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Python Type Checker Comparison: Empty Container Inference
- curiousgal 7mo agoI can't help but find type hints in python to be..goofy? I have a colleague who has a substantial C++ background and now working in python, the code is just littered with TypeAlias, Generic, cast, long Unions etc.. this can't be the way..
- IshKebab 7mo agoI strongly disagree. Python has actually done a decent job of adding type annotations into the language IMO. If you ignore the bit where they don't actually specify their semantics anyway. > this can't be the way.. The alternative is fragile and unmaintainable code. I know which I prefer!
- b00ty4breakfast 7mo agothe alternative should be using a real statically-typed language instead of glorified comments that don't do anything without outside tools. I understand that very large code bases have been built in python and this is a compromise to avoid making them rewrite Ks upon Ks of LoC but as it stands, Python type annotations are akin to putting a Phillip's head screwdriver on a ball peen hammer; the screwdriver is not a real screwdriver and the ergonomics of the hammer have been compromised.
- IshKebab 7mo agoWell yes I agree using Rust or whatever would be better, but if your options are Python or Python with type hints, then the latter gets you closest to proper static typing. They're really not that bad with Pyright in strict mode. Mypy is rubbish.
- tialaramex 7mo agoTyping is a relatively easy way for the human author and the machine to notice if they disagree about what's going on before problems arise. It is unfortunate that Python doesn't do a good job with types, I was reading earlier today about the mess they made of booleans - their bool type is actually just the integers again.
- nubg 7mo ago> I was reading earlier today about the mess they made of booleans Can you elaborate on that?
- IshKebab 7mo agoHe did - booleans are integers: >>> isinstance(False, int) True A related screw-up is implicitly casting everything to bool. A lot of languages made that mistake. Overall I'd say they didn't do an awful job though. The main problems with Python are the absolutely abysmal tooling (which thankfully uv fixes), the abysmal performance (which sometimes isn't an issue, but it usually becomes an issue eventually), and the community's attitude to type checking. Actually type checking code you've written yourself with Pyright in strict mode is quite a pleasant experience. But woe betide you if you want to import any third party libraries. There's at least a 50% chance they have no type annotations at all, and often it's deliberate. Typescript used to have a similar problem but the Javascript community realised a lot quicker than the Python community that type hints are a no-brainer.
- tialaramex 7mo agoBecause Python decided that (for the usual New Jersey reason, simplicity of implementation) bool should just be an integer type the Liskov criterion comes into play. If we can X an integer and we've agreed bool is an integer => we can X a bool. That's not what booleans are but hey, it's sorta close and this was easier to implement. So, can we add two bools together? Adding booleans together is nonsense, but we've said these are a kind of integer so sure, I guess True + True = 2 ? And this cascades into nonsense like ~True being a valid operation in Python and its result is true...
- wiseowise 7mo agoWhat is the way in your opinion?
- maleldil 7mo ago> TypeAlias, Generic This is mitigated by modern (3.12+) generic and `type` syntax, which just looks like any other static language.
- IshKebab 7mo agoI think it would be worth mentioning that in normal use (strict mode) Pyright simply requires you to add type annotations to the declaration. Occasionally mildly annoying but IMO it's clearly the best option.
- veber-alex 7mo agoIt's not "mildly annoying". I don't enable strict mode on multiple projects because people don't want to type anything outside of function signatures. Inferring the type from the first use is 100% the correct choice because this is what users want 99% of the time, for the rest you can provide type information.
- maleldil 7mo agoAnnotating empty collections is one of the few places you need to annotate outside function signatures. It's not a big deal. It doesn't happen that often.
- nomel 7mo agoAnd, when it does, you can just put them when the empty container is assigned: things: set[tuple[str, str, int]] = set() users: list[User] = [] Many people don't seem to know this exists.
- maleldil 7mo agoYes, that's what I was referring to. I get it that Pyrefly wanted to advertise their approach here, but it's weird that they didn't at least acknowledge this. It's what I use because it works on every type check, and I don't need to rely on their particular implementation for this. In fact, I recently migrated a project from Pyright to Pyrefly for performance reasons, and there was very little I had to change between. The most annoying thing was Pyrefly's lack of exhaustive pattern matching for StrEnum and Literal[...]
- 7mo ago
- jez 7mo agoA more complicated version of this problem exists in TypeScript and Ruby, where there are only arrays. Python’s case is considerably simpler by also having tuples, whose length is fixed at the time of assignment. In Python, `x = []` should always have a `list[…]` type inferred. In TypeScript and Ruby, the inferred type needs to account for the fact that `x` is valid to pass to a function which takes the empty tuple (empty array literal type) as well as a function that takes an array. So the Python strategy #1 in the article of defaulting to `list[Any]` does not work because it rejects passing `[]` to a function declared as taking `[]`.
- Boxxed 7mo agoMy favorite part about the type annotations in python is that it steers you into a sane subset of the language. I feel like it's kind of telling that python is this super dynamic language but the type annotations aren't powerful enough to denote all that craziness.
- reubenmorais 7mo agoThat's nice if you're starting from scratch, but if you have existing code to deal with, you don't have the privilege of ignoring the insane subset.
- yunnpp 7mo agoThe type hints are not even enforced at runtime. They are mostly documentation.
- LtWorf 7mo agoThey can be used at runtime though. I wrote typedload, to load external data (json/bson/yaml) into python typed objects. In this way you know that if the data doesn't match the expectations you will have an exception at a specific point in the code, and after that it's safe to use the objects, rather than having to manually check at every access. Now there are several other libraries that do this thing, but at the time (python3.5 and 3.6) it was the only option.
- yunnpp 7mo agoThat seems to handle deserialization? But would it protect you from assigning a value of the wrong type to the object later on?
- LtWorf 7mo agoIt must be used in combination with a static checker to be useful. So you can do like a = typedload.load(json_data, int) and then "a" is considered to be an int and at runtime will be an int. Of course your static checker should prevent you from doing a + "string" later on because that would fail.
- loevborg 7mo agoFWIW, Typescript is using Strategy 2: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABMOcAUEEBMawQLkSgCcQBTASkQG8AoRBxTMAZykQA8XEBeRAbQC6AbnqMuohgHopiAHoB+WmIYxgiDNlzwwVFYwZcAdAAcQLABZoAjBUmMyAGxZl9B42ctoARAEN-3hTK7iz2MvJKAL5AA https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABM... I'm a bit confused by the fact that the array starts out typed as `any[]` (e.g. if you hover over the declaration) but then, later on, the type gets refined to `(string | number)[]`. IMO it would be nicer if the declaration already showed the inferred type on hover.
- bastawhiz 7mo agoIt depends on your tsconfig. An empty array could be typed as never[], forcing you to annotate it.
- loevborg 7mo agoWhich setting specifically? Can you repro in the typescript playground?
- wk_end 7mo agoI don't believe this is correct. There's no settings that correspond to that AFAIK, and it'd actually be quite bad, because you could access the empty array and then get a `never` object, which you're not supposed to be able to do. https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABMOcAUBKAXIgzlAJxjAHNEBvAKEUQgX0TAFMA3Jg3HZtggbQF1EAXkQCA3NVr0oiAB7DGrdrl4AGfhJoEmUEASSyJAX0pA https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABM... `unknown[]` might be more appropriate as a default, but TypeScript does you one better: with OP's settings, although it's typed as `any[]`, it'll error out if you don't do anything to give it more information because of `noImplicitAny`.
- sheept 7mo agoI agree, it's always been unsettling to see any[] on hover, even though it gets typed in the end. I think one reason might be to allow the type to be refined differently in different code paths. For example: function x () { let arr = [] if (Math.random() < 0.5) { arr.push(0) return arr } else { arr.push('0') return arr } } In each branch, arr is typed as number[] and string[], respectively, and x's return type is number[] | string[]. If it decided to retroactively infer the type of arr at declaration, then I'd imagine x's return type would be the less specific (number | string)[].
- nimbus-hn-test 7mo ago[dead]
- tl2do 7mo agoIs there a compile-to-Python language with built-in type safety, similar to how TypeScript transpiles to JavaScript? I'm aware of Mojo and mypyc, but those compile to native code/binaries, not Python source.
- exyi 7mo agoPython does not need that, as it has built-in type annotation support. The annotation is any expression, so you can in theory express anything a custom type-only language would allow you (although you could make it less verbose and easier to read). However, the it IMHO just works much worse than TS because: * many libraries still lack decent annotations * other libraries are impossible to type because of too much dynamic stuff * Python semantics are multiple orders of magnitude more complex than JavaScript. Even just the simplest question: Is `1` allowed in parameter typed `float`? What about numpy float64?
- tl2do 7mo agoThanks for helping me understand. I wasn't aware of Python's type annotation support. I did some quick research and learned that type annotations don't cause compile errors even when there are type errors. Is that why type checkers like Pyrefly exist?
- linsomniac 7mo agoCorrect, currently in Python the type checking is implemented more in a linting phase than in a compiling or runtime phase. Though you can also get it from editors that do LSP, they'll show you type errors while editing the code.
- tl2do 7mo agoThanks linsomniac and exyi. I didn't realize Python's type hints are checked by linters, not the compiler. Learned something today.
- 7mo ago
- Sinidir 7mo agoIn the example given in the article i think the correct behavior would have been to infer the type backwards from the return type of the function. Is that not why mypy actually errors here?
- ocamoss 7mo agoIf you're referring to the `first_three_lines` example in strategy 3, Mypy would give the same error even if we changed the return value to something unrelated like `return ["something"]`.
- brainzap 7mo agoIn early typescript I was too lazy and just set an inital value and then zero the list
- electroglyph 7mo agomy wishlist for pyrefly: when using decorated functions, show the underlying type hints instead of the decorators
- dupdrop 7mo agoOnly Python, is a language soooo dynamic, that the question "Does this code type-checks?" may get the valid response: "With which of the 5 existing type checkers?"
- Daishiman 7mo agoIt's actually a fairly frequent fact of programming language development that type resolution can change across versions. Haskell famously has a ton of extensions that enhance the type system in various, potentially incompatible ways. In fact the question of whether a code type-checks is itself undecidable.