funcsigs is a backport of the PEP 362 function signature features from
Python 3.3's inspect module. The backport is compatible with Python 2.6, 2.7
as well as 3.3 and up. 3.2 was supported by version 0.4, but with setuptools and
pip no longer supporting 3.2, we cannot make any statement about 3.2
compatibility.
The funcsigs backport has been tested against:
- CPython 2.6
- CPython 2.7
- CPython 3.3
- CPython 3.4
- CPython 3.5
- CPython nightlies
- PyPy and PyPy3(currently failing CI)
Continuous integration testing is provided by Travis CI.
Under Python 2.x there is a compatibility issue when a function is assigned to
the __wrapped__ property of a class after it has been constructed.
Similiarily there under PyPy directly passing the __call__ method of a
builtin is also a compatibility issues. Otherwise the functionality is
believed to be uniform between both Python2 and Python3.
Source code for funcsigs is hosted on GitHub. Any bug reports or feature
requests can be made using GitHub's issues system. 
To obtain a Signature object, pass the target function to the
funcsigs.signature function.
>>> from funcsigs import signature
>>> def foo(a, b=None, *args, **kwargs):
... pass
...
>>> sig = signature(foo)
>>> sig
<funcsigs.Signature object at 0x...>
>>> sig.parameters
OrderedDict([('a', <Parameter at 0x... 'a'>), ('b', <Parameter at 0x... 'b'>), ('args', <Parameter at 0x... 'args'>), ('kwargs', <Parameter at 0x... 'kwargs'>)])
>>> sig.return_annotation
<class 'funcsigs._empty'>Note
This section of documentation is a direct reproduction of the Python standard library documentation for the inspect module.
The Signature object represents the call signature of a callable object and its return annotation. To retrieve a Signature object, use the :func:`signature` function.
.. function:: signature(callable)
Return a :class:`Signature` object for the given ``callable``::
>>> from funcsigs import signature
>>> def foo(a, *, b:int, **kwargs):
... pass
>>> sig = signature(foo)
>>> str(sig)
'(a, *, b:int, **kwargs)'
>>> str(sig.parameters['b'])
'b:int'
>>> sig.parameters['b'].annotation
<class 'int'>
Accepts a wide range of python callables, from plain functions and classes to
:func:`functools.partial` objects.
.. note::
Some callables 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.
A Signature object represents the call signature of a function and its return annotation. For each parameter accepted by the function it stores a :class:`Parameter` object in its :attr:`parameters` collection.
Signature objects are immutable. Use :meth:`Signature.replace` to make a modified copy.
.. attribute:: Signature.empty A special class-level marker to specify absence of a return annotation.
.. attribute:: Signature.parameters An ordered mapping of parameters' names to the corresponding :class:`Parameter` objects.
.. attribute:: Signature.return_annotation The "return" annotation for the callable. If the callable has no "return" annotation, this attribute is set to :attr:`Signature.empty`.
.. method:: Signature.bind(*args, **kwargs) Create a mapping from positional and keyword arguments to parameters. Returns :class:`BoundArguments` if ``*args`` and ``**kwargs`` match the signature, or raises a :exc:`TypeError`.
.. method:: Signature.bind_partial(*args, **kwargs) Works the same way as :meth:`Signature.bind`, but allows the omission of some required arguments (mimics :func:`functools.partial` behavior.) Returns :class:`BoundArguments`, or raises a :exc:`TypeError` if the passed arguments do not match the signature.
.. method:: Signature.replace(*[, parameters][, return_annotation])
Create 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
:attr:`Signature.empty`.
::
>>> def test(a, b):
... pass
>>> sig = signature(test)
>>> new_sig = sig.replace(return_annotation="new return anno")
>>> str(new_sig)
"(a, b) -> 'new return anno'"
Parameter objects are immutable. Instead of modifying a Parameter object, you can use :meth:`Parameter.replace` to create a modified copy.
.. attribute:: Parameter.empty A special class-level marker to specify absence of default values and annotations.
.. attribute:: Parameter.name 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``).
.. attribute:: Parameter.default The default value for the parameter. If the parameter has no default value, this attribute is set to :attr:`Parameter.empty`.
.. attribute:: Parameter.annotation The annotation for the parameter. If the parameter has no annotation, this attribute is set to :attr:`Parameter.empty`.
.. attribute:: Parameter.kind
Describes how argument values are bound to the parameter. Possible values
(accessible via :class:`Parameter`, like ``Parameter.KEYWORD_ONLY``):
+------------------------+----------------------------------------------+
| Name | Meaning |
+========================+==============================================+
| *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. |
+------------------------+----------------------------------------------+
| *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.) |
+------------------------+----------------------------------------------+
| *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. |
+------------------------+----------------------------------------------+
| *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. |
+------------------------+----------------------------------------------+
| *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. |
+------------------------+----------------------------------------------+
Example: print all keyword-only arguments without default values::
>>> def foo(a, b, *, c, d=10):
... pass
>>> sig = signature(foo)
>>> for param in sig.parameters.values():
... if (param.kind == param.KEYWORD_ONLY and
... param.default is param.empty):
... print('Parameter:', param)
Parameter: c
.. method:: Parameter.replace(*[, name][, kind][, default][, annotation])
Create a new Parameter instance based on the instance replaced was invoked
on. To override a :class:`Parameter` attribute, pass the corresponding
argument. To remove a default value or/and an annotation from a
Parameter, pass :attr:`Parameter.empty`.
::
>>> from funcsigs import Parameter
>>> param = Parameter('foo', Parameter.KEYWORD_ONLY, default=42)
>>> str(param)
'foo=42'
>>> str(param.replace()) # Will create a shallow copy of 'param'
'foo=42'
>>> str(param.replace(default=Parameter.empty, annotation='spam'))
"foo:'spam'"
Result of a :meth:`Signature.bind` or :meth:`Signature.bind_partial` call. Holds the mapping of arguments to the function's parameters.
.. attribute:: BoundArguments.arguments
An ordered, mutable mapping (:class:`collections.OrderedDict`) of
parameters' names to arguments' values. Contains only explicitly bound
arguments. Changes in :attr:`arguments` will reflect in :attr:`args` and
:attr:`kwargs`.
Should be used in conjunction with :attr:`Signature.parameters` for any
argument processing purposes.
.. note::
Arguments for which :meth:`Signature.bind` or
:meth:`Signature.bind_partial` relied on a default value are skipped.
However, if needed, it is easy to include them.
::
>>> def foo(a, b=10):
... pass
>>> sig = signature(foo)
>>> ba = sig.bind(5)
>>> ba.args, ba.kwargs
((5,), {})
>>> for param in sig.parameters.values():
... if param.name not in ba.arguments:
... ba.arguments[param.name] = param.default
>>> ba.args, ba.kwargs
((5, 10), {})
.. attribute:: BoundArguments.args A tuple of positional arguments values. Dynamically computed from the :attr:`arguments` attribute.
.. attribute:: BoundArguments.kwargs A dict of keyword arguments values. Dynamically computed from the :attr:`arguments` attribute.
The :attr:`args` and :attr:`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)
.. seealso::
:pep:`362` - Function Signature Object.
The detailed specification, implementation details and examples.
funcsigs is a derived work of CPython under the terms of the PSF License Agreement. The original CPython inspect module, its unit tests and documentation are the copyright of the Python Software Foundation. The derived work is distributed under the Apache License Version 2.0.