[Python] Replace pandas column value by previous column value if cell contain a substring

I'm using Python 3.6.8. I have a dataframe df with columns name & sport. I want to replace value of sport if it contains substring "replace this value for".

My dataframe is generated using following code

import pandas as pd
df = pd.DataFrame({'name': ['Bob', 'Jane', 'Alice'],
'sport': ['tennis', 'replace this value for Jane', 'replace this value for Alice']})


So I wrote following code

row_num=df.index[df.iloc[:,1].str.contains('replace this value for')]
for num in row_num:
df.loc[num,1]=df.loc[num,0]


But I'm getting following error

# If we have a listlike key, _check_indexing_error will raise

KeyError: 0


Can you please help me to resolve this error?

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