Argument type is partially unknown for map

[DanielPechersky]

Environment data

• Language Server version: 2021.6.0
• OS and version: darwin x64
• Python version (and distribution if applicable, e.g. Anaconda): python3.10.0b2
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: strict

Expected behaviour

No errors, type checker should know that map(list, list[list[X]]) -> list[list[X]]

Actual behaviour

Argument type is partially unknown for map(list, lst)

Code Snippet / Additional information

lst: list[list[int]] = [[1,2,3], [4]]
lst2: list[list[int]] = list(map(list, lst))
#                            ^^^^^^^^^^^^^^
# Argument type is partially unknown
#   Argument corresponds to parameter "iterable" in function "__init__"
#   Argument type is "map[list[Unknown]]" Pylance(reportUnknownArgumentType)

[erictraut]

This behavior is a known limitation of bidirectional type inference. In the first case (with variable lst), the type engine is able to use the type annotation on the LHS (left-hand side) of the assignment to inform the inferred type of the RHS expression. In the second case (with variable lst2), this cannot be done because bidirectional type inference cannot be applied to argument expressions used within a call expression. Therefore, the inferred type of map(list, lst) is list[unknown].

In cases like this, you have several options:

1. Explicitly specialize the list argument to the map call:

lst2 = list(map(list[int], lst))

2. Use a temporary variable with an annotation:

lst2_temp: map[list[int]] = map(list, lst)
lst2 = list(lst2_temp)

3. Use a cast (not a great option because it overrides type checker logic):

lst2 = list(cast(map[list[int]], map(list, lst)))

4. Use a "# type: ignore" comment to suppress the error (also not a great option).

lst2: list[list[int]] = list(map(list, lst)) # type: ignore

[DanielPechersky]

Alright, that sounds good. Specializing list sounds like the best approach here.

[DanielPechersky]

Actually, why doesn't specialization work for this case?

lst = [[1,2,3],[4]]
tup: tuple[tuple[int, ...], ...] = tuple(map(tuple[int, ...], lst))
#                                        ^^^^^^^^^^^^^^^^^^^^^^^^^
# No overloads for "__init__" match the provided arguments
#   Argument types: (Type[tuple[int, ...]], list[list[int]]) Pylance(reportGeneralTypeIssues)

[erictraut]

That's a bug! Thanks for reporting it. This will be fixed in the next release.

[jakebailey]

This issue has been fixed in version 2021.6.1, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202161-9-june-2021