API Reference (in progress) — Python-Future documentation

# API Reference (in progress)

NOTE: This page is still a work in progress… We need to go through our docstrings and make them sphinx-compliant, and figure out how to improve formatting with the sphinx-bootstrap-theme plugin. Pull requests would be very welcome.

## future.builtins Interface

A module that brings in equivalents of the new and modified Python 3 builtins into Py2. Has no effect on Py3.

See the docs here (`docs/what-else.rst`) for more information.

## Backported types from Python 3

This module contains backports the data types that were significantly changed in the transition from Python 2 to Python 3.

- an implementation of Python 3’s bytes object (pure Python subclass of Python 2’s builtin 8-bit str type)

- an implementation of Python 3’s str object (pure Python subclass of Python 2’s builtin unicode type)

- a backport of the range iterator from Py3 with slicing support

It is used as follows:

    from __future__ import division, absolute_import, print_function
    from builtins import bytes, dict, int, range, str

to bring in the new semantics for these functions from Python 3. And then, for example:

    b = bytes(b'ABCD')
    assert list(b) == [65, 66, 67, 68]
    assert repr(b) == "b'ABCD'"
    assert [65, 66] in b

    # These raise TypeErrors:
    # b + u'EFGH'
    # b.split(u'B')
    # bytes(b',').join([u'Fred', u'Bill'])

    s = str(u'ABCD')

    # These raise TypeErrors:
    # s.join([b'Fred', b'Bill'])
    # s.startswith(b'A')
    # b'B' in s
    # s.find(b'A')
    # s.replace(u'A', b'a')

    # This raises an AttributeError:
    # s.decode('utf-8')

    assert repr(s) == 'ABCD'      # consistent repr with Py3 (no u prefix)

    for i in range(10**11)[:10]:
        pass

and:

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

### For more information:

- future.types.newbytes

- future.types.newdict

- future.types.newint

- future.types.newobject

- future.types.newrange

- future.types.newstr

#### Notes

### range()

`range` is a custom class that backports the slicing behaviour from Python 3 (based on the `xrange` module by Dan Crosta). See the `newrange` module docstring for more details.

### super()

`super()` is based on Ryan Kelly’s `magicsuper` module. See the `newsuper` module docstring for more details.

### round()

Python 3 modifies the behaviour of `round()` to use “Banker’s Rounding”. See http://stackoverflow.com/a/10825998. See the `newround` module docstring for more details.

## future.standard_library Interface

Python 3 reorganized the standard library (PEP 3108). This module exposes several standard library modules to Python 2 under their new Python 3 names.

It is designed to be used as follows:

    from future import standard_library
    standard_library.install_aliases()

And then these normal Py3 imports work on both Py3 and Py2:

    import builtins
    import copyreg
    import queue
    import reprlib
    import socketserver
    import winreg    # on Windows only
    import test.support
    import html, html.parser, html.entities
    import http, http.client, http.server
    import http.cookies, http.cookiejar
    import urllib.parse, urllib.request, urllib.response, urllib.error, urllib.robotparser
    import xmlrpc.client, xmlrpc.server

    import _thread
    import _dummy_thread
    import _markupbase

    from itertools import filterfalse, zip_longest
    from sys import intern
    from collections import UserDict, UserList, UserString
    from collections import OrderedDict, Counter, ChainMap     # even on Py2.6
    from subprocess import getoutput, getstatusoutput
    from subprocess import check_output              # even on Py2.6

(The renamed modules and functions are still available under their old names on Python 2.)

This is a cleaner alternative to this idiom (see http://docs.pythonsprints.com/python3_porting/py-porting.html):

    try:
        import queue
    except ImportError:
        import Queue as queue

### Limitations

We don’t currently support these modules, but would like to:

    import dbm
    import dbm.dumb
    import dbm.gnu
    import collections.abc  # on Py33
    import pickle     # should (optionally) bring in cPickle on Python 2

class future.standard_library.RenameImport(old_to_new)

  A class for import hooks mapping Py3 module names etc. to the Py2 equivalents.

future.standard_library.cache_py2_modules()

  Currently this function is unneeded, as we are not attempting to provide import hooks for modules with ambiguous names: email, urllib, pickle.

future.standard_library.detect_hooks()

  Returns True if the import hooks are installed, False if not.

future.standard_library.disable_hooks()

  Deprecated. Use remove_hooks() instead. This will be removed by `future` v1.0.

future.standard_library.enable_hooks()

  Deprecated. Use install_hooks() instead. This will be removed by `future` v1.0.

class future.standard_library.exclude_local_folder_imports(*args)

  A context-manager that prevents standard library modules like configparser from being imported from the local python-future source folder on Py3.

  (This was need prior to v0.16.0 because the presence of a configparser folder would otherwise have prevented setuptools from running on Py3. Maybe it’s not needed any more?)

future.standard_library.from_import(module_name, *symbol_names, **kwargs)
  Example use:

        >>> HTTPConnection = from_import('http.client', 'HTTPConnection')
        >>> HTTPServer = from_import('http.server', 'HTTPServer')
        >>> urlopen, urlparse = from_import('urllib.request', 'urlopen', 'urlparse')

  Equivalent to this on Py3:

      >>> from module_name import symbol_names[0], symbol_names[1], ...

  and this on Py2:

      >>> from future.moves.module_name import symbol_names[0], ...

  or:

      >>> from future.backports.module_name import symbol_names[0], ...

  except that it also handles dotted module names such as `http.client`.

class future.standard_library.hooks

  Acts as a context manager. Saves the state of sys.modules and restores it after the ‘with’ block.

  Use like this:

      >>> from future import standard_library
      >>> with standard_library.hooks():
      ...     import http.client
      >>> import requests

  For this to work, http.client will be scrubbed from sys.modules after the ‘with’ block. That way the modules imported in the ‘with’ block will continue to be accessible in the current namespace but not from any imported modules (like requests).

future.standard_library.import_(module_name, backport=False)

  Pass a (potentially dotted) module name of a Python 3 standard library module. This function imports the module compatibly on Py2 and Py3 and returns the top-level module.

  Example use:

        >>> http = import_('http.client')
        >>> http = import_('http.server')
        >>> urllib = import_('urllib.request')

  Then:

        >>> conn = http.client.HTTPConnection(...)
        >>> response = urllib.request.urlopen('http://mywebsite.com')
        >>> # etc.

  Use as follows:

        >>> package_name = import_(module_name)

  On Py3, equivalent to this:

      >>> import module_name

  On Py2, equivalent to this if backport=False:

      >>> from future.moves import module_name

  or to this if backport=True:

      >>> from future.backports import module_name

  except that it also handles dotted module names such as `http.client` The effect then is like this:

      >>> from future.backports import module
      >>> from future.backports.module import submodule
      >>> module.submodule = submodule

  Note that this would be a SyntaxError in Python:

      >>> from future.backports import http.client

future.standard_library.install_aliases()

  Monkey-patches the standard library in Py2.6/7 to provide aliases for better Py3 compatibility.

future.standard_library.install_hooks()

  This function installs the future.standard_library import hook into sys.meta_path.

future.standard_library.is_py2_stdlib_module(m)

  Tries to infer whether the module m is from the Python 2 standard library. This may not be reliable on all systems.

future.standard_library.remove_hooks(scrub_sys_modules=False)

  This function removes the import hook from sys.meta_path.

future.standard_library.restore_sys_modules(scrubbed)

  Add any previously scrubbed modules back to the sys.modules cache, but only if it’s safe to do so.

future.standard_library.scrub_future_sys_modules()

  Deprecated.

future.standard_library.scrub_py2_sys_modules()

  Removes any Python 2 standard library modules from `sys.modules` that would interfere with Py3-style imports using import hooks. Examples are modules with the same names (like urllib or email).

  (Note that currently import hooks are disabled for modules like these with ambiguous names anyway …)

class future.standard_library.suspend_hooks

  Acts as a context manager. Use like this:

      >>> from future import standard_library
      >>> standard_library.install_hooks()
      >>> import http.client
      >>> # ...
      >>> with standard_library.suspend_hooks():
      >>>     import requests     # incompatible with ``future``'s standard library hooks

  If the hooks were disabled before the context, they are not installed when the context is left.

## future.utils Interface

A selection of cross-compatible functions for Python 2 and 3.

This module exports useful functions for 2/3 compatible code:

> - bind_method: binds functions to classes

> - `native_str_to_bytes` and `bytes_to_native_str`

> - `native_str`: always equal to the native platform string object (because this may be shadowed by imports from future.builtins)

> - lists: lrange(), lmap(), lzip(), lfilter()

> - iterable method compatibility:

>     - iteritems, iterkeys, itervalues

>     - viewitems, viewkeys, viewvalues

>     These use the original method if available, otherwise they use items, keys, values.

> - types:

>   > - text_type: unicode in Python 2, str in Python 3

>   > - string_types: basestring in Python 2, str in Python 3

>   > - binary_type: str in Python 2, bytes in Python 3

>   > - integer_types: (int, long) in Python 2, int in Python 3

>   > - class_types: (type, types.ClassType) in Python 2, type in Python 3

> - bchr(c):

>     Take an integer and make a 1-character byte string

> - bord(c)

>     Take the result of indexing on a byte string and make an integer

> - tobytes(s)

>     Take a text string, a byte string, or a sequence of characters taken from a byte string, and make a byte string.

> - raise_from()

> - raise_with_traceback()

This module also defines these decorators:

> - `python_2_unicode_compatible`

> - `with_metaclass`

> - `implements_iterator`

Some of the functions in this module come from the following sources:

> - Jinja2 (BSD licensed: see https://github.com/mitsuhiko/jinja2/blob/master/LICENSE)

> - Pandas compatibility module pandas.compat

> - six.py by Benjamin Peterson

> - Django

future.utils.as_native_str(encoding='utf-8')

  A decorator to turn a function or method call that returns text, i.e. unicode, into one that returns a native platform str.

  Use it as a decorator like this:

      from __future__ import unicode_literals

      class MyClass(object):
          @as_native_str(encoding='ascii')
          def __repr__(self):
              return next(self._iter).upper()

future.utils.binary_type

  alias of `bytes`

future.utils.bind_method(cls, name, func)

  Bind a method to class, python 2 and python 3 compatible.

  ### Parameters

  clstype

    class to receive bound method

  namebasestring

    name of method on class instance

  funcfunction

    function to be bound as method

  ### Returns

  None

future.utils.exec_(source, globals=None, locals=None, /, *, closure=None)

  Execute the given source in the context of globals and locals.

  The source may be a string representing one or more Python statements or a code object as returned by compile(). The globals must be a dictionary and locals can be any mapping, defaulting to the current globals and locals. If only globals is given, locals defaults to it. The closure must be a tuple of cellvars, and can only be used when source is a code object requiring exactly that many cellvars.

future.utils.implements_iterator(cls)

  From jinja2/_compat.py. License: BSD.

  Use as a decorator like this:

      @implements_iterator
      class UppercasingIterator(object):
          def __init__(self, iterable):
              self._iter = iter(iterable)
          def __iter__(self):
              return self
          def __next__(self):
              return next(self._iter).upper()

future.utils.is_new_style(cls)

  Python 2.7 has both new-style and old-style classes. Old-style classes can be pesky in some circumstances, such as when using inheritance. Use this function to test for whether a class is new-style. (Python 3 only has new-style classes.)

future.utils.isbytes(obj)
  Deprecated. Use::

        >>> isinstance(obj, bytes)

  after this import:

        >>> from future.builtins import bytes

future.utils.isidentifier(s, dotted=False)

  A function equivalent to the str.isidentifier method on Py3

future.utils.isint(obj)

  Deprecated. Tests whether an object is a Py3 `int` or either a Py2 `int` or `long`.

  Instead of using this function, you can use:

      >>> from future.builtins import int
      >>> isinstance(obj, int)

  The following idiom is equivalent:

      >>> from numbers import Integral
      >>> isinstance(obj, Integral)

future.utils.isnewbytes(obj)

  Equivalent to the result of `type(obj) == type(newbytes)` in other words, it is REALLY a newbytes instance, not a Py2 native str object?

  Note that this does not cover subclasses of newbytes, and it is not equivalent to ininstance(obj, newbytes)

future.utils.istext(obj)
  Deprecated. Use::

        >>> isinstance(obj, str)

  after this import:

        >>> from future.builtins import str

future.utils.iteritems(obj, **kwargs)

  Use this only if compatibility with Python versions before 2.7 is required. Otherwise, prefer viewitems().

future.utils.iterkeys(obj, **kwargs)

  Use this only if compatibility with Python versions before 2.7 is required. Otherwise, prefer viewkeys().

future.utils.itervalues(obj, **kwargs)

  Use this only if compatibility with Python versions before 2.7 is required. Otherwise, prefer viewvalues().

future.utils.native(obj)

  On Py3, this is a no-op: native(obj) -> obj

  On Py2, returns the corresponding native Py2 types that are superclasses for backported objects from Py3:

      >>> from builtins import str, bytes, int

      >>> native(str(u'ABC'))
      u'ABC'
      >>> type(native(str(u'ABC')))
      unicode

      >>> native(bytes(b'ABC'))
      b'ABC'
      >>> type(native(bytes(b'ABC')))
      bytes

      >>> native(int(10**20))
      100000000000000000000L
      >>> type(native(int(10**20)))
      long

  Existing native types on Py2 will be returned unchanged:

      >>> type(native(u'ABC'))
      unicode

future.utils.native_bytes

  alias of `bytes`

future.utils.native_str

  alias of `str`

future.utils.native_str_to_bytes(s, encoding='utf-8')

  On Py3, returns an encoded string. On Py2, returns a newbytes type, ignoring the `encoding` argument.

future.utils.old_div(a, b)

  DEPRECATED: import `old_div` from `past.utils` instead.

  Equivalent to `a / b` on Python 2 without `from __future__ import division`.

  TODO: generalize this to other objects (like arrays etc.)

future.utils.python_2_unicode_compatible(cls)

  A decorator that defines __unicode__ and __str__ methods under Python 2. Under Python 3, this decorator is a no-op.

  To support Python 2 and 3 with a single code base, define a __str__ method returning unicode text and apply this decorator to the class, like this:

      >>> from future.utils import python_2_unicode_compatible

      >>> @python_2_unicode_compatible
      ... class MyClass(object):
      ...     def __str__(self):
      ...         return u'Unicode string: 孔子'

      >>> a = MyClass()

  Then, after this import:

      >>> from future.builtins import str

  the following is `True` on both Python 3 and 2:

      >>> str(a) == a.encode('utf-8').decode('utf-8')

  True

  and, on a Unicode-enabled terminal with the right fonts, these both print the Chinese characters for Confucius:

      >>> print(a)
      >>> print(str(a))

  The implementation comes from django.utils.encoding.

future.utils.raise_(tp, value=None, tb=None)

  A function that matches the Python 2.x `raise` statement. This allows re-raising exceptions with the cls value and traceback on Python 2 and 3.

future.utils.raise_with_traceback(exc, traceback=Ellipsis)

  Raise exception with existing traceback. If traceback is not passed, uses sys.exc_info() to get traceback.

future.utils.reraise(tp, value=None, tb=None)

  A function that matches the Python 2.x `raise` statement. This allows re-raising exceptions with the cls value and traceback on Python 2 and 3.

future.utils.text_type

  alias of `str`

future.utils.tobytes(s)

  Encodes to latin-1 (where the first 256 chars are the same as ASCII.)

future.utils.viewitems(obj, **kwargs)

  Function for iterating over dictionary items with the same set-like behaviour on Py2.7 as on Py3.

  Passes kwargs to method.

future.utils.viewkeys(obj, **kwargs)

  Function for iterating over dictionary keys with the same set-like behaviour on Py2.7 as on Py3.

  Passes kwargs to method.

future.utils.viewvalues(obj, **kwargs)

  Function for iterating over dictionary values with the same set-like behaviour on Py2.7 as on Py3.

  Passes kwargs to method.

future.utils.with_metaclass(meta, *bases)

  Function from jinja2/_compat.py. License: BSD.

  Use it like this:

      class BaseForm(object):
          pass

      class FormType(type):
          pass

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

  This requires a bit of explanation: the basic idea is to make a dummy metaclass for one level of class instantiation that replaces itself with the actual metaclass. Because of internal type checks we also need to make sure that we downgrade the custom metaclass for one level to something closer to type (that’s why __call__ and __init__ comes back from type etc.).

  This has the advantage over six.with_metaclass of not introducing dummy classes into the final MRO.

## past.builtins Interface

A resurrection of some old functions from Python 2 for use in Python 3. These should be used sparingly, to help with porting efforts, since code using them is no longer standard Python 3 code.

This module provides the following:

1. Implementations of these builtin functions which have no equivalent on Py3:

- apply

- chr

- cmp

- execfile

2. Aliases:

- intern <- sys.intern

- raw_input <- input

- reduce <- functools.reduce

- reload <- imp.reload

- unichr <- chr

- unicode <- str

- xrange <- range

3. List-producing versions of the corresponding Python 3 iterator-producing functions:

- filter

- map

- range

- zip

4. Forward-ported Py2 types:

- basestring

- dict

- str

- long

- unicode

class past.builtins.basestring

  A minimal backport of the Python 2 basestring type to Py3

past.builtins.chr(i)

  Return a byte-string of one character with ordinal i; 0 <= i <= 256

past.builtins.cmp(x, y) → integer

  Return negative if x<y, zero if x==y, positive if x>y. Python2 had looser comparison allowing cmp None and non Numerical types and collections. Try to match the old behavior

past.builtins.dict

  alias of `olddict`

past.builtins.execfile(filename, myglobals=None, mylocals=None)

  Read and execute a Python script from a file in the given namespaces. The globals and locals are dictionaries, defaulting to the current globals and locals. If only globals is given, locals defaults to it.

past.builtins.filter(function or None, sequence) → list, tuple, or string

  Return those items of sequence for which function(item) is true. If function is None, return the items that are true. If sequence is a tuple or string, return the same type, else return a list.

past.builtins.intern(string, /)

  ``Intern’’ the given string.

  This enters the string in the (global) table of interned strings whose purpose is to speed up dictionary lookups. Return the string itself or the previously interned string object with the same value.

past.builtins.long

  alias of `int`

past.builtins.map(function, sequence[, sequence, ...]) → list

  Return a list of the results of applying the function to the items of the argument sequence(s). If more than one sequence is given, the function is called with an argument list consisting of the corresponding item of each sequence, substituting None for missing values when not all sequences have the same length. If the function is None, return a list of the items of the sequence (or a list of tuples if more than one sequence).

  Test cases: >>> oldmap(None, ‘hello world’) [‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘ ‘, ‘w’, ‘o’, ‘r’, ‘l’, ‘d’]

      >>> oldmap(None, range(4))
      [0, 1, 2, 3]

  More test cases are in test_past.test_builtins.

past.builtins.raw_input(prompt='', /)

  Read a string from standard input. The trailing newline is stripped.

  The prompt string, if given, is printed to standard output without a trailing newline before reading input.

  If the user hits EOF (*nix: Ctrl-D, Windows: Ctrl-Z+Return), raise EOFError. On *nix systems, readline is used if available.

past.builtins.reduce(function, iterable[, initial]) → value

  Apply a function of two arguments cumulatively to the items of a sequence or iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty.

past.builtins.reload(module)

  Reload the module and return it.

  The module must have been successfully imported before.

past.builtins.str

  alias of `oldstr`

past.builtins.unichr(i)

  Return a byte-string of one character with ordinal i; 0 <= i <= 256

past.builtins.unicode

  alias of `str`

past.builtins.xrange

  alias of `range`

## Forward-ported types from Python 2

Forward-ports of types from Python 2 for use with Python 3:

- `basestring`: equivalent to `(str, bytes)` in `isinstance` checks

- `dict`: with list-producing .keys() etc. methods

- `str`: bytes-like, but iterating over them doesn’t product integers

- `long`: alias of Py3 int with `L` suffix in the `repr`

- `unicode`: alias of Py3 str with `u` prefix in the `repr`

class past.types.basestring

  A minimal backport of the Python 2 basestring type to Py3

past.types.long

  alias of `int`

class past.types.olddict

  A backport of the Python 3 dict object to Py2

  has_key(k) → True if D has a key k, else False
  items() → a set-like object providing a view on D's items
  iteritems()

    D.items() -> a set-like object providing a view on D’s items

  iterkeys()

    D.keys() -> a set-like object providing a view on D’s keys

  itervalues()

    D.values() -> an object providing a view on D’s values

  keys() → a set-like object providing a view on D's keys
  values() → an object providing a view on D's values
  viewitems()

    D.items() -> a set-like object providing a view on D’s items

  viewkeys()

    D.keys() -> a set-like object providing a view on D’s keys

  viewvalues()

    D.values() -> an object providing a view on D’s values

class past.types.oldstr

  A forward port of the Python 2 8-bit string object to Py3

past.types.unicode

  alias of `str`