Python函数传递pandas Dataframe属性不起作用

def modify_dataframe(df):
    df['new_column'] = df['existing_column'] * 2

df = pd.DataFrame({'existing_column': [1, 2, 3]})
modify_dataframe(df)
print(df)

   existing_column  new_column
0                1           2
1                2           4
2                3           6

def modify_dataframe(df):
    df['new_column'] = df['existing_column'] * 2
    return df

df = pd.DataFrame({'existing_column': [1, 2, 3]})
df = modify_dataframe(df)
print(df)

   existing_column  new_column
0                1           2
1                2           4
2                3           6