Default values for positional function arguments in Python?

Question

I was going trough some code, and I came across the following function:

def foo(self, arg1=1, *, arg2=2):
    pass

I was surprised to see keyword arguments on the left side of the *, the positional arguments. I notice then that I can call foo in both of the following ways:

>>> foo(1)
>>> foo(arg1=1)

I think I would be expecting the second call to fail as I am calling foo using a named argument by providing the keyword arg1.

With that said, am I using positional arguments in both scenarios, or is the second call to foo a named argument?

Answer 1

The best sentence that I found that best describes this is:

"The trick here is to realize that a “keyword argument” is a concept of the call site, not the declaration. But a “keyword only argument” is a concept of the declaration, not the call site."

Below is a concise explanation copied from here in case the link dies at some point.

def bar(a,    # <- this parameter is a normal python parameter
        b=1,  # <- this is a parameter with a default value
        *,    # <- all parameters after this are keyword only
        c=2,  # <- keyword only argument with default value
        d):   # <- keyword only argument without default value
    pass

Answer 2

The arg1 argument is allowed to be called as either a positional argument, or a keyword argument.

As of Python 3.8, it is possible to specify some arguments as positional only. See PEP 570. Prior to 3.8, this isn't possible unless you write a python C extension.

The 3.8 syntax looks like this (directly from the PEP):

def name(positional_only_parameters, /, positional_or_keyword_parameters,
         *, keyword_only_parameters): ...

...prior to 3.8, the only legal syntax is this:

def name(positional_or_keyword_parameters, *, keyword_only_parameters): ...