Question

I have a dict, which I need to pass key/values as keyword arguments.. For example..

d_args = {'kw1': 'value1', 'kw2': 'value2'} example(**d_args) 

This works fine, but if there are values in the d_args dict that are not accepted by the example function, it obviously dies.. Say, if the example function is defined as def example(kw2):

This is a problem since I don’t control either the generation of the d_args, or the example function.. They both come from external modules, and example only accepts some of the keyword-arguments from the dict..

Ideally I would just do

parsed_kwargs = feedparser.parse(the_url) valid_kwargs = get_valid_kwargs(parsed_kwargs, valid_for = PyRSS2Gen.RSS2) PyRSS2Gen.RSS2(**valid_kwargs) 

I will probably just filter the dict, from a list of valid keyword-arguments, but I was wondering: Is there a way to programatically list the keyword arguments the a specific function takes?

Answer 1

A little nicer than inspecting the code object directly and working out the variables is to use the inspect module.

>>> import inspect >>> def func(a,b,c=42, *args, **kwargs): pass >>> inspect.getargspec(func) (['a', 'b', 'c'], 'args', 'kwargs', (42,)) 

If you want to know if its callable with a particular set of args, you need the args without a default already specified. These can be got by:

def get_required_args(func):     args, varargs, varkw, defaults = inspect.getargspec(func)     if defaults:         args = args[:-len(defaults)]     return args   # *args and **kwargs are not required, so ignore them. 

Then a function to tell what you are missing from your particular dict is:

def missing_args(func, argdict):     return set(get_required_args(func)).difference(argdict) 

Similarly, to check for invalid args, use:

def invalid_args(func, argdict):     args, varargs, varkw, defaults = inspect.getargspec(func)     if varkw: return set()  # All accepted     return set(argdict) - set(args) 

And so a full test if it is callable is :

def is_callable_with_args(func, argdict):     return not missing_args(func, argdict) and not invalid_args(func, argdict) 

(This is good only as far as python’s arg parsing. Any runtime checks for invalid values in kwargs obviously can’t be detected.)