[Python] When does python 3 execute the beginning of a generator function?

Python 3 seems not to execute the beginning of a generator function, from its first line up to the first yield at the time when the function is first called. Python seems to defer the initial execution until the first time next(agenerator) is invoked. Is that correct? How does python know to do that?

Example:

def fib():
print("Executing the top of fib")
a, b = 0, 1
while True:
yield a
a, b = b, a + b


usage:

f=fib()
print("called fib()")
print([next(f) for i in range( 5)])


result:

called fib()
Executing the top of fib
[0, 1, 1, 2, 3]

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