for i in range(10):
i += 1
print(i)
is equivalent to
iterator = iter(range(10))
try:
while True:
i = next(iterator)
i += 1
print(i)
except StopIteration:
pass
The iterator that iter(range(10)) produces will yield values 0, 1, 2... 8 and 9 each time next is called with it, then raise StopIteration on the 11th call.
Thus, you can see that i gets overwritten in each iteration with a new value from the range(10), and not incremented as one would see in e.g. C-style for loop.
