title: pandas: filter out column values containing empty list
body: <p>I have the following data frame <code>my_df</code>:</p>
  
  <pre><code>col_A    col_B
  ---------------
  John     []
  Mary     ['A','B','C']
  Ann      ['B','C']
  </code></pre>
  
  <p>I want to delete the rows where <code>col_B</code> has an empty list. i.e. I want the new data frame to be:</p>
  
  <pre><code>col_A    col_B
  ---------------
  Mary     ['A','B','C']
  Ann      ['B','C']
  </code></pre>
  
  <p>Below is what I did:</p>
  
  <pre><code>my_df[ len(my_df['col_B']) &gt;0 ]
  </code></pre>
  
  <p>But I got the following errors:</p>
  
  <hr>
  
  <pre><code>KeyError                                  Traceback (most recent call last)
  /usr/local/lib/python3.4/dist-packages/pandas/indexes/base.py in get_loc(self, key, method, tolerance)
     2133             try:
  -&gt; 2134                 return self._engine.get_loc(key)
     2135             except KeyError:
  
  pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4164)()
  
  pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4028)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13166)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13120)()
  
  KeyError: True
  
  During handling of the above exception, another exception occurred:
  
  KeyError                                  Traceback (most recent call last)
  &lt;ipython-input-27-75da0b0af6a1&gt; in &lt;module&gt;()
  ----&gt; 1 records_df_pair_count[ len(records_df_pair_count['stable_seq']) &gt;0 ]
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/frame.py in __getitem__(self, key)
     2057             return self._getitem_multilevel(key)
     2058         else:
  -&gt; 2059             return self._getitem_column(key)
     2060 
     2061     def _getitem_column(self, key):
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/frame.py in _getitem_column(self, key)
     2064         # get column
     2065         if self.columns.is_unique:
  -&gt; 2066             return self._get_item_cache(key)
     2067 
     2068         # duplicate columns &amp; possible reduce dimensionality
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/generic.py in _get_item_cache(self, item)
     1384         res = cache.get(item)
     1385         if res is None:
  -&gt; 1386             values = self._data.get(item)
     1387             res = self._box_item_values(item, values)
     1388             cache[item] = res
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/internals.py in get(self, item, fastpath)
     3539 
     3540             if not isnull(item):
  -&gt; 3541                 loc = self.items.get_loc(item)
     3542             else:
     3543                 indexer = np.arange(len(self.items))[isnull(self.items)]
  
  /usr/local/lib/python3.4/dist-packages/pandas/indexes/base.py in get_loc(self, key, method, tolerance)
     2134                 return self._engine.get_loc(key)
     2135             except KeyError:
  -&gt; 2136                 return self._engine.get_loc(self._maybe_cast_indexer(key))
     2137 
     2138         indexer = self.get_indexer([key], method=method, tolerance=tolerance)
  
  pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4164)()
  
  pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4028)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13166)()
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:13120)()
  
  KeyError: True
  </code></pre>
  
  <p>Any idea what I did wrong here? Thanks!</p>