Why does Python allow function calls with wrong number of arguments?

Question

Python is my first dynamic language. I recently coded a function call incorrectly supplying a wrong number of arguments. This failed with an exception at the time that function was called. I expected that even in a dynamic language, this kind of error can be detected when the source file is parsed.

I understand that the type of actual arguments is not known until the function is called, because the same variable may contain values of any type at different times. But the number of arguments is known as soon as the source file is parsed. It is not going to change while the program is running.

So that this is not a philosophical question

To keep this in scope of Stack Overflow, let me phrase the question like this. Is there some feature, that Python offers, that requires it to delay checking the number of arguments in a function call until the code actually executes?

Answer 1

Python cannot know up-front what object you'll end up calling, because being dynamic, you can swap out the function object. At any time. And each of these objects can have a different number of arguments.

Here is an extreme example:

import random  def foo(): pass def bar(arg1): pass def baz(arg1, arg2): pass  the_function = random.choice([foo, bar, baz]) print(the_function()) 

The above code has a 2 in 3 chance of raising an exception. But Python cannot know a-priori if that'll be the case or not!

And I haven't even started with dynamic module imports, dynamic function generation, other callable objects (any object with a __call__ method can be called), or catch-all arguments (*args and **kwargs).

But to make this extra clear, you state in your question:

It is not going to change while the program is running.

This is not the case, not in Python, once the module is loaded you can delete, add or replace any object in the module namespace, including function objects.