What do you call this type of parameter?

[league55]

In some code I've seen stuff like this:

def some_function(a -> int):
    #do something

I thought a -> int was called a decorator, but a decorator seems to be this:

def some_function(func):
    def some_func(*args):
        #do something to func
        return func(*args)
    return some_func

@some_function
def some_other_function(params):
    #do something

So if a -> int is not a decorator, what's it called and what does it do?

[snippsat]

It's type hints added in Python 3.5.
So it's hints and not forced yet maybe in 3.7.

So if run in Python 3.6 no error massage is generated,
have to use 3-party library like mypy to get a error message about wrong types.
Example:
hello.py

def greeting(name: str) -> str:
    return f'Hello {name}'

print(greeting('Kent'))

hello_int.py

def greeting(name: str) -> str:
    return f'Hello {name}'

print(greeting(100))

With mypy pip install mypy

E:\1
λ mypy hello.py
# No error

E:\1
λ mypy hello_int.py
hello_int.py:4: error: Argument 1 to "greeting" has incompatible type "int"; expected "str"

[league55]

Can I use the type hints to do type checking without mypy? (Or I guess I can actually try it to find out haha.)