Python best practices

Instead of:

  tot = x + y

Better:

  sum_ = x + y

And I can't see nothing wrong with:

  freqs = {}
  for c in "abracadabra":
      try:
          freqs[c] += 1
      except KeyError:
          freqs[c] = 1

Or:

  nested = [[1, 2, 3], [4], [5, 6]]
  flattened = sum(nested, [])

It is worth to mention dict.setdefault():

  indices = {}
  for i, c in enumerate("abracadabra"):
      indices.setdefault(c, []).append(i)