Assume that I have a function which converts Python data-types to Postgres data-types like this:
def map_type(input):
if isinstance(input, int):
return MyEnum(input)
elif isinstance(input, str):
return MyCustomClass(str)
I could type-hint this as:
def map_type(input: Union[int, str]) -> Union[MyEnum, MyCustomClass]: ...
But then code like the following would fail to type-check even though it is correct:
myvar = map_type('foobar')
print(myvar.property_of_my_custom_class)
Complete example (working code, but errors in type-hinting):
from typing import Union
from enum import Enum
class MyEnum(Enum):
VALUE_1 = 1
VALUE_2 = 2
class MyCustomClass:
def __init__(self, value: str) -> None:
self.value = value
@property
def myproperty(self) -> str:
return 2 * self.value
def map_type(value: Union[int, str]) -> Union[MyEnum, MyCustomClass]:
if isinstance(value, int):
return MyEnum(value)
elif isinstance(value, str):
return MyCustomClass(value)
raise TypeError('Invalid input type')
myvar1 = map_type(1)
print(myvar1.value, myvar1.name)
myvar2 = map_type('foobar')
print(myvar2.myproperty)
I'm aware that I could split up the mapping into two functions, but the aim is to have a generic type-mapping function.
I was also thinking about working with classes and polymorphism, but then how would I type-hint the topmost class methods? Because their output type would depend on the concrete instance type.
See Question&Answers more detail:
os 与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…