[issue38834] TypedDict: no way to tell which (if any) keys are optional at runtime

[Zac Hatfield-Dodds]

New submission from Zac Hatfield-Dodds <zac.hatfield.dodds at gmail.com>:

Consider the following cases:

```python
class A(typing.TypedDict):
    a: int  # a is required

class B(A, total=False):
    b: bool  # a is required, b is optional

class C(B):
    c: str  # a is required, b is optional, c is required again
```

PEP-589 is clear about the semantics, and this is obvious enough when reading the code.  At runtime the __annotations__ attribute of each class gives us the set of allowed keys and the type of each corresponding value, but we have a problem:

- C has __total__==True, but b is not actually required.
- B has __total__==False, but a *is* required.
- I can't see any way to get the parent classes of a TypedDict class!

The _TypedDictMeta metaclass updates the attributes, but leaves no record of the parent type - at runtime A, B, and C all appear to inherit directly from dict.

After discussion on the typing-sig mailing list, I propose to add __required_keys__ and __optional_keys__ attributes to TypedDict subclasses, as frozensets of strings.

This will be very useful for Hypothesis' `from_type()` strategy, as well as for type-based validation frameworks like pydantic or typeguard.

----------
components: Library (Lib)
messages: 356836
nosy: Zac Hatfield-Dodds, levkivskyi
priority: normal
severity: normal
status: open
title: TypedDict: no way to tell which (if any) keys are optional at runtime
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38834>
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