python positional arguments

Question

I am trying to understand how the parameters to this function are interpreted:

def f(a, *, b):
   return a, b

It appears this function forces the caller to call f() with exactly 2 params and the second param should always be a named b= param. How do I decipher this from the function signature? Why does it not allow me to specify a middle argument for the *?

score 4 · Accepted Answer · answered Jun 18 '17 at 00:27

How do I decipher this from the function signature?

Arguments with no default value must be passed.
Arguments after a * must be passed by keyword if they are passed at all.
Extra arguments cannot be passed to "fill up" the * unless an argument name accompanies the *.

Since b has no default value it must be passed. Since it is after the * it must be passed by keyword. Since the * is "bare" (i.e., it is just the * placeholder and not a vararg like *args), no other positional arguments can be passed as "middle" arguments.

See PEP 3102 for a description of the keyword-only-argument syntax.

Great explanation, I understood it better than the official documentation. — user2399453, Jun 18 '17 at 17:31

score 3 · Answer 2 · answered Jun 18 '17 at 00:33

The * alone is a Python3-only way to express that the following parameters are named arguments and can only be passed to the function as such.

From the documentation:

Parameters after “*” or “*identifier” are keyword-only parameters and may only be passed used keyword arguments.

python positional arguments

2 Answers2