Pandas: update column values from another column if criteria [duplicate]

Question

I have a DataFrame:

   A B

1: 0 1
2: 0 0 
3: 1 1
4: 0 1
5: 1 0

I want to update each item column A of the DataFrame with values of column B if value from column A equals 0.

DataFrame I want to get:

   A B

1: 1 1
2: 0 0 
3: 1 1
4: 1 1
5: 1 0

I've already tried this code

df['A'] = df['B'].apply(lambda x: x if df['A'] == 0 else df['A'])

It raise an error :The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Answer 1

df['A'] = df.apply(lambda x: x['B'] if x['A']==0 else x['A'], axis=1)

Output

    A  B
1:  1  1
2:  0  0
3:  1  1
4:  1  1
5:  1  0