pandas replace rows in dataframe with other dataframe

Question

I have a pandas dataframe and I want to index a subset of rows using two conditions, then replace that subset with a new dataframe I've created. However as you can see below it only replaces the rows that have the same indices. How do I align the indices of the dataframes so the replacement will work for all rows?

df = pd.DataFrame({'A': [-4, 9, 6, -3],
                   'B': ['y', 'b', 'b','x']})

df
Out[581]: 
   A  B
0 -4  y
1  9  b
2  6  b
3 -3  x

replacement = pd.DataFrame({'A':[-7, -4], 'B':['y','x']})

replacement
Out[583]: 
   A  B
0 -7  y
1 -4  x

df.loc[(df['A']>0) & (df['B']=='b')] = replacement

df
Out[585]: 
     A    B
0 -4.0    y
1 -4.0    x
2  NaN  NaN
3 -3.0    x

Answer 1

IIUC you want

>>> df.loc[(df['A']>0) & (df['B']=='b')] = replacement.values
>>> df
   A  B
0 -4  y
1 -7  y
2 -4  x
3 -3  x