Signature

Signature 

 import inspect 
 def foo(a, b, x='blah'): 
   pass 
 print(signature(foo)) 
 # output:
 (a, b, x='blah') 

Annotations are documentation / comment for arguments while decorator transform the function. By itself, Python does not attach any particular meaning or significance to annotations. 

 def foo(a: 'x', b: 5 + 6, c: list) -> max(2, 9):

Combining Python Annotations and Decorators 
https://code.tutsplus.com/tutorials/python-3-function-annotations--cms-25689:

def check_range(f):
    def decorated(*args, **kwargs):
        for name, range in f.__annotations__.items():
            min_value, max_value = range
            if not (min_value <= kwargs[name] <= max_value):
                msg = 'argument {} is out of range [{} - {}]'
                raise ValueError(msg.format(name, min_value, max_value))
        return f(*args, **kwargs)
    return decorated

@check_range 
def foo(a: (0, 8), b: (5, 9), c: (10, 20)): 
    return a * b - c
print(foo(a=4, b=6, c=15) ) 
# 9
print(foo(a=4, b=6, c=105) ) 
# output :
# ValueError: argument c is out of range [10 - 20]

A big bonus Here ! :

def countdown(n:int):
    for i in range(n):
        print(i)


countdown(3.2)
# output :
# TypeError: 'float' object cannot be interpreted as an integer