Examples

import numpy as np
import pandas as pd

df = pd.DataFrame([[3.0, 2.0],
                   [4.0, np.nan],
                   [2.0, 0.0]],
                   columns=list('XY'))
df

	X	Y
0	3.0	2.0
1	4.0	NaN
2	2.0	0.0

By default, iterates over rows and finds the maximum in each column:

df.cummax()

	X	Y
0	3.0	2.0
1	4.0	NaN
2	4.0	2.0

To iterate over columns and find the maximum in each row, use axis=1:

df.cummax(axis=1)

	X	Y
0	3.0	3.0
1	4.0	NaN
2	2.0	2.0