A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_indexer,col_indexer] = value instead

# Error:
# SettingWithCopyWarning: A value is trying to be set on a copy of a
# slice from a DataFrame

# As explained in the Source, this warning is usually safe to ignore. You
# can disable it by running the following:

import pandas as pd
pd.options.mode.chained_assignment = None  # default='warn'

Here is what the above code is Doing:
1. The first line imports the pandas module.
2. The second line tells pandas to ignore the warning.

If you want to learn more about this warning, you can read the documentation
here:
http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy