Why is this python generator returning the same value everytime?

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You are reusing the same list object. Your generator returns the one object over and over again, manipulating it as it goes, but any other references to it see those same changes:

>>> r = list(gen())
>>> r
[[4], [4], [4], [4], [4]]
>>> r[0] is r[1]
True
>>> r[0][0] = 42
>>> r
[[42], [42], [42], [42], [42]]

Yield a copy of the list or create a new fresh list object instead of manipulating one.

def gen_copy():
    state = [None]

    for i in range(5):
        state[0] = i
        yield state.copy()  # <- copy

def gen_new():
    for i in range(5):
        state = [i]  # <- new list object every iteration
        yield state

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