title: How to filter a nested dictionary (pythonic way) for a specific value using map or filter instead of list comprehensions?
body: <p>I've a nested dictionary.</p>
  
  <pre><code>&gt;&gt;&gt; foo = {'m': {'a': 10}, 'n': {'a': 20}}
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>I'd like to filter specific values, based on the values of 'a'.</p>
  
  <p>I can use list comprehensions for the purpose.</p>
  
  <pre><code>&gt;&gt;&gt; [foo[n] for n in foo if foo[n]['a'] == 10]
  [{'a': 10}]
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Using list alone gives me the elements from foo (and not the values of the elements) - as expected:</p>
  
  <pre><code>&gt;&gt;&gt; list(filter(lambda x: foo[x] if foo[x]['a']==10 else None,foo))
  ['m']
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Using map returns me unwanted 'None' values:</p>
  
  <pre><code>&gt;&gt;&gt; list(map(lambda x: foo[x] if foo[x]['a']==10 else None,foo))
  [{'a': 10}, None]
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Combining these two, I can fetch the desired value. But I guess foo is iterated twice - once each for filter and map. The list comprehension solution needs me to iterate just once.</p>
  
  <pre><code>&gt;&gt;&gt; list(map(lambda t: foo[t], filter(lambda x: foo[x] if foo[x]['a']==10 else None,foo)))
  [{'a': 10}]
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Here's another approach using just filter. This gives me the desired values but I'm not sure if iterating over values of a dictionary is a good/pythonic approach:</p>
  
  <pre><code>&gt;&gt;&gt; list(filter(lambda x: x if x['a'] == 10 else None, foo.values()))
  [{'a': 10}]
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>I'd like to know if:</p>
  
  <ol>
  <li>What's the pythonic/recommended approach for this scenario?</li>
  <li>If the last example using filter on dictionary values is an acceptable?</li>
  </ol>
  
  <p>Regards</p>
  
  <p>Sharad</p>