How to use reveal_type in mypy

Question

I have read that I can reveal the type of variables by using a function called reveal_type, but I can't find how to use it or from where to import it.

Answer 1

I found out in the end how to use it: You should just put and use the reveal_type in the code, and run it with the mypy program. Then, it will log a message that look like this:

Revealed type is 'builtins.str*'

From the mypy documentation:

reveal_type is only understood by mypy and doesn’t exist in Python, if you try to run your program. You’ll have to remove any reveal_type calls before you can run your code. reveal_type is always available and you don’t need to import it.

For more reading: here.

Answer 2

To add to the current answer, you can use the TYPE_CHECKING flag from typing to have Python skip code that is only for mypy, or to have mypy skip code you don't want it to check.

# myfile.py
from typing import TYPE_CHECKING


a = 'spam' and 5
print(a)

if TYPE_CHECKING:
    reveal_type(a)  # python skips this line
else:
    a = 1/2  # mypy can't see this type error
print(a)

❯ python3 myfile.py 
5
0.5

❯ mypy myfile.py 
myfile.py:8: note: Revealed type is "Union[builtins.str, builtins.int]"
Success: no issues found in 1 source file