Python counting item's appearances in list

Question

Given

listEx = ['cat', 'dog', 'cat', 'turtle', 'apple', 'bird', 'bird']

for i in listEx:
    if listEx.count(i) > 1:
        print "this item appears more than once", i
    else:
        print "this item appears only once", i

I want it to print that cat and bird appear more than once (or just produce ['cat', 'bird']). How can I do this?

Answer 1

The collections.Counter tool makes this sort of task really easy:

>>> from collections import Counter
>>> listEx = ['cat', 'dog', 'cat', 'turtle', 'apple', 'bird', 'bird']
>>> [k for k, cnt in Counter(listEx).items() if cnt > 1]
['bird', 'cat']

Answer 2

>>> [v for v, r in itertools.groupby(sorted(listEx)) if len(list(r)) > 1]
['bird', 'cat']

Answer 3

All previous answers are great. I just wanted to point out, that you could also simply use a dictionary to count your items:

>>> listEx = ['cat', 'dog', 'cat', 'turtle', 'apple', 'bird', 'bird']
>>> d = dict()
>>> for v in listEx: d[v] = d[v] + 1 if v in d else 1
...
>>> d
{'turtle': 1, 'bird': 2, 'apple': 1, 'dog': 1, 'cat': 2}
>>> for v in d:
...     if d[v] > 1:
...             print v
...
bird
cat
>>>

Answer 4

You can also use a collections.defaultdict to do the counting:

from collections import defaultdict

animals = ["cat", "dog", "cat", "turtle", "apple", "bird", "bird"]

animal_counts = defaultdict(int)
for animal in animals:
    animal_counts[animal] += 1

print([animal for animal, count in animal_counts.items() if count > 1])
# ['cat', 'bird']

Answer 5

[word for word in set(listEx) if listEx.count(word) > 1]