title: Pandas: check the unique values of a column with datatype 'set'
body: <p>I have a pandas data frame <code>my_df</code>. The column <code>animals</code> in <code>my_df</code> has the datatype 'set'. Then I tried to use the following code to check how many different values (sets) in this column <code>animals</code>:</p>
  
  <pre><code>print(my_df.animals.unique())
  </code></pre>
  
  <p>But got the following error:</p>
  
  <pre><code>---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-13-e41c02e0e954&gt; in &lt;module&gt;()
  
       11 
  ---&gt; 12 print(my_df.animals.unique())
  
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/series.py in unique(self)
     1237     @Appender(base._shared_docs['unique'] % _shared_doc_kwargs)
     1238     def unique(self):
  -&gt; 1239         result = super(Series, self).unique()
     1240         if is_datetime64tz_dtype(self.dtype):
     1241             # to return array of Timestamp with tz
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/base.py in unique(self)
      971         else:
      972             from pandas.core.nanops import unique1d
  --&gt; 973             result = unique1d(values)
      974         return result
      975 
  
  /usr/local/lib/python3.4/dist-packages/pandas/core/nanops.py in unique1d(values)
      809     else:
      810         table = _hash.PyObjectHashTable(len(values))
  --&gt; 811         uniques = table.unique(_ensure_object(values))
      812     return uniques
      813 
  
  pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.unique (pandas/hashtable.c:14383)()
  
  TypeError: unhashable type: 'set'
  </code></pre>