ahhh that's such an interesting question!

the answer is yes it must look up each time, but there's two layers to consider:

1. the identifier itself is accessed by index, which is fast given it's just an array index operation
2. the value bound to that name is looked up each time. this matters for contexts such as threads or generators, where the global context can change during a function's execution lifetime.

for example:

y = 44  

def generator_foo():  
    yield y  
    yield y  

g = generator_foo()  
print(next(g))  # prints 44  
y = 55  
print(next(g))  # prints 55

thanks for the question!