for循环中的函数是否被多次调用？

Question

I want to know if this loop call multiple times "hello".upper() (or any other method/function) while iterating:

for i in "hello".upper():  
    #DO SOMETHING  

Considering that "hello" doesn't change in the loop, would this be better performance wise?:

string = "hello".upper()  
for i in string:  
    #DO SOMETHING  

Answer 1

The loop target is evaluated only once, after which an iterator is obtained. Then, next() on that iterator is called for every iteration of the loop.

You can think of for a in b as:

_iter = iter(b)  # Evaluates b only ONCE
while True:
    try:
        a = next(_iter)
    except StopIteration:
        break
    # loop body ...

Answer 2

The argument to the for statement is evaluated once. So it won't be executed multiple times.