PEP: 362 Title: Function Signature Object Version: $Revision$
Last-Modified: $Date$ Author: Brett Cannon <brett@python.org>, Jiwon Seo
<seojiwon@gmail.com>, Yury Selivanov <yury@edgedb.com>, Larry Hastings
<larry@hastings.org> Status: Final Type: Standards Track Content-Type:
text/x-rst Created: 21-Aug-2006 Python-Version: 3.3 Post-History:
04-Jun-2012 Resolution:
https://mail.python.org/pipermail/python-dev/2012-June/120682.html

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 "return" annotation for the function. If the function has no
        "return" annotation, this attribute is set to Signature.empty.

-   

    parameters : OrderedDict

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

-   

    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.

-   

    replace(parameters=<optional>, *, return_annotation=<optional>) -> Signature

        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 Signature.empty.

        Note that the '=<optional>' notation, means that the argument is
        optional. This notation applies to the rest of this PEP.

Signature objects are immutable. Use Signature.replace() to make a
modified copy: :

    >>> def foo() -> None:
    ...     pass
    >>> sig = signature(foo)

    >>> new_sig = sig.replace(return_annotation="new return annotation")
    >>> new_sig is not sig
    True
    >>> new_sig.return_annotation != sig.return_annotation
    True
    >>> new_sig.parameters == sig.parameters
    True

    >>> new_sig = new_sig.replace(return_annotation=new_sig.empty)
    >>> new_sig.return_annotation is Signature.empty
    True

There are two ways to instantiate a Signature class:

-   

    Signature(parameters=<optional>, *, return_annotation=Signature.empty)

        Default Signature constructor. Accepts an optional sequence of
        Parameter objects, and an optional return_annotation. Parameters
        sequence is validated to check that there are no parameters with
        duplicate names, and that the parameters are in the right order,
        i.e. positional-only first, then positional-or-keyword, etc.

-   

    Signature.from_function(function)

        Returns a Signature object reflecting the signature of the
        function passed in.

It's possible to test Signatures for equality. Two signatures are equal
when their parameters are equal, their positional and positional-only
parameters appear in the same order, and they have equal return
annotations.

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

Signature also implements __str__: :

    >>> str(Signature.from_function((lambda *args: None)))
    '(*args)'

    >>> str(Signature())
    '()'

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.

A Parameter object has the following public attributes and methods:

-   

    name : str

        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.)

-   

    default : object

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

-   

    annotation : object

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

-   

    kind

        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 built-in 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
            "**kwargs" parameter in a Python function definition.

        Always use Parameter.* constants for setting and checking value
        of the kind attribute.

-   

    replace(*, name=<optional>, kind=<optional>, default=<optional>, annotation=<optional>) -> Parameter

        Creates a new Parameter instance based on the instance replaced
        was invoked on. To override a Parameter attribute, pass the
        corresponding argument. To remove an attribute from a Parameter,
        pass Parameter.empty.

Parameter constructor:

-   

    Parameter(name, kind, *, annotation=Parameter.empty, default=Parameter.empty)

        Instantiates a Parameter object. name and kind are required,
        while annotation and default are optional.

Two parameters are equal when they have equal names, kinds, defaults,
and annotations.

Parameter objects are immutable. Instead of modifying a Parameter
object, you can use Parameter.replace() to create a modified copy like
so: :

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

    >>> str(param.replace())
    'foo=42'

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

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. Contains only explicitly bound arguments. Arguments for
        which bind() relied on a default value are skipped.

-   

    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)

Arguments which could be passed as part of either *args or **kwargs will
be included only in the BoundArguments.args attribute. Consider the
following example: :

    def test(a=1, b=2, c=3):
        pass

    sig = signature(test)
    ba = sig.bind(a=10, c=13)

    >>> ba.args
    (10,)

    >>> ba.kwargs:
    {'c': 13}

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 it
-   If it has a __wrapped__ attribute, return
    signature(object.__wrapped__)
-   If the object is an instance of FunctionType, construct and return a
    new Signature for it
-   If the object is a bound method, construct and return a new
    Signature object, with its first parameter (usually self or cls)
    removed. (classmethod and staticmethod are supported too. Since both
    are descriptors, the former returns a bound method, and the latter
    returns its wrapped function.)
-   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 MRO, return a
        Signature object for it
    -   If the object has a __init__ method defined in its MRO, 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. However, the user can manually cache a Signature
by storing it in the __signature__ attribute.

An implementation for Python 3.3 can be found at[1]. The python issue
tracking the patch is[2].

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

Some functions may not be introspectable

Some functions 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. Adding support for them is out of
scope for this PEP.

Signature and Parameter equivalence

We assume that parameter names have semantic significance--two
signatures are equal only when their corresponding parameters are equal
and have the exact same names. Users who want looser equivalence tests,
perhaps ignoring names of VAR_KEYWORD or VAR_POSITIONAL parameters, will
need to implement those themselves.

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

        @classmethod
        def spam(cls, a):
            return a


    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, **kwargs):
                full_args = shared_args + args
                return f(*full_args, **kwargs)

            # Override signature
            sig = signature(f)
            sig = sig.replace(tuple(sig.parameters.values())[1:])
            wrapper.__signature__ = sig

            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(Foo.spam)
    '(a)'

    >>> 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
            type_ = param.annotation
            if type_ is param.empty or not inspect.isclass(type_):
                # Missing annotation or 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.
            if param.default is not param.empty and not isinstance(param.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
            return_type = sig.return_annotation
            if (return_type is not sig._empty and
                    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

Acceptance

PEP 362 was accepted by Guido, Friday, June 22, 2012[3] . The reference
implementation was committed to trunk later that day.

References

Copyright

This document has been placed in the public domain.



  Local Variables: mode: indented-text indent-tabs-mode: nil
  sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

[1] pep362 branch (https://bitbucket.org/1st1/cpython/overview)

[2] issue 15008 (http://bugs.python.org/issue15008)

[3] "A Desperate Plea For Introspection (aka: BDFAP Needed)"
(https://mail.python.org/pipermail/python-dev/2012-June/120682.html)