python-peps/pep-0362.txt

PEP: 362
Title: Function Signature Object
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett@python.org>, Jiwon Seo <seojiwon@gmail.com>,
        Yury Selivanov <yselivanov@sprymix.com>, Larry Hastings <larry@hastings.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Aug-2006
Python-Version: 3.3
Post-History: 04-Jun-2012


Abstract
========

Python has always supported powerful introspection capabilities,
including introspecting functions and methods (for the rest of
this PEP, "function" refers to both functions and methods).  By
examining a function object you can fully reconstruct the function's
signature.  Unfortunately this information is stored in an inconvenient
manner, and is spread across a half-dozen deeply nested attributes.

This PEP proposes a new representation for function signatures.
The new representation contains all necessary information about a function
and its parameters, and makes introspection easy and straightforward.

However, this object does not replace the existing function
metadata, which is used by Python itself to execute those
functions.  The new metadata object is intended solely to make
function introspection easier for Python programmers.


Signature Object
================

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

A Signature object has the following public attributes and methods:

* return_annotation : object
    The annotation for the return type of the function if specified.
    If the function has no annotation for its return type, this
    attribute is not set.
* parameters : OrderedDict
    An ordered mapping of parameters' names to the corresponding
    Parameter objects (keyword-only arguments are in the same order
    as listed in ``code.co_varnames``).
* bind(\*args, \*\*kwargs) -> BoundArguments
    Creates a mapping from positional and keyword arguments to
    parameters.  Raises a ``TypeError`` if the passed arguments do
    not match the signature.
* bind_partial(\*args, \*\*kwargs) -> BoundArguments
    Works the same way as ``bind()``, but allows the omission
    of some required arguments (mimics ``functools.partial``
    behavior.)  Raises a ``TypeError`` if the passed arguments do
    not match the signature.
* format(...) -> str
    Formats the Signature object to a string.  Optional arguments allow
    for custom render functions for parameter names,
    annotations and default values, along with custom separators.

Signature implements the ``__str__`` method, which fallbacks to the
``Signature.format()`` call.

It's possible to test Signatures for equality.  Two signatures
are equal when they have equal parameters and return annotations.

Changes to the Signature object, or to any of its data members,
do not affect the function itself.


Parameter Object
================

Python's expressive syntax means functions can accept many different
kinds of parameters with many subtle semantic differences.  We
propose a rich Parameter object designed to represent any possible
function parameter.

The structure of the Parameter object is:

* name : str
    The name of the parameter as a string.

* default : object
    The default value for the parameter, if specified.  If the
    parameter has no default value, this attribute is not set.

* annotation : object
    The annotation for the parameter if specified.  If the
    parameter has no annotation, this attribute is not set.

* kind : str
    Describes how argument values are bound to the parameter.
    Possible values:

       * ``Parameter.POSITIONAL_ONLY`` - value must be supplied
         as a positional argument.

         Python has no explicit syntax for defining positional-only
         parameters, but many builtin and extension module functions
         (especially those that accept only one or two parameters)
         accept them.

       * ``Parameter.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.)

       * ``Parameter.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.

       * ``Parameter.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.

       * ``Parameter.VAR_KEYWORD`` - a dict of keyword arguments
         that aren't bound to any other parameter. This corresponds
         to a "\*\*kwds" parameter in a Python function definition.

* implemented : bool
    True if the parameter is implemented for use.  Some platforms
    implement functions but can't support specific parameters
    (e.g. "mode" for ``os.mkdir``).  Passing in an unimplemented
    parameter may result in the parameter being ignored,
    or in NotImplementedError being raised.  It is intended that
    all conditions where ``implemented`` may be False be
    thoroughly documented.

Two parameters are equal when all their attributes are equal.


BoundArguments Object
=====================

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

Has the following public attributes:

* arguments : OrderedDict
    An ordered, mutable mapping of parameters' names to arguments' values.
    Does not contain arguments' default values.
* args : tuple
    Tuple of positional arguments values.  Dynamically computed from
    the 'arguments' attribute.
* kwargs : dict
    Dict of keyword arguments values. Dynamically computed from
    the 'arguments' attribute.

The ``arguments`` attribute should be used in conjunction with
``Signature.parameters`` for any arguments processing purposes.

``args`` and ``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)


Implementation
==============

The implementation adds a new function ``signature()`` to the ``inspect``
module.  The function is the preferred way of getting a ``Signature`` for
a callable object.

The function implements the following algorithm:

    - If the object is not callable - raise a TypeError

    - If the object has a ``__signature__`` attribute and if it
      is not ``None`` - return a deepcopy of it

        - If it is ``None`` and the object is an instance of
          ``BuiltinFunction``, raise a ``ValueError``

    - If it has a ``__wrapped__`` attribute, return
      ``signature(object.__wrapped__)``

    - If the object is a an instance of ``FunctionType`` construct
      and return a new ``Signature`` for it

    - If the object is a method or a classmethod, construct and return
      a new ``Signature`` object, with its first parameter (usually
      ``self`` or ``cls``) removed

    - If the object is a staticmethod, construct and return
      a new ``Signature`` object

    - If the object is an instance of ``functools.partial``, construct
      a new ``Signature`` from its ``partial.func`` attribute, and
      account for already bound ``partial.args`` and ``partial.kwargs``

    - If the object is a class or metaclass:

        - If the object's type has a ``__call__`` method defined in
          its MRO, return a Signature for it

        - If the object has a ``__new__`` method defined in its class,
          return a Signature object for it

        - If the object has a ``__init__`` method defined in its class,
          return a Signature object for it

    - Return ``signature(object.__call__)``

Note, that the ``Signature`` object is created in a lazy manner, and
is not automatically cached.  If, however, the Signature object was
explicitly cached by the user, ``signature()`` returns a new deepcopy
of it on each invocation.

An implementation for Python 3.3 can be found at [#impl]_.
The python issue tracking the patch is [#issue]_.


Design Considerations
=====================

No implicit caching of Signature objects
----------------------------------------

The first PEP design had a provision for implicit caching of ``Signature``
objects in the ``inspect.signature()`` function.  However, this has the
following downsides:

  * If the ``Signature`` object is cached then any changes to the function
    it describes will not be reflected in it.  However, If the caching is
    needed, it can be always done manually and explicitly

  * It is better to reserve the ``__signature__`` attribute for the cases
    when there is a need to explicitly set to a ``Signature`` object that
    is different from the actual one


Examples
========

Visualizing Callable Objects' Signature
---------------------------------------

Let's define some classes and functions:

::

    from inspect import signature
    from functools import partial, wraps


    class FooMeta(type):
        def __new__(mcls, name, bases, dct, *, bar:bool=False):
            return super().__new__(mcls, name, bases, dct)

        def __init__(cls, name, bases, dct, **kwargs):
            return super().__init__(name, bases, dct)


    class Foo(metaclass=FooMeta):
        def __init__(self, spam:int=42):
            self.spam = spam

        def __call__(self, a, b, *, c) -> tuple:
            return a, b, c


    def shared_vars(*shared_args):
        """Decorator factory that defines shared variables that are
           passed to every invocation of the function"""

        def decorator(f):
            @wraps(f)
            def wrapper(*args, **kwds):
                full_args = shared_args + args
                return f(*full_args, **kwds)
            # Override signature
            sig = wrapper.__signature__ = signature(f)
            for __ in shared_args:
                sig.parameters.popitem(last=False)
            return wrapper
        return decorator


    @shared_vars({})
    def example(_state, a, b, c):
        return _state, a, b, c


    def format_signature(obj):
        return str(signature(obj))


Now, in the python REPL:

::

    >>> format_signature(FooMeta)
    '(name, bases, dct, *, bar:bool=False)'

    >>> format_signature(Foo)
    '(spam:int=42)'

    >>> format_signature(Foo.__call__)
    '(self, a, b, *, c) -> tuple'

    >>> format_signature(Foo().__call__)
    '(a, b, *, c) -> tuple'

    >>> format_signature(partial(Foo().__call__, 1, c=3))
    '(b, *, c=3) -> tuple'

    >>> format_signature(partial(partial(Foo().__call__, 1, c=3), 2, c=20))
    '(*, c=20) -> tuple'

    >>> format_signature(example)
    '(a, b, c)'

    >>> format_signature(partial(example, 1, 2))
    '(c)'

    >>> format_signature(partial(partial(example, 1, b=2), c=3))
    '(b=2, c=3)'


Annotation Checker
------------------
::

    import inspect
    import functools

    def checktypes(func):
        '''Decorator to verify arguments and return types

        Example:

            >>> @checktypes
            ... def test(a:int, b:str) -> int:
            ...     return int(a * b)

            >>> test(10, '1')
            1111111111

            >>> test(10, 1)
            Traceback (most recent call last):
              ...
            ValueError: foo: wrong type of 'b' argument, 'str' expected, got 'int'
        '''

        sig = inspect.signature(func)

        types = {}
        for param in sig.parameters.values():
            # Iterate through function's parameters and build the list of
            # arguments types
            try:
                type_ = param.annotation
            except AttributeError:
                continue
            else:
                if not inspect.isclass(type_):
                    # Not a type, skip it
                    continue

                types[param.name] = type_

                # If the argument has a type specified, let's check that its
                # default value (if present) conforms with the type.
                try:
                    default = param.default
                except AttributeError:
                    continue
                else:
                    if not isinstance(default, type_):
                        raise ValueError("{func}: wrong type of a default value for {arg!r}". \
                                         format(func=func.__qualname__, arg=param.name))

        def check_type(sig, arg_name, arg_type, arg_value):
            # Internal function that encapsulates arguments type checking
            if not isinstance(arg_value, arg_type):
                raise ValueError("{func}: wrong type of {arg!r} argument, " \
                                 "{exp!r} expected, got {got!r}". \
                                 format(func=func.__qualname__, arg=arg_name,
                                        exp=arg_type.__name__, got=type(arg_value).__name__))

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            # Let's bind the arguments
            ba = sig.bind(*args, **kwargs)
            for arg_name, arg in ba.arguments.items():
                # And iterate through the bound arguments
                try:
                    type_ = types[arg_name]
                except KeyError:
                    continue
                else:
                    # OK, we have a type for the argument, lets get the corresponding
                    # parameter description from the signature object
                    param = sig.parameters[arg_name]
                    if param.kind == param.VAR_POSITIONAL:
                        # If this parameter is a variable-argument parameter,
                        # then we need to check each of its values
                        for value in arg:
                            check_type(sig, arg_name, type_, value)
                    elif param.kind == param.VAR_KEYWORD:
                        # If this parameter is a variable-keyword-argument parameter:
                        for subname, value in arg.items():
                            check_type(sig, arg_name + ':' + subname, type_, value)
                    else:
                        # And, finally, if this parameter a regular one:
                        check_type(sig, arg_name, type_, arg)

            result = func(*ba.args, **ba.kwargs)
            # The last bit - let's check that the result is correct
            try:
                return_type = sig.return_annotation
            except AttributeError:
                # Looks like we don't have any restriction on the return type
                pass
            else:
                if isinstance(return_type, type) and not isinstance(result, return_type):
                    raise ValueError('{func}: wrong return type, {exp} expected, got {got}'. \
                                     format(func=func.__qualname__, exp=return_type.__name__,
                                            got=type(result).__name__))
            return result

        return wrapper


Render Function Signature to HTML
---------------------------------

::

    import inspect

    def format_to_html(func):
        sig = inspect.signature(func)

        html = sig.format(token_params_separator='<span class="t-comma">,</span>',
                          token_colon='<span class="t-colon">:</span>',
                          token_eq='<span class="t-eq">=</span>',
                          token_return_annotation='<span class="t-ra">-&gt;</span>',
                          token_left_paren='<span class="t-lp">(</span>',
                          token_right_paren='<span class="t-lp">)</span>',
                          token_kwonly_separator='<span class="t-ast">*</span>',
                          format_name=lambda name: '<span class="name">'+name+'</span>')

        return '<span class="py-func">{}</span>'.format(html)


References
==========

.. [#impl] pep362 branch (https://bitbucket.org/1st1/cpython/overview)
.. [#issue] issue 15008 (http://bugs.python.org/issue15008)


Copyright
=========

This document has been placed in the public domain.


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