TypeVar defined within if/else

Question

While this code works:

from typing import TypeAlias, TypeVar

T = TypeVar("T", complex, float, str)

z: TypeAlias = tuple[T, ...] | list[T]

defining T conditionally does not.

from typing import Generic, TypeAlias, TypeVar

try:
    import somepackage
    use_complex = True
except:
    use_complex = False

if use_complex:
    T = TypeVar("T", complex, float, str)
else:
    T = TypeVar("T", float, str)

z: TypeAlias = tuple[T, ...] | list[T]

I am getting: Variable not allowed in type expression

Is there a way to tell the typechecker that use_complex is a constant and therefore it is either one branch or the other, T is defined only once and does not change?

Answer 1

mypy and Pyright¹ both support defining compile-time constants to enable control flow analysis like this; the behaviour is similar to sys.version_info, sys.platform, or typing.TYPE_CHECKING guards.

Put the following in your mypy configuration file:

[mypy]

# Only specify ONE of the following:

# * When developing against the package
always_true = use_complex

# * When not developing against the package
always_false = use_complex

Then do one of the following:

• Define the constant in a dedicated module

  # constants.py
  
  try:
      import somepackage
      use_complex = True
  except:
      use_complex = False
  

  >>> import constants
  >>>
  >>> if constants.use_complex: ...
  ...     ...
  

• Define the constant in the same module and use it. This is the same as your example in the question (mypy playground demo: always_true = use_complex, always_false = use_complex)

  try:
       import somepackage
       use_complex = True
  except:
       use_complex = False
  
  if use_complex:
       T = TypeVar("T", complex, float, str)
  else:
       T = TypeVar("T", float, str)
  
  z: TypeAlias = tuple[T, ...] | list[T]
  
  num: z[complex]  = [8j]  # OK when `always_true = use_complex`, else errors when `always_false = use_complex`
  

During development, you'd just change your configuration file to switch between the use_complex modes.

Note that the try...except doesn't actually do anything for mypy here, that's only for your runtime behaviour.

---

1. See the defineConstant configuration of https://github.com/microsoft/pyright/blob/main/docs/configuration.md