[Python-Dev] PEP 362: 4th edition

Fri Jun 15 21:50:25 CEST 2012

Hello,
The new revision of PEP 362 has been posted:
http://www.python.org/dev/peps/pep-0362/
Summary:
1. Signature's 'is_args', 'is_kwargs', 'is_keyword_only' were all 
replaced with a single 'kind' attribute (Nick, I borrowed your
description of 'kind' attribute and its possible values.)
2. 'signature()' checks for the '__wrapped__' attribute on all
callables.
3. 'POSITIONAL_ONLY' parameters should be fully supported by 
'bind()' and other Signature class methods.
Open questions:
1. Should we keep 'Parameter.implemented' or not. *Please vote*
2. Should 'Signature.format()' instead of 7 or so arguments accept
just a SignatureFormatter class, that implements all the formatting
logic, that is easy to override?
All in all, I think that the PEP is almost ready for the acceptance.
As 3.3 beta 1 is just a week away, I want to ask for BDFAP (Nick?)
Please also do the patch review: http://bugs.python.org/issue15008
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 ``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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