title: Python Dataframe strange behavior
body: <p>This is quite strange. I have no idea what is happening.
  I have this dataframe:</p>
  <pre><code>df
  Out[52]: 
        userID itemID  rating
  0        196    242     3.0
  1        186    302     3.0
  2         22    377     1.0
  3        244     51     2.0
  4        166    346     1.0
       ...    ...     ...
  99995    880    476     3.0
  99996    716    204     5.0
  99997    276   1090     1.0
  99998     13    225     2.0
  99999     12    203     3.0
  
  
  type(df)
  Out[55]: pandas.core.frame.DataFrame
  
  df.columns
  Out[56]: Index(['userID', 'itemID', 'rating'], dtype='object')
  </code></pre>
  <p>However, when I ask</p>
  <pre><code>df[df['userID']==196 ]
  Out[53]: 
  Empty DataFrame
  Columns: [userID, itemID, rating]
  Index: []
  </code></pre>
  <p>Or</p>
  <pre><code>df.loc[df['userID']==196 ]
  Out[54]: 
  Empty DataFrame
  Columns: [userID, itemID, rating]
  Index: []
  </code></pre>
  <p>Note that there is the 'userID'==196 from the extract above.</p>
  <p>To be precise, this dataframe comes from the library Surprise:</p>
  <pre><code>from surprise import Dataset
  data = Dataset.load_builtin('ml-100k')
  df = pd.DataFrame(data.__dict__['raw_ratings'], columns=['user_id','item_id','rating','timestamp'])
  df.drop(columns=['timestamp'],inplace=True)
  df.rename({'user_id':'userID','item_id':'itemID'},axis=1,inplace=True)
  </code></pre>
  <p>What is happening here?</p>