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This problem originated when I tried to apply a more functional approach to problems in python. What I tried to do is simply square a list of numbers, no biggie.
from operator import pow
from functools import partial
squared = list(map(partial(pow, b=2), range(10))
As it turns out, this didn't work. TypeError: pow() takes no keyword arguments
Confused I checked if pow(b=2, a=3) did. It didn't.
I've checked the operator source code, nothing suspicious.
Confused, I've begun to doubt my own python knowledge, I made a pow function myself.
def pow(a, b):
return a ** b
Then I tried doing the same thing with my function and surprisingly, everything worked.
I'm not going to guess what is the cause of the problem, what I'm asking is simply why is this a thing and if there exists a workaround.
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However, in Python 3, there is a simple way to enforce it! By adding a * in the function arguments, we force all succeeding arguments to be named. E.g. by having the first argument be * , we force all arguments to be named.
Functions do not have declared return types. A function without an explicit return statement returns None . In the case of no arguments and no return value, the definition is very simple. Calling the function is performed by using the call operator () after the name of the function.
Python allows functions to be called using keyword arguments. When we call functions in this way, the order (position) of the arguments can be changed.
When a function is invoked, all formal (required and default) arguments are assigned to their corresponding local variables as given in the function declaration. The remaining non-keyword variable arguments are inserted in order into a tuple for access.
If you check the signature of the built-in pow() or operator.pow() using the help() function in the interactive shell, you'll see that they require positional-only parameters (note the trailing slashes):
pow(x, y, z=None, /)
pow(a, b, /)
The reason is that both functions are implemented in C and don't have names for their arguments. You have to provide the arguments positionally. As a workaround, you can create a pure Python pow() function:
def pow(a, b):
return a ** b
See also What does the slash(/) in the parameter list of a function mean? in the FAQ.
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