title: Add Counters keeping zero values
body: <p>I try to sum the value of a key present in other dictionaries with this code:</p>
  <pre><code>import functools
  import operator
  import collections
  
  my_dict = [{'a':0, 'b':1, 'c':5}, {'b':3, 'c':2}, {'b':1, 'c':1}]
  sum_key_value = functools.reduce(operator.add, map(collections.Counter, my_dict))
  
  print(sum_key_value)
  
  # Output
  # Counter({'c': 8, 'b': 5})
  
  </code></pre>
  <p>My question is if I want the output to keep all dictionary keys, even if the key does not appear in all the dictionaries like <code>a</code> in my case, what is the best way to do that without using a loop ?</p>