Get max value in previous rows for matching rows

Say I have a dataframe that records temperature measurements for various sensors:

import pandas as pd

df = pd.DataFrame({'sensor': ['A', 'C', 'A', 'C', 'B', 'B', 'C', 'A', 'A', 'A'],
                   'temperature': [4.8, 12.5, 25.1, 16.9, 20.4, 15.7, 7.7, 5.5, 27.4, 17.7]})

I would like to add a column max_prev_temp that will show the previous maximum temperature for the corresponding sensor. So this works:

df["max_prev_temp"] = df.apply(
    lambda row: df[df["sensor"] == row["sensor"]].loc[: row.name, "temperature"].max(),
    axis=1,
)

It returns:

  sensor  temperature  max_prev_temp
0      A          4.8            4.8
1      C         12.5           12.5
2      A         25.1           25.1
3      C         16.9           16.9
4      B         20.4           20.4
5      B         15.7           20.4
6      C          7.7           16.9
7      A          5.5           25.1
8      A         27.4           27.4
9      A         17.7           27.4

Problem is: my actual data set contains over 2 million rows, so this is excruciatingly slow (it probably will take about 2 hours). I understand that rolling is a better method, but I don’t see to use it for this specific case.

Any hint would be appreciated.

>Solution :

Use Series.expanding per groups with remove first level by Series.droplevel:

df["max_prev_temp"] = df.groupby('sensor')["temperature"].expanding().max().droplevel(0)
print (df)
  sensor  temperature  max_prev_temp
0      A          4.8            4.8
1      C         12.5           12.5
2      A         25.1           25.1
3      C         16.9           16.9
4      B         20.4           20.4
5      B         15.7           20.4
6      C          7.7           16.9
7      A          5.5           25.1
8      A         27.4           27.4
9      A         17.7           27.4