Spurious `duplicate-keyword-argument` error for `str.format` calls with `self` keyword argument

[panhania]

Consider the following snippet:

print("{self}".format(self=42))

When type-checking it with pytype we will get the following error:

File "(...)", line 1, in <module>: function str.format got multiple values for keyword argument 'self' [duplicate-keyword-argument]
         Expected: (self, *args, **kwargs)
  Actually passed: (self, self)

Note that mypy does not complain about this code. It also executes as expected:

$ python3 -c 'print("{self}".format(self=42))'
42

[panhania]

Interesting that indeed self cannot be used for methods in general. For example, the following:

class Foo:

    def bar(self, **kwargs):
        print(kwargs)


Foo().bar(self=42)

will fail to execute:

  File "(...)", line 7, in <module>
    Foo().bar(self=42)
TypeError: bar() got multiple values for argument 'self'

and pytype will correctly catch it:

File "(...)", line 7, in <module>: function Foo.bar got multiple values for keyword argument 'self' [duplicate-keyword-argument]
         Expected: (self, **kwargs)
  Actually passed: (self, self)

We can fix it with the PEP 570 syntax (positional-only arguments):

class Foo:

    def bar(self, /, **kwargs):
        print(kwargs)

Foo().bar(self=42)

and execution will succeed and pytype will be happy as well:

Success: no errors found

Interesting that mypy will report an error in neither of these cases (which is wrong!).