Hello everybody! In a personnal project I feel the need (or the desire) to implement something like this: assert isinstance(1, PositiveInteger) assert not isinstance(-1, PositiveInteger) So I began looking a lot in the abc module, and I end unp using an __instancehook__ special method wich is called by __instancechek__ in the corresponding metaclass, just like the __subclasshook__ special method called by __subclasscheck__. The core idea is something like this: class MyMeta(type): def __instancecheck__(cls, instance): return cls.__instancehook__(instance) class PositiveInteger(metaclass=MyMeta): @classmethod def __instancehook__(cls, instance): return isinstance(instance, int) and instance > 0 Of course, the real implemention is more detailed... What do you think about that ?
On 26 December 2017 at 09:13, Yahya Abou 'Imran via Python-ideas <python-ideas@python.org> wrote:
To me, this seems like an over-use of classes for something they are not really appropriate for (in Python, at least). In my experience, it's people coming from other languages that are more strongly class based, such as Java, that prefer this type of construct. In Python, I'd write this as assert isinstance(1, int) and 1 > 0 assert not isinstance(-1, int) or -1 < 0 # or maybe you meant isinstance(-1, int) and -1 < 0 for the second one...? That reads far more naturally to me in Python.
I don't think it's needed - it feels to me like a solution to a problem that Python doesn't have in practice. (Of course, there may be more complex or specialised cases where it would be useful, but as you've demonstrated, you can write the implementation yourself for the rare cases it might be needed, so that may well be sufficient). Thanks for the idea - it's interesting to see how other people approach problems like this even if it turns out not to be something worth adding. Paul
Well, I fact, I was playing with metaclasses and descriptors, and I was thinking about a way to use the famous variable annotation to instatiate decrptors like this: class Point(metaclass=MyMeta): x: int y: int class Circle(metaclass=MyMeta): center: Point radius: PositiveInteger Here the radius must be [strictly] positive, but since I'm using the annotations to get the type to test in the __set__ method of the descriptor, I didn't found a more elegant way than that... And it was before I knew the existance of dataclasses!!! I was pretty shocked by the way. Maybe I can make this compatible with it anyway.
On 26 December 2017 at 09:13, Yahya Abou 'Imran via Python-ideas <python-ideas@python.org> wrote:
To me, this seems like an over-use of classes for something they are not really appropriate for (in Python, at least). In my experience, it's people coming from other languages that are more strongly class based, such as Java, that prefer this type of construct. In Python, I'd write this as assert isinstance(1, int) and 1 > 0 assert not isinstance(-1, int) or -1 < 0 # or maybe you meant isinstance(-1, int) and -1 < 0 for the second one...? That reads far more naturally to me in Python.
I don't think it's needed - it feels to me like a solution to a problem that Python doesn't have in practice. (Of course, there may be more complex or specialised cases where it would be useful, but as you've demonstrated, you can write the implementation yourself for the rare cases it might be needed, so that may well be sufficient). Thanks for the idea - it's interesting to see how other people approach problems like this even if it turns out not to be something worth adding. Paul
Well, I fact, I was playing with metaclasses and descriptors, and I was thinking about a way to use the famous variable annotation to instatiate decrptors like this: class Point(metaclass=MyMeta): x: int y: int class Circle(metaclass=MyMeta): center: Point radius: PositiveInteger Here the radius must be [strictly] positive, but since I'm using the annotations to get the type to test in the __set__ method of the descriptor, I didn't found a more elegant way than that... And it was before I knew the existance of dataclasses!!! I was pretty shocked by the way. Maybe I can make this compatible with it anyway.
participants (2)
-
Paul Moore -
Yahya Abou 'Imran