Deduced actual type of variable not used in lambda context

[oldium]

Environment data

• Language Server version: 2021.2.4 (pyright a18e6fb4)
• OS and version: Windows 10
• Python version (& distribution if applicable, e.g. Anaconda): Python 3.4.1, typing 3.6.2 (from PyPi)

Expected behaviour

Deduced actual type (reduced by isinstance's) is recognised in lambda context.

Actual behaviour

When typed argument is used inside lambda, the original argument type is used instead of the one reduced by isinstance's.

Warning:

(parameter) value: int | (*args: Unknown, **kwargs: Unknown) -> Any
Object of type "int" is not callablePylancereportGeneralTypeIssues)

Logs

This is reported from a company notebook, so I cannot provide logs (data collection restrictions).

Code Snippet / Additional information

This fails, see the last line and a value() call.:

from typing import Callable
def int_to_value(value: 'int | Callable') -> 'dict':
  if isinstance(value, int):
    return dict(hex="0x{:X}".format(value))
  else:
    return dict(hex=lambda: "0x{:X}".format(value()))

Please note that the following works correctly:

from typing import Callable
def int_to_value(value: 'int | Callable') -> 'dict':
  if isinstance(value, int):
    return dict(hex="0x{:X}".format(value))
  else:
    return dict(hex="0x{:X}".format(value()))

[erictraut]

Type narrowing is not applied to variables captured within a lambda context because captured symbols can be reassigned a new value (with a different type) by the time the lambda is executed. In this particular case, it is safe, but in general a type checker cannot make this assumption.

The workaround is to assign the narrowed value to a local variable that has a non-union callable type.

from typing import Callable
def int_to_value(value: 'int | Callable') -> 'dict':
  if isinstance(value, int):
    return dict(hex="0x{:X}".format(value))
  else:
    callable_value = value
    return dict(hex=lambda: "0x{:X}".format(callable_value()))

[oldium]

Perfect, thanks for really fast response, it works fine with this workround.

[erictraut]

I found a way to handle type narrowing of local variables and parameters captured by inner-scoped functions and lambdas, so the above workaround will no longer be necessary in this case.

[jakebailey]

This issue has been fixed in version 2021.9.4, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202194-29-september-2021

[oldium]

Works great. Thanks!