Pandas- Fill nans up until first non NULL value

Bit of a trick using pandas.DataFrame.ffill with notna and where:

df.where(df.ffill().notna(), 0)

Or using pandas.DataFrame.interpolate:

df.interpolate('zero', fill_value=0, limit_direction='backward')

Output:

   A    B    C
0  1  0.0  0.0
1  2  0.0  5.0
2  3  3.0  NaN
3  4  NaN  NaN

This would be done using where or mask.

df.mask(df.notna().cumsum().eq(0), 0)
# or,
df.where(df.notna().cumsum().ne(0), 0)

   A    B    C
0  1  0.0  0.0
1  2  0.0  5.0
2  3  3.0  NaN
3  4  NaN  NaN

Many ways to skin a cat here :-)