title: How to count occurrences of key in list of dictionaries
body: <p>I know this is a frequently asked question, however I do not have access to the Counter module as I'm using v2.6 of Python. I want to count the number of time a specific key appears in a list of dictionaries.</p>
  
  <p>If my dictionary looks like this:</p>
  
  <pre><code>data = [{'a':1, 'b':1}, {'a':1, 'c':1}, {'b':1, 'c':1}, {'a':1, 'c':1}, {'a':1, 'd':1}]
  </code></pre>
  
  <p>How would I find out how many times "a" appears? I've tried using len, but that only returns the number of values for one key.</p>
  
  <pre><code>len(data['a'])
  </code></pre>