Python 输入，如果 none 给出 return none

Question

是否可以在 python 中编写一个类型提示来保证如果将 None 提供给函数然后返回 None？

例如，这是可能的：

from typing import Dict, Union
def getMaybe(dictionary: Optional[Dict], key: str) -> Optional[str]:
    if dictionary is None:
        return dictionary

    return dictionary.get(key)

但是即使我知道参数有值，类型签名也不能保证输出。例如：

def printer(msg: str):
    print(msg)

data = {'a': 'a'}
result = getMaybe(data, 'a')
printer(result)

报错：

error: Argument of type "str | None" cannot be assigned to parameter "msg" of type "str" in function "printer"
  Type "str | None" cannot be assigned to type "str"
    Type "None" cannot be assigned to type "str" (reportGeneralTypeIssues)

是否可以在类型签名中编码，当 None 作为参数给出时，返回 None？

Answer 1

typing.overload就是你想要的：

from typing import Dict, Union, Optional, overload

@overload
def getMaybe(dictionary: None, key: str) -> None: ...

@overload
def getMaybe(dictionary: Dict, key: str) -> str: ...
    
def getMaybe(dictionary: Optional[Dict], key: str) -> Optional[str]:
    if dictionary is None:
        return dictionary

    return dictionary.get(key)
    

reveal_type(getMaybe(None, "")) # None
reveal_type(getMaybe({}, "")) # str

Python 输入，如果 none 给出 return none

Python typing, if none is given return none

python

python-typing