Dictionary error: cannot convert dictionary update sequence element #0 to a sequence

I’m trying to count the frequency from a list. I received an type error

mylist = [1, 1, 1, 2, 3, 3, 4]
def frequency(x):
    counts = dict()
    if x in counts:
        counts[x] += 1
    elif len(counts) < 3:
        counts.update([x, 1])            
    return counts

for x in mylist:
    frequency(x)

I received TypeError: cannot convert dictionary update sequence element #0 to a sequence on counts.update([x, 1]). I checked what this error is about, but I cannot see I violated it.

>Solution :

Fix

The counts should be created outside, so before, if you create it in the method, that will be new one each time

You may update a dict with another dict, not a list

counts.update({x: 1})

Improve logic

• simply do counts[x] = 1 instead, that’s cheaper
• it woul be better that the method does the whole frequency computation, from a list

counts = {}
def frequency(values):
    for x in values:
        if x in counts:
            counts[x] += 1
        else:
            counts[x] = 1
    return counts

mylist = [1, 1, 1, 2, 3, 3, 4]
counts = frequency(mylist)
print(counts)  # {1: 3, 2: 1, 3: 2, 4: 1}

Improve code

There is method that could help you get nicer code

• collections.defaultdict that handles if the key isn’t present (set 0, as we told him int)

  def frequency(values):
      counts = defaultdict(int)
      for x in values:
          counts[x] += 1
      return counts
  

• the master of all collections.Counter

  def frequency(values):
      return Counter(values)