pyhton list - remove dictionares with certain key-CodePudding

When running this python code:

shapes = [{"label": "bad_stuff"}, {"label": "cat"}, {"label": "cat"},{"label": "cat"}, {"label": "bad_stuff"}, {"label": "bad_stuff"}]
for elem in shapes:
    if elem['label'] == "bad_stuff":
        shapes.remove(elem)

... I get this result:

[{'label': 'cat'}, {'label': 'cat'}, {'label': 'cat'}, {'label': 'bad_stuff'}]

Why does not the code remove the last element in the list and how can I solve it?

CodePudding user response：

Because you are modifying list during for loop. Let's see what's happening in your code:

On 1st interation it removes shapes[0] item. 2, 3, 4 iterations do not satisfy condition, so passed. On 5th iteration the elem is {"label": "bad_stuff"}, and your shapes looks like this:

[{"label": "cat"}, {"label": "cat"},{"label": "cat"}, **{"label": "bad_stuff"}**, {"label": "bad_stuff"}]

So when you remove(elem) on this step, it removes last item in the list. And since you removed last item - iteation stops.

There is solution to your problem:

shapes = [{"label": "bad_stuff"}, {"label": "cat"}, {"label": "cat"},{"label": "cat"}, {"label": "bad_stuff"}, {"label": "bad_stuff"}]

cat_shapes = []
for elem in shapes:
    if elem['label'] != "bad_stuff":
        cat_shapes.append(elem)

Another solution mentioned in comments by DanielB :

cat_shapes = [elem for elem in shapes if elem['label'] != "bad_stuff"]

CodePudding user response：

You can do this with filter,

list(filter(lambda x:x['label'] != 'bad_stuff', shapes))
# [{'label': 'cat'}, {'label': 'cat'}, {'label': 'cat'}]