Is there a way to mark variables as constant / unchangeable in pyright?

Question

I tried googling but I haven't come across a way to use a type annotation to mark a variable as constant in pyright. I think the type Constant is more like scalar constant rather than the C/C++ keyword const.

Any thoughts appreciated.

Thanks.

Answer 1

Since Python 3.8, you can type-hint a variable as Final(API docs, PEP591), to determine that it should not be changed beyond this point.

# type1.py
from typing import Final
foo: Final = 1
foo = 2 # raises an error

$ mypy type1.py --pretty
type1.py:3: error: Cannot assign to final name "foo"  [misc]
    foo = 2
    ^~~~~~~
Found 1 error in 1 file (checked 1 source file)

You can also add more type information by enclosing the type in square brackets: bar: Final[float] = 2