Issue #15151: PEP 362 — document signature and related classes for inspect.py

Initial patch by Yury Selivanov

Doc/library/inspect.rst

@@ -395,6 +395,267 @@ Retrieving source code
    onwards is removed.  Also, all tabs are expanded to spaces.
 
 
+.. _inspect-signature-object:
+
+Introspecting callables with Signature Object
+---------------------------------------------
+
+Signature object represents the call signature of a callable object and its
+return annotation.  To get a Signature object use the :func:`signature`
+function.
+
+
+.. versionadded:: 3.3
+
+.. seealso::
+
+   :pep:`362` - Function Signature Object.
+      The detailed specification, implementation details and examples.
+
+
+.. function:: signature(callable)
+
+   Returns 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)
+
+      Creates 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])
+
+      Creates 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.                                  |
+      +------------------------+----------------------------------------------+
+
+      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])
+
+      Creates 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 arguments 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's 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
+
+      Tuple of positional arguments values. Dynamically computed
+      from the :attr:`arguments` attribute.
+
+   .. attribute:: BoundArguments.kwargs
+
+      Dict of keyword arguments values. Dynamically computed
+      from the :attr:`arguments` attribute.
+
+   :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)
+
+
 .. _inspect-classes-functions:
 
 Classes and functions
@@ -443,6 +704,10 @@ Classes and functions
 
    The first four items in the tuple correspond to :func:`getargspec`.
 
+   .. note::
+      Consider using the new :ref:`Signature Object <inspect-signature-object>`
+      interface, which provides a better way of introspecting functions.
+
 
 .. function:: getargvalues(frame)
 
@@ -505,6 +770,9 @@ Classes and functions
 
    .. versionadded:: 3.2
 
+   .. note::
+      Consider using the new :meth:`Signature.bind` instead.
+
 
 .. function:: getclosurevars(func)