How can I read a function's signature including default argument values?

Question

import inspect

def foo(a, b, x='blah'):
    pass

print(inspect.signature(foo))
# ArgSpec(args=['a', 'b', 'x'], varargs=None, keywords=None, defaults=('blah',))

Python 3.5+ recommends inspect.signature().

Arguably the easiest way to find the signature for a function would be help(function):

>>> def function(arg1, arg2="foo", *args, **kwargs): pass
>>> help(function)
Help on function function in module __main__:

function(arg1, arg2='foo', *args, **kwargs)

Also, in Python 3 a method was added to the inspect module called signature, which is designed to represent the signature of a callable object and its return annotation:

>>> from inspect 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'>

#! /usr/bin/env python

import inspect
from collections import namedtuple

DefaultArgSpec = namedtuple('DefaultArgSpec', 'has_default default_value')

def _get_default_arg(args, defaults, arg_index):
    """ Method that determines if an argument has default value or not,
    and if yes what is the default value for the argument

    :param args: array of arguments, eg: ['first_arg', 'second_arg', 'third_arg']
    :param defaults: array of default values, eg: (42, 'something')
    :param arg_index: index of the argument in the argument array for which,
    this function checks if a default value exists or not. And if default value
    exists it would return the default value. Example argument: 1
    :return: Tuple of whether there is a default or not, and if yes the default
    value, eg: for index 2 i.e. for "second_arg" this function returns (True, 42)
    """
    if not defaults:
        return DefaultArgSpec(False, None)

    args_with_no_defaults = len(args) - len(defaults)

    if arg_index < args_with_no_defaults:
        return DefaultArgSpec(False, None)
    else:
        value = defaults[arg_index - args_with_no_defaults]
        if (type(value) is str):
            value = '"%s"' % value
        return DefaultArgSpec(True, value)

def get_method_sig(method):
    """ Given a function, it returns a string that pretty much looks how the
    function signature would be written in python.

    :param method: a python method
    :return: A string similar describing the pythong method signature.
    eg: "my_method(first_argArg, second_arg=42, third_arg='something')"
    """

    # The return value of ArgSpec is a bit weird, as the list of arguments and
    # list of defaults are returned in separate array.
    # eg: ArgSpec(args=['first_arg', 'second_arg', 'third_arg'],
    # varargs=None, keywords=None, defaults=(42, 'something'))
    argspec = inspect.getargspec(method)
    arg_index=0
    args = []

    # Use the args and defaults array returned by argspec and find out
    # which arguments has default
    for arg in argspec.args:
        default_arg = _get_default_arg(argspec.args, argspec.defaults, arg_index)
        if default_arg.has_default:
            args.append("%s=%s" % (arg, default_arg.default_value))
        else:
            args.append(arg)
        arg_index += 1
    return "%s(%s)" % (method.__name__, ", ".join(args))


if __name__ == '__main__':
    def my_method(first_arg, second_arg=42, third_arg='something'):
        pass

    print get_method_sig(my_method)
    # my_method(first_argArg, second_arg=42, third_arg="something")

Try calling help on an object to find out about it.

>>> foo = [1, 2, 3]
>>> help(foo.append)
Help on built-in function append:

append(...)
    L.append(object) -- append object to end

Maybe a bit late to the party, but if you also want to keep the order of the arguments and their defaults, then you can use the Abstract Syntax Tree module (ast).

Here's a proof of concept (beware the code to sort the arguments and match them to their defaults can definitely be improved/made more clear):

import ast

for class_ in [c for c in module.body if isinstance(c, ast.ClassDef)]:
    for method in [m for m in class_.body if isinstance(m, ast.FunctionDef)]:
        args = []
        if method.args.args:
            [args.append([a.col_offset, a.id]) for a in method.args.args]
        if method.args.defaults:
            [args.append([a.col_offset, '=' + a.id]) for a in method.args.defaults]
        sorted_args = sorted(args)
        for i, p in enumerate(sorted_args):
            if p[1].startswith('='):
                sorted_args[i-1][1] += p[1]
        sorted_args = [k[1] for k in sorted_args if not k[1].startswith('=')]

        if method.args.vararg:
            sorted_args.append('*' + method.args.vararg)
        if method.args.kwarg:
            sorted_args.append('**' + method.args.kwarg)

        signature = '(' + ', '.join(sorted_args) + ')'

        print method.name + signature