Can't update value of list element in python

Question

Can someone explain why the below mentioned behavior happens, in debug mode, why cannot I update a list element value:

I don't get, what I'm doing wrong? My code:

 if request.method == 'GET':
        coordinates = mongo_harassments_utils.get_geolocated({})
        count =  coordinates.count()
        for i in range(coordinates.count()):
            first = coordinates[i]["story"]
            coordinates[i]["story"] = "Test"
            second = coordinates[i]["story"]

@idjaw I did that only for reasons that in that picture you can see the values on debug mode — EgzEst
– EgzEst, Commented Mar 11, 2016 at 13:03
@Lafexlos as u can see in the picture, u see that even after I assign a string value "Test" in coordinates[i]['story'] the value still remain the same — EgzEst
– EgzEst, Commented Mar 11, 2016 at 13:06
Yeah, I saw that later and that's why I removed my comment but seems like I was slow. — Lafexlos
– Lafexlos, Commented Mar 11, 2016 at 13:07

Sergei Lebedev · Accepted Answer · 2016-03-11 13:11:43Z

1

In your example coordinates is not a list, but a pymongo.cursor.Cursor. You need to explicitly coerce it to a list for the code to work:

if request.method == 'GET':
    coordinates = list(mongo_harassments_utils.get_geolocated({}))
    count = len(coordinates)
    for i in range(len(coordinates)):
        first = coordinates[i]["story"]
        coordinates[i]["story"] = "Test"
        second = coordinates[i]["story"]

Also, explicit indexing is often considered an anti-pattern in Python. For your case enumerate is perfectly applicable

for i, coordinate in enumerate(coordinates):
    first = coordinate["story"]
    coordinate["story"] = "Test"
    second = coordinate["story"]

Note that with enumerate you no longer need to make coordinates a list.

edited Mar 11, 2016 at 13:11

answered Mar 11, 2016 at 13:05

Sergei Lebedev

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Comments

pdepmcp · Accepted Answer · 2016-03-11 13:09:41Z

0

I suspect that the issue is that it's not a list. Not all iterables are lists, and in your particular situation "coordinates" looks like a mongodb cursor.

answered Mar 11, 2016 at 13:09

pdepmcp

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