title: Python: updating a large dictionary using another large dictionary
body: <p>I am trying to update some values of a large dictionary using values from another dictionary where they have similar keys (the same date but in a different format). The process I'm currently using is too slow and I want to reduce the bottleneck.  </p>
  
  <p>This is my current solution (it writes the updated dict to a file):</p>
  
  <pre><code>from dateutil import parser
  File = open(r'E:Test1.txt','w')
  
  b = {'1946-1-1':0,..........,'2012-12-31':5}
  d = {'1952-12-12':5,........,'1994-7-2':10}
  
  for key1, val1 in b.items():
      DateK1 = parser.parse(key1)
      Value = val1
      for key2, val2 in d.items():
          DateK2 = parser.parse(key2)
          if DateK1 == DateK2:
              d[key2] = Value        
  
  Order= sorted(d.items(), key=lambda t: t[0])
  
  for item in Order:
      File.write('%s,%s\n' % item)
  File.close()
  </code></pre>