check for valid arguments

Question

So let's define a few functions:

def x(a, b, c): pass
def y(a, b=1, c=2): pass
def z(a=1, b=2, c=3): pass

Now, what's the best way to, given a pointer to x, y, or z (p), a tuple of args (a) and a dictionary of kwargs (k), check to see if

p(*a, **kw)

would produce any exception regarding having not enough arguments or incorrect arguments, etc—without actually calling p(*a, **kw) and then catching the exceptions raised.

Example

def valid(ptr, args, kwargs): ... #implementation here

valid(x, ("hello", "goodbye", "what?"), {}) # => True
valid(x, ("hello", "goodbye"), {}) # => False
valid(y, ("hello", "goodbye", "what?"), {}) # => True
valid(y, (), {"a": "hello", "b": "goodbye", "c": "what"}) #=> True
valid(y, ("hello", "goodbye"), {"c": "what?"}) #=> True

Answer 1

You can use the inspect module's getargspec function to determine the functions of the arguments and their defaults, if any, as well as if the function accepts varargs or keywordargs. This should be enough information to determine if a given tuple will satisfy the argument requirements.

Here's a start (have to run, but I figured posting a start was better than nothing).

def valid(f, *args, **kwargs):
    specs = getargspec(f)
    required = specs.args[:-len(specs.defaults)] if (specs.defaults != None) else specs.args 
    #Now just check that your required arguments list is fulfilled by args and kwargs
    #And check that no args are left over, unless the argspec has varargs or kwargs defined.