Return max of zero or value for a pandas DataFrame column

You could use the clip method:

import pandas as pd
import numpy as np
df = pd.DataFrame({'value': np.arange(-5,5)})
df['value'] = df['value'].clip(0, None)
print(df)

yields

   value
0      0
1      0
2      0
3      0
4      0
5      0
6      1
7      2
8      3
9      4