Is pyright too picky about empty containers?

[dimaqq]

a = lambda: {42}
b = lambda: set() # E: Return type of lambda, "set[Unknown]", is partially unknown

Same for literal empty dict {}, frozenset, etc.

I can work around that using lambda: typing.cast(typing.Set[str], set()) but that doesn't seem too elegant.

What am I doing wrong?

[erictraut]

It looks like you have enabled the reportUnknownLambda type checking rules, which is off be default. Or more likely you've enabled "strict" mode. In this case, pyright will report when a lambda expression is evaluated with a partially-unknown type.

If you don't want pyright to report errors of this sort, I recommend not using strict mode.

If you want to use strict mode, you will need to provide additional information to the type checker in cases like this. There are a couple of options here:

1. When calling the set constructor, provide explicit specialization such as set[int]().
2. Provide a type annotation for the variable you are assigning the lambda to. Pyright will then use bidirectional type inference to infer the intended type of the set.

b: Callable[[], set[int]] = lambda: set()