title: filter() a dict in python - return just the key
body: <p>I want to filter a dictionary, where I get the keys when they have a specific value, and just want to return the keys from the dict. Here is my code:</p>
  
  <pre><code>def smaller_than(d, value):
      result = filter(lambda x: x[1]&lt;=3, d.items())
      return result
  
  number_key = {35135135: 5, 60103513: 3, 10981179: 2, 18637724 : 4}
  number = smaller_than(number_key, 3)
  print(number)
  </code></pre>
  
  <p>There I get a result like [(60103513, 3), (10981179, 2)], but I just want to get it like [60103513, 10981179]. I think it is a easy answer but I don't know how to do this. </p>