pandas - Repeat the previous row in a database based on conditions using python

Question

I've been stuck on this for a while now. I want to repeat the previous row and iterate the time to one second before the next row (Please see the output below). In my code, I've attempted the task with pandas. The condition for this repeat command should be when "a" column reads "Power_change" AND when the "indicator" column reads above 1 then repeat previous row.

The Input:

a	P0	P1	time	Indicator
ISALIVE	325	350	10:59:40	31
POWER_CHANGE	200	42333	10:59:52	11

question from:https://stackoverflow.com/questions/65952140/repeat-the-previous-row-in-a-database-based-on-conditions-using-python

Answer 1

fileter the data, use index - 1 to get the previous row. then append the origin dataframe.

cond = (df['a'] == "POWER_CHANGE") & (df['Indicator'] > 1)

# calculate the time is 1 second before POWER_CHANGE
obj_time = (pd.to_datetime(df.loc[cond, 'time']) - 
            pd.to_timedelta('00:00:01')).dt.strftime('%T')

# get the previous record before 'POWER_CHANGE'
idx = df.loc[cond].index - 1
dfn = df.loc[idx].copy()

# reassign the time, use `.values` to ignore the index
dfn['time'] = obj_time.values

df.append(dfn).sort_index()