How to get method parameter names?

Given that a function a_method has been defined like

def a_method(arg1, arg2):
    pass

Starting from a_method itself, how can I get the argument names – for example, as a tuple of strings, like ("arg1", "arg2")?

Answer

Take a look at the inspect module – this will do the inspection of the various code object properties for you.

>>> inspect.getfullargspec(a_method)
(['arg1', 'arg2'], None, None, None)

The other results are the name of the *args and **kwargs variables, and the defaults provided. ie.

>>> def foo(a, b, c=4, *arglist, **keywords): pass
>>> inspect.getfullargspec(foo)
(['a', 'b', 'c'], 'arglist', 'keywords', (4,))

Note that some callables may not be introspectable in certain implementations of Python. For Example, in CPython, some built-in functions defined in C provide no metadata about their arguments. As a result, you will get a ValueError if you use inspect.getfullargspec() on a built-in function.

Since Python 3.3, you can use inspect.signature() to see the call signature of a callable object:

>>> inspect.signature(foo)
<Signature (a, b, c=4, *arglist, **keywords)>