I've done some more thinking more about Serhiy's worry about changing a
factory function to a class, and how that would change the meaning of
type-hints. Say:
def spam(x: Eggs, y:Cheese) -> _Aardvark:
# actual factory function implementation
# later, we use it as a function protocol
def myfunction(a: int, callback: spam) -> str:
...
If spam changes implementation from a factory function to the actual
class of _Aardvark, keeping the name:
class spam:
# body of _Ardvark goes here
that would completely change the meaning of the myfunction type
declaration.
I still think that is not a scenario we need to care about. In my mind,
that counts as a user-applied footgun. But if people disagree, and
Serhiy's argument persuades them, then we can still use functions as
their own prototype.
@Callable
def spam(x: Eggs, y: Cheese) -> _Aardvark:
# actual factory function implementation
The Callable decorator would just flag the function as *permitted* to be
used as a prototype, with no other change. Or perhaps we could have
@FunctionPrototype.
Alternatively, we could write the consumer's annotation like this:
def myfunction(a: int, callback: Prototype[spam]) -> str:
...
where Prototype[x] uses:
- the signature of x if x is a function or method;
- the signature of `x.__call__` if x is a callable instance;
- the signature of `x.__new__` or `x.__init__` is x is a class;
as the prototype.
However it is spelled, we might require functions to opt-in before they
can be used as prototypes, in other words the decorator is the author's
promise that they aren't going to change the function into a class, or
change its signature, without the usual deprecation warnings etc.
--
Steve
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