PEP 728: TypedDict with Typed Extra Items

PEPs
115

That’s fair, though _ as a key is still probably quite rare. Your proposal seems pretty reasonable.

The issue with key conflicts got me thinking about a more general solution to work around key naming limitations. We already don’t support all possible keys when using the class-based syntax, and thus we probably can’t deprecate the functional syntax. What if we’d add a new way of specifying arbitrary string keys using the class-based-syntax? Here’s one idea:

class Foo(TypedDict):
    name: str  # Regular item
    _: bool    # Type of extra items
    __items__ = {
        "_": int,   # Literal "_" as a key
        "class": str,  # Keyword as a key
        "tricky.name?": float,  # Arbitrary str key
    }

(The name __items__ is just the first thing that came to my mind – the specifics of the name aren’t important.)

This may go beyond the original goals of the PEP, but this would have some nice properties:

  • Arbitrary key names can be supported using the class-based syntax.
  • We can hopefully deprecate the functional TypedDict syntax.
  • We can support forward references in the extra item type without escaping.
  • All TypedDict items (including the extra ones) are defined using a similar syntax in the common case where __items__ is not needed.
  • We have the option of adding arbitrary magic key names such as _, since name conflicts can be worked around easily.

This would make the proposal a bit bigger, but on the other hand, we could deprecate the functional syntax, so arguably this would simplify the overall non-deprecated TypedDict functionality.

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