PEP 362 – Function Signature Object | peps.python.org (original) (raw)

Author:

Brett Cannon , Jiwon Seo , Yury Selivanov , Larry Hastings

Status:

Final

Type:

Standards Track

Created:

21-Aug-2006

Python-Version:

3.3

Post-History:

04-Jun-2012

Resolution:

Python-Dev message

Table of Contents

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:

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:

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:

Parameter constructor:

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:

The arguments attribute should be used in conjunction withSignature.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 **kwargswill 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 inspectmodule. The function is the preferred way of getting a Signature for a callable object.

The function implements the following algorithm:

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 Signatureobjects in the inspect.signature() function. However, this has the following downsides:

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

This document has been placed in the public domain.