python - Pandas Dataframe exists outside of method. Why -

lets presume have simple dataframe df , simple method dataframe

def alterdf(df):      df1['new column'] = df['some column'] + x     return df1 

in above method modify entire column x , save new variable name...inside method! however, when inspect original dataframe (i.e. df) see has new column added it...

i aware original dataframe created exists outside of method. expect alterations occur inside method, should remain there, unless save changes via return block in method.

however, know wrong...the changes applied within method, occur outside of method. how can be? why so?

probably because have line this

df1 = df # doing copy reference 

if want copy dataframe use

df1 = df.copy()  

instead