Questionable annotation behaviors

I discovered a few questionable behaviors recently around typing and annotations. In each case, backwards compatibility probably means we can’t change the behavior, so I am not proposing any concrete change, but I want to make more people aware of these issues.

The three issues are explained in more detail here:

In summary, the three issues are:

Annotations on NamedTuple and TypedDict subclasses are converted

>>> class X(NamedTuple):
...     a: None
...     b: "str"
>>> X.__annotations__
{'a': <class 'NoneType'>, 'b': ForwardRef('str')}

Unlike normal classes, None is turned into NoneType and strings are turned into ForwardRef objects.

TypedDict __annotations__ include those on base classes

>>> class X(TypedDict):
...     a: int
>>> class Y(X):
...     b: str
>>> Y.__annotations__
{'a': <class 'int'>, 'b': <class 'str'>}

On normal classes (and NamedTuples), __annotations__ only includes the class’s own annotations; on TypedDicts it also includes those of base classes.

Parenthesized names don’t show up in __annotations__

>>> class X:
...     a: int
...     (b): str
>>> X.__annotations__
{'a': <class 'int'>}

And if you ever wondered why the AnnAssign AST node has a simple field, this is why.

I posted a PR (annotations: expand documentation on "simple" assignment targets by JelleZijlstra · Pull Request #120535 · python/cpython · GitHub) making the documentation for this behavior more explicit.

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The first behavior I know I’ve hit before and added a special case in my runtime type inspection logic to deal with it. The other two behaviors I don’t think I’ve ever noticed. The second behavior I tend to merge type annotations of parent classes anyway to know all attributes while 3rd feels even harder to hit naturally.

I would generally be happy to have behaviors like first “fixed” and that annotation value not have extra undocumented rules.

I think the second and third behaviors are intentional and arguably justified.

TypedDicts get their “base class” annotations because they don’t actually have base classes (other than dict); their “inheritance” is rather a syntactic shortcut for constructing a new independent TypedDict that gets the fields from the “bases”. The “bases” aren’t actually found in the MRO of the new TypedDict, so if the annotations weren’t copied, it would be difficult/unintuitive to actually introspect the full type signature of a TypedDict at runtime. (Technically it would be possible via __orig_bases__ but that’s pretty obscure.)

(I’m not sure that this is the best/only way for TypedDict to be implemented at runtime, but it’s what we have.)

For the third, it seems reasonable that annotations of things that aren’t just a name wouldn’t go into __annotations__, since it’s not clear what the key would be. Maybe a single parenthesized name could be special-cased, but it’s not clear who would practically benefit.

The first behavior seems to me like an outright bug that I would prefer to just fix, but the backward-compat consequences might indeed be too great.


That’s true, I hadn’t realized that the __bases__ of a TypedDict. A problem with merging the annotations is that robust runtime introspection tools need to know which scope each annotation came from (in order to resolve string annotations). Fortunately, the first behavior makes this a little easier, because annotations in TypedDicts get resolved to ForwardRefs that are aware of their defining module. If this wasn’t the case, it would not be possible to resolve string annotations in TypedDict fields at all.

Of course, the implementation of PEP 649 will make this a lot more robust.

I agree that it makes sense that annotations for a.b: T or a[b]: T are not stored anywhere. What I find questionable is that (a): T is also discarded: I would have expected it to be equivalent to a: T. The implementation needs special casing to make it so that annotations are not stored in this case.


Oh, I didn’t realize this. I agree with you; I don’t see much rationale for special-casing to avoid storing annotations in the case of (a): T.

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