dataframe_selection_done_right.py

import pandas as pd

data = {"colA": [1, 2 , 3, 4, 5, 6, 7 ,8], 
        "colB": ["one", "two", "one", "two", "one", "two", "one", "two"]}
df = pd.DataFrame(data)

# Always use the pattern
# df.loc[<selector>, column_to_change] = value
# selector is usually df['a_column'] == "something"
# the column can be one or more.

df.loc[df['colB'] == "one", "colC"] = "new_value"
df.loc[df['colB'] == "two", "colC"] = "old_value"

# Note the old way was
# df[df['colB'] == "one"]]['colC'] = "value"
# This is wrong do not use it anymore
# Check this: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy