在 Python 中的 function 定义上声明多个返回类型

Question

I've being having some trouble when commentating functions or methods that returns multiple variables as example:

def separate_dfs(dataframe: pd.DataFrame) -> ?:

    there_is_only_one_column = False
    df1 = dataframe["column1"]
    
    try:
       df2 = dataframe["column2"]
    except KeyError:
       df2 = None
       there_is_only_one_column = True
    
    return df1, df2, there_is_only_one_column

In the example, the returned variables can be of 3 types: pd.DataFrame, None and bool. But i couldn't find anywhere in the pep8 guide that explained how to handle that.

I also don't know what to do when one variable can be more than one type (like df2 that might be pd.DataFrame or None).

Answer 1

The exact return type would be

from typing import Optional, Tuple
def separate_dfs(dataframe: pd.DataFrame) -> Tuple[pd.DataFrame, Optional[pd.DataFrame], bool]
...

When you are using Python > 3.8, You can use the builtin tuple to write your annotation. When you are using Python 3.10 you can write it even more compact as

def separate_dfs(dataframe: pd.DataFrame) -> tuple[pd.DataFrame, pd.DataFrame | None, bool]
...