Any and typevar make the automatic completion of IDE better

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author ：kingname

source ： Unheard of Code

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It is necessary to read this article 6 minute .

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I believe many students are writing Python When , Will use type annotations to improve code readability , It also helps IDE Realize automatic completion .

Suppose we now have an object , This object can be a list or a generator , I write a function , Get its first element . The code is simple ：

from typing import Iterator
from contextlib import suppress


class People:
    def __init__(self, name):
        self.name = name

    def eat(self):
        print(f'{self.name} I am eating ')

    def walk(self):
        print(f'{self.name} Walking ')


def get_first_element(ele_list):
    if isinstance(ele_list, list):
        return ele_list[0] if ele_list else None
    if isinstance(ele_list, Iterator):
        with suppress(Exception):
            value = next(ele_list)
            return value
    return None


if __name__ == '__main__':
    kingname = People('kingname')
    pm = People('pm')
    people_list = [kingname, pm]

    obj = get_first_element(people_list)
    if obj:
        print(obj.)

The code is written , But when I get the first element , When you want to print the data in it , I found that I had forgotten People What properties does this class have , And then PyCharm The automatic completion of is also invalid , I had to turn the code back , Go looking for People Defined location , Very inefficient . As shown in the figure below .

If we use type notation , We can solve this problem ：

This general usage , Everyone must know .

Now comes the question , We have nothing but People class , also Cat class , And the elements in the list may be all People Class , Maybe it's all Cat Class instance , What about this situation ？

First you have the first problem ,get_first_element How to write the type label of the parameter of ？

You might write like this ：

def get_first_element(ele_list: Union[List[Union[People, Cat]], Iterator[Union[People, Cat]]])

If there is another one Dog Class? ？

To simplify the operation , You may use Any, type , therefore get_first_element It's like this ：

def get_first_element(ele_list: Union[List[Any], Iterator[Any]]) -> Optional[Any]:
    if isinstance(ele_list, list):
        return ele_list[0] if ele_list else None
    if isinstance(ele_list, Iterator):
        with suppress(Exception):
            value = next(ele_list)
            return value
    return None

Now you find the problem again ,PyCharm The automatic completion of is broken again . because Any It's any kind of , So before the code runs , It doesn't know what you're returning . As shown in the figure below ：

In this case , You need to use Python In type annotations Generic 了 . We know , Generics are a concept in static languages ,Python Because of the use of type annotations , There are also types . So I borrowed this concept .

Let's see how to use it ：

from typing import TypeVar

T = TypeVar('T')

Notice the variable name here T and TypeVar Parameters of 'T' It can be written as any string at the same time , But the variable name should be consistent with the parameter . for example ：

GodType = TypeVar('GodType')

And then put T As Any Just use the same . Let's see the effect ：

You can see ,PyCharm It can be completed automatically again . Use TypeVar, You can tell PyCharm, The returned type is the same as that in the passed in parameter T The type of corresponding position shall be consistent . For example, in the passed in parameter ,T stay List[T] perhaps Generator[T] in , Therefore, the returned parameters should be consistent with the elements in the list or the element types in the generator .

We use it Cat Generator to test , Discovery can also be completed automatically ：

There's more , If my list has Cat Example , And then there is People What about the instance of ？ This is the time ,PyCharm I will list all the possible complements of the two examples ：

End

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