How to specify multiple return types using type-hints

问题

I have a function in python that can either return a bool or a list. Is there a way to specify the return types using type hints.

For example, Is this the correct way to do it?

def foo(id) -> list or bool:
      ...

回答1:

From the documentation

class typing.Union

Union type; Union[X, Y] means either X or Y.

Hence the proper way to represent more than one return data type is

from typing import Union


def foo(client_id: str) -> Union[list,bool]

---

But do note that typing is not enforced. Python continues to remain a dynamically-typed language. The annotation syntax has been developed to help during the development of the code prior to being released into production. As PEP 484 states, "no type checking happens at runtime."

>>> def foo(a:str) -> list:
...     return("Works")
... 
>>> foo(1)
'Works'

As you can see I am passing a int value and returning a str. However the __annotations__ will be set to the respective values.

>>> foo.__annotations__ 
{'return': <class 'list'>, 'a': <class 'str'>}

---

Please Go through PEP 483 for more about Type hints. Also see What are Type hints in Python 3.5?

Kindly note that this is available only for Python 3.5 and upwards. This is mentioned clearly in PEP 484.

回答2:

The statement def foo(client_id: str) -> list or bool: when evaluated is equivalent to def foo(client_id: str) -> list: and will therefore not do what you want.

The native way to describe a "either A or B" type hint is Union (thanks to Bhargav Rao):

def foo(client_id: str) -> Union[list, bool]:

I do not want to be the "Why do you want to do this anyway" guy, but maybe having 2 return types isn't what you want:

If you want to return a bool to indicate some type of special error-case, consider using Exceptions instead. If you want to return a bool as some special value, maybe an empty list would be a good representation. You can also indicate that None could be returned with Optional[list]

来源：https://stackoverflow.com/questions/33945261/how-to-specify-multiple-return-types-using-type-hints