python3 TypeVar(“A”, bound="") 怎么理解

A nice question about Python type hints! 😊

In Python 3, TypeVar is a way to create a type variable, which is a special kind of type that can be used as a placeholder for a specific type. The syntax you mentioned, TypeVar("A", bound="<ClassName>"), is used to create a type variable A that is bounded by a specific class ClassName.

Here’s a breakdown of the syntax:

• TypeVar: This is a function from the typing module that creates a type variable.
• "A": This is the name of the type variable. You can choose any valid Python identifier as the name.
• bound="<ClassName>": This specifies the bound of the type variable. In this case, A is bounded by the class ClassName, which means that A can only be instantiated with a type that is a subclass of ClassName.

In other words, TypeVar("A", bound="<ClassName>") means that A is a type variable that can only be replaced with a type that is a subclass of ClassName.

Here’s an example to illustrate this:

from typing import TypeVar

class Animal: pass

class Dog(Animal): pass
class Cat(Animal): pass

A = TypeVar("A", bound="Animal")

def process_animal(a: A) -> A:
    return a

my_dog: Dog = Dog()
result: Dog = process_animal(my_dog)  # OK!

my_cat: Cat = Cat()
result: Cat = process_animal(my_cat)  # OK!

my_int: int = 5
result: int = process_animal(my_int)  # Error! int is not a subclass of Animal

In this example, A is a type variable bounded by Animal. The process_animal function takes an argument of type A and returns an object of the same type. Because Dog and Cat are subclasses of Animal, we can pass instances of these classes to the process_animal function, and the type checker will be happy.

However, if we try to pass an int object to the function, the type checker will raise an error, because int is not a subclass of Animal.

By using TypeVar with a bound, we can create more flexible and reusable functions that work with a range of related types, while still maintaining type safety.