Pandas replace values of a column with comparison to another Dataframe

Question

I want to replace in PF column value by a value in another Dataframe if exists In yellow no correspondance, so leave value as it is):

and Dataframe with old value comparison and new value:

I tried to do this but does not work unicite['CustomerID'] = np.where(unicite['CustomerId'] == Fidclients['CustomerId'],Fidclients['Newvalue'], unicite['CustomerID'])

Answer 1

If I'm understanding the question correctly, you want to replace the values in CustomerID in the table unicite with the values in the column Newvalue if they exist in the column CustomerID within the table Fidclients .

I believe you'll have to merge the tables to achieve this. For example,

unicite = pd.DataFrame({'CustomerID': ['a', 'b', 'c']})
print(unicite)

    CustomerID
0   a
1   b
2   c

Fidclients = pd.DataFrame({'CustomerID': ['c', 'f', 'g'], 'Newvalue': ['x', 'y', 'z']})
print(Fidclients)

  CustomerID Newvalue
0          c        x
1          f        y
2          g        z

merged = unicite.merge(Fidclients, on='CustomerID', how='left')
merged.loc[merged.Newvalue.notnull(), 'CustomerID'] = merged.Newvalue
merged.drop('Newvalue', axis=1)

    CustomerID
0   a
1   b
2   x