df_sort = df.sort_values(['user_id', 'event_time'])

diff_timestamp = df_sort.groupby('user_id')['event_time'].diff().reset_index()

 

for i in range(len(diff_timestamp)):

    diff_timestamp.loc[i,'second'] = (diff_timestamp.loc[i,'event_time'].seconds)

   

new_session = (diff_timestamp['second'].isnull()) | (diff_timestamp['second'] > 600)

df_sort['session_id'] = df_sort.loc[new_session, ['user_id', 'event_time']].groupby('user_id').rank(method='first').astype(int)

df_sort['session_id'] = df_sort['session_id'].fillna(method='ffill') #.astype(int)