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PEP 544: failing example
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
