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