Calculating the mode in a multimodal list in Python

Question

I'm trying to calculate the mode (most frequent value) of a list of values in Python. I came up with a solution, which gave out the wrong answer anyway, but I then realised that my data may be mutlimodal;

ie 1,1,2,3,4,4 mode = 1 & 4

Here is what I came up with so far:

def mode(valueList):
  frequencies = {}
  for value in valueList:
    if value in frequencies:
      frequencies[value] += 1
    else:
      frequencies[value] = 1
  mode = max(frequencies.itervalues())
  return mode

I think the problem here is that I'm outputting the value rather than the pointer of the maximum value. Anyway can anyone suggest a better way of doing this that could work where there is more than one mode? Or failing that how I can fix what I've got so far and identify a single mode?

As you can probably tell I'm very new to python, thanks for the help.

edit: should have mentioned I'm in Python 2.4

Answer 1

In Python >=2.7, use collections.Counter for frequency tables.

from collections import Counter
from itertools import takewhile

data = [1,1,2,3,4,4]
freq = Counter(data)
mostfreq = freq.most_common()
modes = list(takewhile(lambda x_f: x_f[1] == mostfreq[0][1], mostfreq))

Note the use of an anonymous function (lambda) that checks whether a pair (_, f) has the same frequency as the most frequent element.

Answer 2

Note that starting in Python 3.8, the standard library includes the statistics.multimode function to return a list of the most frequently occurring values in the order they were first encountered:

from statistics import multimode

multimode([1, 1, 2, 3, 4, 4])
# [1, 4]