Replace values in dataframe with another dataframes values based on condition
The easiest way to do it for your case is something like this:

In [1]: import pandas as pd                                                                                                         

In [2]: df1 = pd.DataFrame([0, 1, 2, 3, 1, 2, 3, 1, 2], columns=[&quot;A&quot;])                                                              

In [3]: df2 = pd.DataFrame([9] * len(df1), columns=[&quot;A&quot;])                                                                           

In [4]: df1                                                                                                                         
Out[4]: 
   A
0  0
1  1
2  2
3  3
4  1
5  2
6  3
7  1
8  2

In [5]: df2                                                                                                                         
Out[5]: 
   A
0  9
1  9
2  9
3  9
4  9
5  9
6  9
7  9
8  9

Wherever the row of column A in df1 is equal to 1, insert 1 into the same row of column A in df2
In [6]: df2.A[df1.A == 1] = 1                                                                                                       

In [7]: df2                                                                                                                         
Out[7]: 
   A
0  9
1  1
2  9
3  9
4  1
5  9
6  9
7  1
8  9



You could also use the pd.DataFrame.where method, to replace the part df1.A == 1.
There are also many other ways to get the same thing done, but my example is the most straighforward :)