whatever.py

In [16]: df = pd.DataFrame({'A': [1, 2, 1, 2, 1, 2, 3, 3]})                                                                                                                 

In [17]: df2 = df[df['A'].isin([1, 2])]                                                                                                                                     

In [18]: df2.loc[df2['A'] == 1, 'A'] = 4                                                                                                                                    
/home/glemaitre/miniconda3/lib/python3.7/site-packages/pandas/core/indexing.py:189: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self._setitem_with_indexer(indexer, value)
/home/glemaitre/miniconda3/bin/ipython:1: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  #!/home/glemaitre/miniconda3/bin/python