[Python-Dev] PEP 362 Third Revision
Yury Selivanov
yselivanov.ml at gmail.com
Thu Jun 14 04:52:43 CEST 2012
Hello,
The new revision of PEP 362 has been posted:
http://www.python.org/dev/peps/pep-0362/
Summary:
1. Signature object now represents the call signature of a
function. That said, it doesn't have 'name' and 'qualname'
attributes anymore, and can be tested for equality against
other signatures.
2. signature() function support all kinds of callables:
classes, metaclasses, methods, class- & staticmethods,
'functools.partials', and callable objects. If a callable
object has a '__signature__' attribute it does a deepcopy
of it before return.
3. No implicit caching to __signature__.
4. Added 'Signature.bind_partial' and 'Signature.format'
methods.
A patch with the PEP implementation is attached to the
issue 15008. It should be ready for code review.
Thank you!
PEP: 362
Title: Function Signature Object
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett at python.org>, Jiwon Seo <seojiwon at gmail.com>,
Yury Selivanov <yselivanov at sprymix.com>, Larry Hastings <larry at 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 ``BindError`` (subclass of ``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.)
* 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.
* is_keyword_only : bool
True if the parameter is keyword-only, else False.
* is_args : bool
True if the parameter accepts variable number of arguments
(``*args``-like), else False.
* is_kwargs : bool
True if the parameter accepts variable number of keyword
arguments (``**kwargs``-like), else False.
* is_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 ``is_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 the object is a an instance of ``FunctionType``:
- If it has a ``__wrapped__`` attribute, return
``signature(object.__wrapped__)``
- Or else construct a new ``Signature`` object and return 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
---------------------------------------
::
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
print('FooMeta >', str(signature(FooMeta)))
print('Foo >', str(signature(Foo)))
print('Foo.__call__ >', str(signature(Foo.__call__)))
print('Foo().__call__ >', str(signature(Foo().__call__)))
print('partial(Foo().__call__, 1, c=3) >',
str(signature(partial(Foo().__call__, 1, c=3))))
print('partial(partial(Foo().__call__, 1, c=3), 2, c=20) >',
str(signature(partial(partial(Foo().__call__, 1, c=3), 2, c=20))))
The script will output:
::
FooMeta > (name, bases, dct, *, bar:bool=False)
Foo > (spam:int=42)
Foo.__call__ > (self, a, b, *, c) -> tuple
Foo().__call__ > (a, b, *, c) -> tuple
partial(Foo().__call__, 1, c=3) > (b, *, c=3) -> tuple
partial(partial(Foo().__call__, 1, c=3), 2, c=20) > (*, c=20) -> tuple
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=sig.qualname, arg=param.name))
def check_type(sig, arg_name, arg_type, arg_value):
# Internal function that incapsulates 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=sig.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.is_args:
# 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.is_kwargs:
# 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=sig.qualname, exp=return_type.__name__,
got=type(result).__name__))
return result
return wrapper
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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