Question

How can I find the number of arguments of a Python function? I need to know how many normal arguments it has and how many named arguments.

Example:

def someMethod(self, arg1, kwarg1=None):
    pass

This method has 2 arguments and 1 named argument.

Answer 1

The previously accepted answer has been deprecated as of Python 3.0. Instead of using inspect.getargspec you should now opt for the Signature class which superseded it.

Creating a Signature for the function is easy via the signature function:

from inspect import signature

def someMethod(self, arg1, kwarg1=None):
    pass

sig = signature(someMethod)

Now, you can either view its parameters quickly by string it:

str(sig)  # returns: '(self, arg1, kwarg1=None)'

or you can also get a mapping of attribute names to parameter objects via sig.parameters.

params = sig.parameters 
print(params['kwarg1']) # prints: kwarg1=20

Additionally, you can call len on sig.parameters to also see the number of arguments this function requires:

print(len(params))  # 3

Each entry in the params mapping is actually a Parameter object that has further attributes making your life easier. For example, grabbing a parameter and viewing its default value is now easily performed with:

kwarg1 = params['kwarg1']
kwarg1.default # returns: None

similarly for the rest of the objects contained in parameters.

---

As for Python 2.x users, while inspect.getargspec isn't deprecated, the language will soon be :-). The Signature class isn't available in the 2.x series and won't be. So you still need to work with inspect.getargspec.

As for transitioning between Python 2 and 3, if you have code that relies on the interface of getargspec in Python 2 and switching to signature in 3 is too difficult, you do have the valuable option of using inspect.getfullargspec. It offers a similar interface to getargspec (a single callable argument) in order to grab the arguments of a function while also handling some additional cases that getargspec doesn't:

from inspect import getfullargspec

def someMethod(self, arg1, kwarg1=None):
    pass

args = getfullargspec(someMethod)

As with getargspec, getfullargspec returns a NamedTuple which contains the arguments.

print(args)
FullArgSpec(args=['self', 'arg1', 'kwarg1'], varargs=None, varkw=None, defaults=(None,), kwonlyargs=[], kwonlydefaults=None, annotations={})