accessed as attributes of the six module object.

• string_types / text_type / binary_type
  wrong

  isinstance(value, str)

  correct

  import six
  isinstance(value, six.string_types)

  On Python 2 string_types is (basestring,); on Python 3 it is (str,). Using bare str misses unicode on Py2.

• six.moves (stdlib remaps)
  wrong

  import six.moves

  correct

  from six.moves import urllib
  from six.moves import range
  from six.moves import configparser

  six.moves is a virtual module implemented via a PEP 302 meta path hook inside six.py — there is no moves.py on disk. Direct 'import six.moves' can fail with ModuleNotFoundError in some environments (frozen importers, vendored copies, Python 3.12+). Always use 'from six.moves import <name>'.

• with_metaclass
  wrong

  class MyClass(object, metaclass=Meta): pass  # Py3-only syntax

  correct

  from six import with_metaclass
  class MyClass(with_metaclass(Meta, Base)): pass

  Use with_metaclass() or the @add_metaclass decorator only when the codebase must remain Py2-compatible; on Py3-only code use the native metaclass= keyword directly.

• add_metaclass

  from six import add_metaclass
  @add_metaclass(Meta)
  class MyClass(object): pass

  Decorator alternative to with_metaclass; produces an equivalent class but applied post-construction.

• reraise

  from six import reraise
  reraise(tp, value, tb=None)

  Wraps Python 3's 'raise value.with_traceback(tb)' syntax which is a SyntaxError on Python 2.

• six.moves.urllib
  wrong

  from six.moves import urllib
  urllib.parse.urlencode(…)

  correct

  from six.moves.urllib.parse import urlencode
  from six.moves.urllib.request import urlopen

  six.moves.urllib sub-namespaces (parse, request, response, error, robotparser) must be imported individually; accessing them as attributes of the moves.urllib object is unreliable without first importing the sub-module.

Quickstart verified last tested: 2026-04-23

Demonstrate the most common six patterns: version guards, type checks, and stdlib moves.

import six
from six.moves import urllib
from six.moves.urllib.parse import urlencode

# Version guard
if six.PY2:
    print("Running on Python 2")
else:
    print("Running on Python 3")

# Cross-version type check
value = u"hello"
assert isinstance(value, six.text_type)        # unicode on Py2, str on Py3
assert isinstance(b"bytes", six.binary_type)  # str on Py2, bytes on Py3
assert isinstance(value, six.string_types)    # catches both str/unicode on Py2

# Stdlib move: urllib.parse.urlencode works on both versions
params = urlencode({"key": "value", "foo": "bar"})
print("Encoded:", params)

# Integer types (int + long on Py2, int only on Py3)
assert isinstance(42, six.integer_types)

# Metaclass compatibility
from six import with_metaclass
class Meta(type):
    pass
class MyBase(with_metaclass(Meta, object)):
    pass
print("Metaclass:", type(MyBase))