and heterogeneously typed ducts: https://peps.python.org/pep-0589/
Traditionally, duck typing is used to inject types into a library that isn’t expecting extension at that particular point — eg, substituting a test class for a real class in a data object that normally wouldn’t be a protocol.
I’m not seeing how that PEP addresses that use case.
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Similarly, the heading on the dictionary says “for a fixed set of keys” — but what if I want a dynamic heterodox dict? Eg, unpacking JSON.
You just end up shoving “any” all over the place. At which point, are the types helping?
Though, protocols do help the dict case for typing:
dict : Hashable -> Any
https://peps.python.org/pep-0544/#:~:text=Structural%20subty....
> substituted-for class to be defined as a protocol.
... which is false
> Similarly, the heading on the dictionary says “for a fixed set of keys” — but what if I want a dynamic heterodox dict? Eg, unpacking JSON.
You can quite obviously decode to recursive types (e.g. using pydantic), not sure what the problem is.
This section seems to agree with me, where it explains why normal classes can’t be subclassed to protocols:
> Now, C is a subtype of Proto, and Proto is a subtype of Base. But C cannot be a subtype of Base (since the latter is not a protocol). This situation would be really weird. In addition, there is an ambiguity about whether attributes of Base should become protocol members of Proto.
https://peps.python.org/pep-0544/#protocols-subclassing-norm...
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I’m not sure why you think it’s “obvious” that you can use a third party library to solve the problem — or why that addresses my complaint that the built-in type system doesn’t work for that.
If anything, the existence of a third party library hints the standard library doesn’t cover the use case.
edit: To be more precise: The article specifically mentions that accessing invalid members give you a type error, but posits that people will probably not bother and just use 'any' instead. How is that not saying 'too much discipline'?
I’m also generally pro-types, but I think it’s worth having a discussion about this type system in the context of Pythonisms.
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If you’re using “any” for most of your types, then it’s not providing value — an untyped statement implicitly has the type “any”.
There’s a reason I brought up the JSON example: loading and manipulating JSON of varying structure is something I do a lot at work.
>There’s a reason I brought up the JSON example: loading and manipulating JSON of varying structure is something I do a lot at work.
Do you have a specific example that you find troublesome? For JSON with a fixed, regular structure (i.e. no subtyping) something like Pydantic works well, otherwise using unions + protocols with custom validation code has covered even my most esoteric use cases so far.
If we're talking about unstable JSON, a recursive union type works well enough in my experience, though defining that type gets a bit ugly if your type checker doesn't directly support recursive types.
Python types don’t support a common Pythonism used frequently in my work.
In fact, you admit that even fixed JSON can be difficult without a 3rd party library.
You’re lecturing me like I don’t understand types without realizing that you repeated my point about a gap in the current type system.
JSON = Union[
None,
bool,
int,
float,
str,
List['JSON'],
Dict[str, 'JSON'],
]
>In fact, you admit that even fixed JSON can be difficult without a 3rd party library.
Sure, but that's just because the standard library (for now?) doesn't offer deserialization with runtime validation. Java doesn't have that either, if you want to parse JSON you'll have to either roll your own or rely on a third party library.
Don't get me wrong, the situation isn't perfect, but especially if you use the 'right' tooling, you can in my experience already get a perfectly serviceable experience, which to me is indicative that the type system itself is fine and pythonic, it's just the tooling that is lagging a bit behind at the moment.
> Sure, but that's just because the standard library (for now?) doesn't offer deserialization with runtime validation.
> it's just the tooling that is lagging a bit behind at the moment
So… the standard library has exactly the problem I articulated, and you’re violently agreeing while speaking down to me.
I think that reflects poorly on you.
If you think that's violently agreeing and talking down to you, then there must be some communication barrier that I don't know how to overcome.
This does not define the type of a valid json object. (fe None is not a valid json object, neither is '')
Duck typing and heterogeneous dictionaries are (and have been) standard Python.
Adding a type system which doesn’t respect duck typing by trying to access the member even when the types don’t match or which can’t express standard idioms used in Python seems a poor architectural choice.
It’s not saying that Python’s type system is “too much discipline”, but rather that the type system doesn’t encode typical Pythonisms.