No error for identical overloaded functions

[shreydesai]

Mypy doesn't raise any errors for this set up:

from typing import overload

class AbstractClass(object):
    @overload
    def f(self) -> None:
        pass

    @overload
    def f(self) -> None:
        pass

By definition, overloaded methods follow the "same name, different parameters" mantra, so allowing two identical overloaded methods should not be allowed.

This could get ugly when a base class inherits from the abstract class; there would be no guarantee the right function would be called.

[gvanrossum]

Actually mypy's attitude towards overloading is more subtle -- it allows overlapping argument types as long as the return types match. PEP 484 is unclear about this. If you want to, you could help by adding some clarifying text to PEP 484 to the github.com/python/peps repo. We could also use some more words about this in mypy's own docs.

[shreydesai]

Glad to help! I'll add the clarifying text soon.

[shreydesai]

@gvanrossum I'm a bit confused about this; could you clarify the result of this scenario? Let's say I have this setup, where B is the base class of the class A:

from typing import overload

class A(object):
  @overload
  def f(self) -> None:
    print('Hello world 1')

  @overload
  def f(self) -> None:
    print('Hello world 2')

class B(A):
  def __init__(self) -> None:
    self.f()

b = B()

When I call self.f() in __init__ for B, which output message would I get – the first or the second? And, why does this happen; how does mypy know which function to call?

[gvanrossum]

Overloading is only for stubs.

On Saturday, July 16, 2016, Shrey Desai [email protected] wrote:

@gvanrossum https://github.com/gvanrossum I'm a bit confused about
this; could you clarify the result of this scenario? Let's say I have this
setup, where B is the base class of the class A:

from typing import overload
class A(object):
@overload
def f(self) -> None:
print('Hello world 1')

@overload
def f(self) -> None:
print('Hello world 2')
class B(A):
def init(self) -> None:
self.f()

When I call self.f() in init for B, which output message would I get
– the first or the second? And, why does this happen; how does mypy
know which function to call?

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[Michael0x2a]

As of f61c2ba, mypy will now report the following error message for the above code snippet: "Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader".

We could maybe improve the error message so it gives more customized feedback when the signatures are literally identical, but tbh I think this is clear enough. Feel free to reopen if anybody disagrees.