conditional replace based off prior value in same column of pandas dataframe python

Question

Feel like I\'ve looked just about everywhere and I know its probably something very simple. I\'m working with a pandas dataframe and looking to fill/replace data in one of

Answer 1

using Where

df['B'] = df.A[0:len(df.A)-1].where((df.A==0 ) & (df.B.shift(-1)==-1),-1)
df['B'] = df['B'].fillna(df.A)