title: Update dictionary and append if key exists
body: <p>I've got a list of dictionarys.</p>
  
  <pre><code>list_of_dicts = [{ a: 1, b:f, c:3}, {a: y, b:q, c:z, d: 1}, ... ] 
  </code></pre>
  
  <p>Now i want to create a new dictionary which looks like: </p>
  
  <pre><code>newDict = { a: [1,y], b: [f,q], c: [3,z], d:[1]}
  </code></pre>
  
  <p>I know i could could make a double for loop, but that is rather slow since I'm dealing with large objects (mostly NumPy arrays) in the dictionaries.</p>
  
  <pre><code>newDict = {}
  for l in list_of_dicts: 
      for k, v in l.items():
          if k in newDict: 
             newDict.append(v)
          else:
             newDict[k] = [v]
  </code></pre>
  
  <p>How to do this faster?</p>