How can I read the function signature, including default argument values?

Question

For a function object, how can I get its signature? For example, for:

def myMethod(firt, second, third='something'): pass 

I would like to get "myMethod(firt, second, third='something')" . "myMethod(firt, second, third='something')"

Answer 1

 import inspect def foo(a, b, x='blah'): pass print(inspect.getargspec(foo)) # ArgSpec(args=['a', 'b', 'x'], varargs=None, keywords=None, defaults=('blah',)) 

However, note that inspect.getargspec() deprecated since Python 3.0.

Python 3.0--3.4 recommends inspect.getfullargspec() .

Python 3.5+ recommends inspect.signature() .

Answer 2

Perhaps the easiest way to find a signature for a function is 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) 

In addition, in Python 3, a method was added to the inspect module, called signature , that is designed to represent the signature of the called object and its reverse 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'> 

Answer 3

 #! /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 ie 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") 

Answer 4

Try calling help on an object to find out.

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

Answer 5

It may be a bit late for the party, but if you also want to keep the order of the arguments and their default values, you can use the Abstract Syntax Tree module (ast) .

Here's a proof of concept (beware of code for sorting arguments and comparing them with their default values, of course, you can improve / make it 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 

Answer 6

If all you are trying to do is print the function, use pydoc.

 import pydoc def foo(arg1, arg2, *args, **kwargs): '''Some foo fn''' pass >>> print pydoc.render_doc(foo).splitlines()[2] foo(arg1, arg2, *args, **kwargs) 

If you are trying to actually parse a function signature, use the argspec verification module. I had to do this while checking the hook function of the script user in the general structure.