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 <br...@python.org>, Jiwon Seo <seoji...@gmail.com>, Yury Selivanov <yseliva...@sprymix.com>, Larry Hastings <la...@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">-></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. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
