Python: removing objects form lists based on object attribute

Question

I have a list of python objects and I'd like to remove duplicates in the list based on the time value. For example:

class MyClass(object):

    identifier = models.CharField(max_length=128)
    label = models.CharField(max_length=128)
    stat_time = models.DateTimeField(auto_now_add=True)
    def __unicode__(self):
        return str(self.label)

My list may have several instances of MyClass with the same label but different stat_times. I'd like to trim the list and have only one instance of the label with the latest stat_time.

>>> my_list
[MyClass: xxx, MyClass: yyy, MyClass: yyy, MyClass: zzz]

I'd like to end up with:

>>> my_list
[MyClass: xxx, MyClass: yyy, MyClass: zzz]

Here my_list should only contain one instance of MyClass with the 'yyy' label with the latest stat_time.

I hope I have made that clear. Any suggestions much appreciated.

I haven't used Django but I imagine it would be easier to make sure copies aren't added in the first place instead of sanitizing the list afterwards. — kylieCatt
– kylieCatt, Commented Jan 3, 2014 at 16:15
Do you want to filter on __unicode__ or stat_time? As your Q currently stands, it's a bit hard to understand. — Steinar Lima
– Steinar Lima, Commented Jan 3, 2014 at 16:34

SimonT · Accepted Answer · 2014-01-03 16:26:28Z

1

One way you could do it is to create a dict mapping values of label to MyClass instances. You would add each the elements of your list to this dict, but only keep the wanted values.

aDict = dict()
for element in myList:
    s = element.label
    if s not in aDict: # the key is not used yet
        aDict[s] = element
    else:
        aDict[s] = max(aDict[s], element, key = lambda x: x.stat_time)
myList = list(aDict.items()) # iteritems() in Python 2

The lambda expression passed into max tells Python which value to compare when computing the max.

edited Jan 3, 2014 at 16:26

answered Jan 3, 2014 at 16:19

SimonT

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3 Comments

9000 Over a year ago

You can rewrite the max clause like element if element[s].stat_time > aDict[s].stat_time else aDict[s] so that it reads almost like plain English.

Kevin Sheahan Over a year ago

Thanks, however will this not just add the first element encountered?

SimonT Over a year ago

@9000 Good suggestion, I'll keep that in mind for the future.

Steinar Lima · Accepted Answer · 2014-01-03 17:00:42Z

0

I'm not sure if you should filter your object based on __unicode__(), but here is how I would have done it.

unique_objs = []

for o in my_list:
    if (o.__unicode__(), o.stat_time) in unique_objs:
        continue
    new_list.append(o)
    unique_objs.append(tuple(o.__unicode__(), o.stat_time))

edited Jan 3, 2014 at 17:00

answered Jan 3, 2014 at 16:22

Steinar Lima

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4 Comments

Kevin Sheahan Over a year ago

Thanks,. However, that will only get me a unique list of objects based on the name - I need to have a unique list based on name AND latest time for stat_time.

Steinar Lima Over a year ago

@KevinSheahan I changed the answer. Does this suit your needs?

Kevin Sheahan Over a year ago

Thanks. unique_objs.add can only take one argument, and there is no comparison for the latest time but the solution will probably look something like this.

Steinar Lima Over a year ago

@KevinSheahan I updated the answer, it should work now

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