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Apparently, if you need to use both keyword and positional arguments while calling your function, you have to use the positional argument first. But the following code results in an error;

def greet(first_name, l_name):
   print(f'Hi, {first_name} {last_name}!')

greet('Holmes',
first_name='Harry')

So does it mean that if you're using both, you have to use the positional argument first in the required order, and only then the keyword argument?

harry
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1 Answers1

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Positional arguments must be passed in order as declared in the function. So if you pass three positional arguments, they must go to the first three arguments of the function, and those three arguments can't be passed by keyword. If you want to be able to pass the first argument out of order by keyword, all your arguments must be passed by keyword (or not at all, if they have defaults).

If it helps, Python's binding mechanism is roughly:

  1. Assign positional arguments one by one to sequential parameters of the function. These parameters are now set.
  2. Assign keyword arguments to remaining parameters in any order. If one of the keyword arguments matches an argument already assigned positionally (or the same keyword argument is passed twice), it's an error.

In your case, what this means is that:

greet('Holmes', first_name='Harry')

first binds 'Holmes' to first_name. Then it saw you tried to pass first_name again as a keyword argument and objected.

ShadowRanger
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  • I am wondering why the binding mechanism does not proceed in the opposite direction: binding keyword arguments first, and remaining positional arguments second. That would allow mixing positional arguments and keyword arguments like this: `f(x=1, 2, y=3, 4, z=5, 6)` for `def f(x, y, z, a, b, c): ...`. – Géry Ogam Oct 05 '22 at 22:04
  • @Maggyero: Two possible reasons (this is a guess, I don't know): 1) Python guarantees left-to-right order of argument evaluation (it's not unspecified like it is in C/C++), and the stack manipulations required to evaluate in order yet keep the positional and keyword arguments separated on the stack (positional first, then keyword) would be a pain. 2) Mixing them makes it harder to read function calls in the code, and Python sometimes limits things that are possible, but make for unreadable code, on principle. – ShadowRanger Oct 06 '22 at 18:42