PEP 692: Using TypedDict for more precise **kwargs typing

PEPs
63

(Speaking for myself, not the SC, although these questions did come out of discussions with the rest of the SC).

I think the use-case for TypedDict for **kwargs is sensible, and a logical extension of both PEP 646 and PEP 589, but I have strong reservations about the syntactic change. Specifically, the PEP introduces a new meaning of ** in an expression (but only in one specific context), something that’s in line with the use of ** for parameter lists but very different from ** elsewhere in expressions. So I have a few pointed questions:

  • How much does using ** actually win here? The alternative is to use Unpack, which doesn’t seem that bad, especially since it’s an established annotation. Is the use of Unpack really confusing enough, or common enough, or error-prone enough to warrant special syntax? Is the use of ** really obvious enough, and discoverable enough, to be less confusing or less error-prone?
  • How bad would be it be to invalidate type annotations on **kwargs that aren’t Unpack? Do people actually want the current behaviour of annotating **kwargs? Regardless of whether they do, is the current behaviour actually desirable?
  • If the current behaviour isn’t desirable, how bad would it be to transition to using TypedDict semantics by default on **kwargs annotations? How many annotations of **kwargs using the old semantics are in fact correct, rather than misunderstandings or overly broad annotations like Any?
  • I feel like at some point in the journey to type-annotate all Python code we will end up discovering mistakes of the past or better ways of doing things, so having transition processes is probably a good idea. Something similar in concept, if not design, as __future__ imports. Considering the simplicity of the Unpack alternative to the new syntax, and assuming a transition is warranted, would this not be a good testing ground for such a process?

Just so it’s clear, the SC hasn’t decided yet on the PEP, one way or another, but I would like some clarity on the questions above to help make the decision.

1 Like