TypedDict: no way to tell which (if any) keys are optional at runtime #83015

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1. Zac-HD commented on Nov 18, 2019

   

   Zac-HDmannequin

   on Nov 18, 2019

   MannequinAuthor

   More actions

   Consider the following cases:

   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.
2. Zac-HD

   added

   3.8 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Nov 18, 2019
3. ilevkivskyi

   closed this as completedon Nov 24, 2019
4. ezio-melotti

   transferred this issue fromon Apr 10, 2022