Typing FAQ: New entry explaining invariance#3073
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| ```py | ||
| def modify(entries: list[Entry]): | ||
| entries.append(File("README.txt")) |
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I think it's sufficiently self-evident that File is a distinct subclass of Entry, but that's entirely implicit and caught me out the first time I read through this -- this is the first time we see File mentioned.
It would be nice if our example only required two types, not three, but I'm not sure if there's a good realistic example with that shape. X vs X | None is one realistic case, but then it requires also understanding unions and subtyping.
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It would be nice if our example only required two types, not three
That's exactly what I said to Alex. The drawback of the class hierarchy example is that it requires three types if you really want to demonstrate a bug.
We did consider using int | None, but also thought that explaining subtyping of unions would make it even more complicated.
I think it's sufficiently self-evident that
Fileis a distinct subclass ofEntry, but that's entirely implicit and caught me out the first time I read through this -- this is the first time we seeFilementioned.
I'll think about this
| ```py | ||
| def modify(entries: list[Entry]): | ||
| entries.append(File("README.txt")) | ||
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| directories: list[Directory] = [Directory("Downloads"), Directory("Documents")] | ||
| modify(directories) # ty emits an error on this call | ||
| ``` |
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I know that it makes the example frustratingly verbose, but I do think being able to copy and paste examples directly into a Python REPL and/or type checker playgrounds is quite valuable. So I would be inclined to defined these classes in the Python snippet:
| ```py | |
| def modify(entries: list[Entry]): | |
| entries.append(File("README.txt")) | |
| directories: list[Directory] = [Directory("Downloads"), Directory("Documents")] | |
| modify(directories) # ty emits an error on this call | |
| ``` | |
| ```py | |
| from dataclasses import dataclass | |
| @dataclass | |
| class Entry: | |
| path: str | |
| class Directory(Entry): | |
| pass | |
| class File(Entry): | |
| pass | |
| def modify(entries: list[Entry]): | |
| entries.append(File("README.txt")) | |
| directories: list[Directory] = [Directory("Downloads"), Directory("Documents")] | |
| modify(directories) # ty emits an error on this call | |
| ``` |
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I thought about adding a playground link somewhere...
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if you want it more concise, you could go for more "stub-like" formatting:
| ```py | |
| def modify(entries: list[Entry]): | |
| entries.append(File("README.txt")) | |
| directories: list[Directory] = [Directory("Downloads"), Directory("Documents")] | |
| modify(directories) # ty emits an error on this call | |
| ``` | |
| ```py | |
| from dataclasses import dataclass | |
| @dataclass | |
| class Entry: | |
| path: str | |
| class Directory(Entry): ... | |
| class File(Entry): ... | |
| def modify(entries: list[Entry]): | |
| entries.append(File("README.txt")) | |
| directories: list[Directory] = [Directory("Downloads"), Directory("Documents")] | |
| modify(directories) # ty emits an error on this call | |
| ``` |
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This would also address my comment about File coming from nowhere.
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I now (ab)used inline annotations to hide the setup code away in a tooltip
3 participants
Summary
Now that we do not union in
Unknowninto element types of invariant collections anymore, this question comes up more frequently. I'm taking a first stab at trying to explain this in a concise way. I am also planning to add a diagnostic hint that would be emitted in those situations, which could refer to this FAQ entry.Rendered