loop over list of list python

Question

I faced an issue with my code where the loop stops running once it removes the list from the list of list.

data=[["why","why","hello"],["why","why","bell"],["why","hi","sllo"],["why","cry","hello"]]

for word_set in data:
    if word_set[-1]!="hello":
        data.remove(word_set)
print(data)

My desired output is

[['why', 'why', 'hello'], ['why', 'cry', 'hello']]

but the output is

[['why', 'why', 'hello'], ['why', 'hi', 'sllo'], ['why', 'cry', 'hello']]

How do I make the loop go on till the end of the list?

Answer 1

That's because, when you remove the second item (whose index is 1), the items after it move forward. In the next iteration, the index is 2. It should have been pointing to ["why","hi","solo"]. But since the items moved forward, it points to ["why","cry","hello"]. That's why you get the wrong result.

It's not recommended to remove list items while iterating over the list.

You can either create a new list (which is mentioned in the first answer) or use the filter function.

def filter_func(item):
    if item[-1] != "hello":
        return False
    return True

new_list = filter(filter_func, old_list)