PEP 544: failing example

[BALOGHBence]

Documentation

The following example fails on Python 3.8.10 for several reasons.

class PColor(Protocol):
    @abstractmethod
    def draw(self) -> str:
        ...
    def complex_method(self) -> int:
        # some complex code here

class NiceColor(PColor):
    def draw(self) -> str:
        return "deep blue"

class BadColor(PColor):
    def draw(self) -> str:
        return super().draw()  # Error, no default implementation

class ImplicitColor:   # Note no 'PColor' base here
    def draw(self) -> str:
        return "probably gray"
    def complex_method(self) -> int:
        # class needs to implement this
        
nice: NiceColor
another: ImplicitColor

def represent(c: PColor) -> None:
    print(c.draw(), c.complex_method())

represent(nice) # OK
represent(another) # Also OK

The first reason why this fails is a syntax error (actually two), namely the methods complex_method are missing the ellipses. If these are corrected, the example still throws a NameError, since 'nice' and 'another' are actually not defined.

The corrected example:

class PColor(Protocol):
    @abstractmethod
    def draw(self) -> str:
        ...
    def complex_method(self) -> int:
        # some complex code here
        ...

class NiceColor(PColor):
    def draw(self) -> str:
        return "deep blue"

class BadColor(PColor):
    def draw(self) -> str:
        return super().draw()  # Error, no default implementation

class ImplicitColor:   # Note no 'PColor' base here
    def draw(self) -> str:
        return "probably gray"
    def complex_method(self) -> int:
        # class needs to implement this
        ...
        
nice: NiceColor = NiceColor()
another: ImplicitColor = ImplicitColor()

def represent(c: PColor) -> None:
    print(c.draw(), c.complex_method())

represent(nice) # OK
represent(another) # Also OK

[AA-Turner]

cc: @ilevkivskyi @JukkaL @ambv

[Loclhost]

Another fails

from typing import Any, Protocol

class ProtoA(Protocol):
    def meth(self, x: int) -> int: ...
class ProtoB(Protocol):
    def meth(self, obj: Any, x: int) -> int: ...

class C:
    def meth(self, x: int) -> int: ...

a: ProtoA = C  # Type check error, signatures don't match!
b: ProtoB = C  # OK

I think the comments in the last lines need to be swapped

[sobolevn]

@Loclhost please, create a new PR with the fix :)

[ilevkivskyi]

@Loclhost
Just to save you time: it is correct in the PEP (you are not the first one, and not even twentieth one, to try to "fix" this "typo"). Please just read example carefully again.

[JelleZijlstra]

(Hint to understand why this is correct: If you want to call C.meth (not C().meth), what is the signature it accepts?)