在python中用格式变量声明变量的目的是什么：class type =？

Question

I am new to programming in general. I would like to know why this symbol is being used in a few codes. What I mean is, what is the purpose of this way of declaring a variable? Is it useful?

For example let's say I define

say_hello1 = "hello everybody"

I could also define the same variable using:

say_hello2 : str = "hello everybody"

Or ..

say_hello3 : float = "hello everybody" # I know this is not a float variable, I'm doing it on purpose

Each variable (say_hello1, say_hello2 and say_hello3 ) will be the same type (string). So what is the purpose of this way of declaring variables? Is it just to say that the variable will be a specific type? Or does it change anything else (ie the efficency of the code)?

Thanks in advance.

Answer 1

It shows what variable you are expecting, which makes your code a lot easier to read and understand. It can also help with linters and other third party tools. Python itself won't look at it afaik.

Take a look here: https://docs.python.org/3/library/typing.html

Answer 2

Python is not a strongly-typed language. The variable : type is just for readability, does not declare strongly typed variables. Later you can change the value, even in another type.

Answer 3

Type hints were introduced in PEP-484 . The goal is to allow additional tools to do static linting based on the acceptable types for a variables. This is included for example in PyCharm, and is used by mypy .

But anyway, Python is a dynamic language, and the interpretor just ignores those declared type hints.

Answer 4

In this case the type is pretty obvious so it is not very helpful but assume that you have a base class BaseClass that some other classes inherit SubClassA and SubClassB , then:

a: SubClassA = SubClassA()  # not very useful
a: BaseClass = SubClassA()  # indicates that you expect a given interface, no necessarily SubClassA

### another dev can safely replace code later on
a: BaseClassA = SubClassB()