Problem with mapping codes to county names in Python | Pandas [duplicate]

I have a dataset containing almost 400 counties from Poland. I need to merge several files containing data related to these counties. Each county is identified by its name and a code. However, there's a challenge: there are 10 pairs of counties with the same name. For example, there is a county called 'powiat brzeski' in both the Małopolskie and Opolskie provinces. To differentiate between these counties, I want to add the name of the province next to the county name using Python. I attempted to do this with the below method, but encountered an issue: after the manipulation, the values in the 'County' column, except for those affected by the manipulation, turned into NaN. Could you assist me with resolving this issue?

  import pandas as pd  # Sample DataFramedata = {'Code': [1202000, 2402000, 802000, 3017000, 3005000, 9999999], # Added an unmatched code 9999999'County': ['Powiat brzeski', 'Powiat bielski', 'Powiat krośnieński', 'Powiat ostrowski', 'Powiat grodziski', 'Powiat ciechanowski']} # Added an unmatched countydf = pd.DataFrame(data)# Print updated DataFrameprint(df)

Result:

     Code               County0  1202000       Powiat brzeski1  2402000       Powiat bielski2   802000   Powiat krośnieński3  3017000     Powiat ostrowski4  3005000     Powiat grodziski5  9999999  Powiat ciechanowskidata = {'Code': [1202000, 2402000, 802000, 3017000, 3005000, 9999999], # Added an unmatched code 9999999'County': ['Powiat brzeski', 'Powiat bielski', 'Powiat krośnieński', 'Powiat ostrowski', 'Powiat grodziski', 'Powiat ciechanowski']} # Added an unmatched countydf = pd.DataFrame(data)# Dictionary mapping codes to county namescode_to_county = {    1202000: "Powiat brzeski_Malopolskie",    2402000: "Powiat bielski_Slaskie",    802000: "Powiat krośnieński_Lubuskie",    3017000: "Powiat ostrowski_Wielkopolskie",    3005000: "Powiat grodziski_Wielkopolskie"}# Update values in "County" column based on values in "Code" columndf['County'] = df['Code'].map(code_to_county)# Print updated DataFrameprint(df)

Result:

      Code                          County0  1202000      Powiat brzeski_Malopolskie1  2402000          Powiat bielski_Slaskie2   802000     Powiat krośnieński_Lubuskie3  3017000  Powiat ostrowski_Wielkopolskie4  3005000  Powiat grodziski_Wielkopolskie5  9999999                             NaN