Why does this python generator act like this?

Question

def upto(n):
    items = []
    for i in range(n):
        items.append(i)
        yield items

print(list(upto(2)))

Why is the print output [[0,1],[0,1]]? When I call next on upto(2) twice, it yields [0] the first time and [0,1] the second time, so shouldn't the result be [[0], [0,1]]?

Answer 1

The thing is,you are referencing the same list(list work by reference) when trying to call list method on a generator. the list you are yielding is updated each time you loop through.so when you list up the generator it gets the updated one. for getting what you want you could do one of the following two things,

1. yield a new list every time you loop through, like yield items[:]
2. iterate through the returned generator, like for i in upto(2): print(i) # here,each time it gets the current one

Answer 2

In [1]:

def upto(n):
    items = []
    for i in range(n):
        items.append(i)
        yield items.copy()

list(upto(2))

Out[1]: [[0], [0, 1]]

Answer 3

you need to yield a copy try yield items[:]