docs/index.rst

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Introducing funcsigs
====================

The Funcsigs Package
--------------------

*funcsigs* is a backport of the `PEP 362`_ function signature features from
Python 3.3's `inspect`_ module. The backport is compatible with Python 2.6, 2.7
as well as 3.2 and up.

.. _PEP 362: http://www.python.org/dev/peps/pep-0362/
.. _inspect: http://docs.python.org/3/library/inspect.html#introspecting-callables-with-the-signature-object

Compatability
`````````````

The *funcsigs* backport has been tested against:

* CPython 2.6
* CPython 2.7
* CPython 3.2
* PyPy 1.9

Continuous integration testing is provided by `Travis CI`_.

Under Python 2.x there is a compatability issue when a function is assigned to
the ``__wrapped__`` property of a class after it has been constructed.
Similiarily there under PyPy directly passing the ``__call__`` method of a
builtin is also a compatability issues.  Otherwise the functionality is
believed to be uniform between both Python2 and Python3.

.. _Travis CI: http://travis-ci.org/

Issues
``````

Source code for *funcsigs* is hosted on `GitHub`_. Any bug reports or feature
requests can be made using GitHub's `issues system`_.

.. _GitHub: https://github.com/aliles/funcsigs
.. _issues system: https://github.com/alies/funcsigs/issues

Introspecting callables with the Signature object
-------------------------------------------------

.. note::

   This section of documentation is a direct repoduction of the Python
   standard library documentation for the inspect module.

The Signature object represents the call signature of a callable object and its
return annotation.  To retrieve a Signature object, use the :func:`signature`
function.

.. function:: signature(callable)

   Return a :class:`Signature` object for the given ``callable``::

      >>> from inspect import signature
      >>> def foo(a, *, b:int, **kwargs):
      ...     pass

      >>> sig = signature(foo)

      >>> str(sig)
      '(a, *, b:int, **kwargs)'

      >>> str(sig.parameters['b'])
      'b:int'

      >>> sig.parameters['b'].annotation
      <class 'int'>

   Accepts a wide range of python callables, from plain functions and classes to
   :func:`functools.partial` objects.

   .. note::

      Some callables may not be introspectable in certain implementations of
      Python.  For example, in CPython, built-in functions defined in C provide
      no metadata about their arguments.


.. class:: Signature

   A Signature object represents the call signature of a function and its return
   annotation.  For each parameter accepted by the function it stores a
   :class:`Parameter` object in its :attr:`parameters` collection.

   Signature objects are *immutable*.  Use :meth:`Signature.replace` to make a
   modified copy.

   .. attribute:: Signature.empty

      A special class-level marker to specify absence of a return annotation.

   .. attribute:: Signature.parameters

      An ordered mapping of parameters' names to the corresponding
      :class:`Parameter` objects.

   .. attribute:: Signature.return_annotation

      The "return" annotation for the callable.  If the callable has no "return"
      annotation, this attribute is set to :attr:`Signature.empty`.

   .. method:: Signature.bind(*args, **kwargs)

      Create a mapping from positional and keyword arguments to parameters.
      Returns :class:`BoundArguments` if ``*args`` and ``**kwargs`` match the
      signature, or raises a :exc:`TypeError`.

   .. method:: Signature.bind_partial(*args, **kwargs)

      Works the same way as :meth:`Signature.bind`, but allows the omission of
      some required arguments (mimics :func:`functools.partial` behavior.)
      Returns :class:`BoundArguments`, or raises a :exc:`TypeError` if the
      passed arguments do not match the signature.

   .. method:: Signature.replace(*[, parameters][, return_annotation])

      Create a new Signature instance based on the instance replace was invoked
      on.  It is possible to pass different ``parameters`` and/or
      ``return_annotation`` to override the corresponding properties of the base
      signature.  To remove return_annotation from the copied Signature, pass in
      :attr:`Signature.empty`.

      ::

         >>> def test(a, b):
         ...     pass
         >>> sig = signature(test)
         >>> new_sig = sig.replace(return_annotation="new return anno")
         >>> str(new_sig)
         "(a, b) -> 'new return anno'"


.. class:: Parameter

   Parameter objects are *immutable*.  Instead of modifying a Parameter object,
   you can use :meth:`Parameter.replace` to create a modified copy.

   .. attribute:: Parameter.empty

      A special class-level marker to specify absence of default values and
      annotations.

   .. attribute:: Parameter.name

      The name of the parameter as a string.  Must be a valid python identifier
      name (with the exception of ``POSITIONAL_ONLY`` parameters, which can have
      it set to ``None``).

   .. attribute:: Parameter.default

      The default value for the parameter.  If the parameter has no default
      value, this attribute is set to :attr:`Parameter.empty`.

   .. attribute:: Parameter.annotation

      The annotation for the parameter.  If the parameter has no annotation,
      this attribute is set to :attr:`Parameter.empty`.

   .. attribute:: Parameter.kind

      Describes how argument values are bound to the parameter.  Possible values
      (accessible via :class:`Parameter`, like ``Parameter.KEYWORD_ONLY``):

      +------------------------+----------------------------------------------+
      |    Name                | Meaning                                      |
      +========================+==============================================+
      | *POSITIONAL_ONLY*      | Value must be supplied as a positional       |
      |                        | argument.                                    |
      |                        |                                              |
      |                        | Python has no explicit syntax for defining   |
      |                        | positional-only parameters, but many built-in|
      |                        | and extension module functions (especially   |
      |                        | those that accept only one or two parameters)|
      |                        | accept them.                                 |
      +------------------------+----------------------------------------------+
      | *POSITIONAL_OR_KEYWORD*| Value may be supplied as either a keyword or |
      |                        | positional argument (this is the standard    |
      |                        | binding behaviour for functions implemented  |
      |                        | in Python.)                                  |
      +------------------------+----------------------------------------------+
      | *VAR_POSITIONAL*       | A tuple of positional arguments that aren't  |
      |                        | bound to any other parameter. This           |
      |                        | corresponds to a ``*args`` parameter in a    |
      |                        | Python function definition.                  |
      +------------------------+----------------------------------------------+
      | *KEYWORD_ONLY*         | Value must be supplied as a keyword argument.|
      |                        | Keyword only parameters are those which      |
      |                        | appear after a ``*`` or ``*args`` entry in a |
      |                        | Python function definition.                  |
      +------------------------+----------------------------------------------+
      | *VAR_KEYWORD*          | A dict of keyword arguments that aren't bound|
      |                        | to any other parameter. This corresponds to a|
      |                        | ``**kwargs`` parameter in a Python function  |
      |                        | definition.                                  |
      +------------------------+----------------------------------------------+

      Example: print all keyword-only arguments without default values::

         >>> def foo(a, b, *, c, d=10):
         ...     pass

         >>> sig = signature(foo)
         >>> for param in sig.parameters.values():
         ...     if (param.kind == param.KEYWORD_ONLY and
         ...                        param.default is param.empty):
         ...         print('Parameter:', param)
         Parameter: c

   .. method:: Parameter.replace(*[, name][, kind][, default][, annotation])

      Create a new Parameter instance based on the instance replaced was invoked
      on.  To override a :class:`Parameter` attribute, pass the corresponding
      argument.  To remove a default value or/and an annotation from a
      Parameter, pass :attr:`Parameter.empty`.

      ::

         >>> from inspect import Parameter
         >>> param = Parameter('foo', Parameter.KEYWORD_ONLY, default=42)
         >>> str(param)
         'foo=42'

         >>> str(param.replace()) # Will create a shallow copy of 'param'
         'foo=42'

         >>> str(param.replace(default=Parameter.empty, annotation='spam'))
         "foo:'spam'"


.. class:: BoundArguments

   Result of a :meth:`Signature.bind` or :meth:`Signature.bind_partial` call.
   Holds the mapping of arguments to the function's parameters.

   .. attribute:: BoundArguments.arguments

      An ordered, mutable mapping (:class:`collections.OrderedDict`) of
      parameters' names to arguments' values.  Contains only explicitly bound
      arguments.  Changes in :attr:`arguments` will reflect in :attr:`args` and
      :attr:`kwargs`.

      Should be used in conjunction with :attr:`Signature.parameters` for any
      argument processing purposes.

      .. note::

         Arguments for which :meth:`Signature.bind` or
         :meth:`Signature.bind_partial` relied on a default value are skipped.
         However, if needed, it is easy to include them.

      ::

        >>> def foo(a, b=10):
        ...     pass

        >>> sig = signature(foo)
        >>> ba = sig.bind(5)

        >>> ba.args, ba.kwargs
        ((5,), {})

        >>> for param in sig.parameters.values():
        ...     if param.name not in ba.arguments:
        ...         ba.arguments[param.name] = param.default

        >>> ba.args, ba.kwargs
        ((5, 10), {})


   .. attribute:: BoundArguments.args

      A tuple of positional arguments values.  Dynamically computed from the
      :attr:`arguments` attribute.

   .. attribute:: BoundArguments.kwargs

      A dict of keyword arguments values.  Dynamically computed from the
      :attr:`arguments` attribute.

   The :attr:`args` and :attr:`kwargs` properties can be used to invoke
   functions::

      def test(a, *, b):
         ...

      sig = signature(test)
      ba = sig.bind(10, b=20)
      test(*ba.args, **ba.kwargs)


.. seealso::

   :pep:`362` - Function Signature Object.
      The detailed specification, implementation details and examples.

Copyright
---------

*funcsigs* is a derived work of CPython under the terms of the `PSF License
Agreement`_. The original CPython inspect module, its unit tests and
documentation are the copyright of the Python Software Foundation. The derived
work is distributed under the `Apache License Version 2.0`_.

.. _PSF License Agreement: http://docs.python.org/3/license.html#terms-and-conditions-for-accessing-or-otherwise-using-python
.. _Apache License Version 2.0: http://opensource.org/licenses/Apache-2.0