[FIXED] Type annotations: tuple type vs union type

Issue

def func(df_a: pd.DataFrame, df_b: pd.DataFrame) -> (pd.DataFrame, pd.DataFrame):

Pylance is advising to modify this line with two solution proposed. What would be the pros and cons of each one if there is any significant difference?

Tuple expression not allowed in type annotation

Use Tuple[T1, ..., Tn] to indicate a tuple type or Union[T1, T2] to indicate a union type

Solution

They mean different things:

• Tuple[A, B, C] means that your functions returns a three-element tuple with the A B C data types:

def f() -> Tuple[str, int, float]:
    return 'hello', 1, 3.33

• Union[A, B] means that your function returns an object of either A or B data type:

import random 

def f() -> Union[str, int]:
    if random.random() > 0.5:
        return 'hello'
    else:
        return 10

In your case, it looks like you want to use Tuple[pd.DataFrame, pd.DataFrame].

Answered By - jfaccioni