Why would an extra column (unnamed: 0) appear after saving the df and then reading it through pd.read_csv? - pandas

Question

My code to save the df is:
fdi_out_vdem.to_csv("fdi_out_vdem.csv")
To read the df into python is :
fdi_out_vdem = pd.read_csv("C:/Users/asus/Desktop/classen/fdi_out_vdem.csv")
The df:
Unnamed: 0
country_name
value
1
Spain
190
2
Spain
311

Answer 1

Your df has two columns, but also an index with "0" and "1". When writing it to csv it looks like this:
,country_name,value
0,Spain,190
1,Spain,311
When importing it with pandas you it is considered as df with 3 columns (and the first has no name)
You have two possibilities here:
Save it without index column:
df.to_csv("fdi_out_vdem.csv", index=False)
df = pd.read_csv("C:/Users/asus/Desktop/classen/fdi_out_vdem.csv")
or save it with index column and define an index col when reading it with pd.read_csv
df.to_csv("fdi_out_vdem.csv")
df = pd.read_csv("C:/Users/asus/Desktop/classen/fdi_out_vdem.csv", index_col=[0])
UPDATE
As recommended by #ouroboros1 in the comments you could also name your index before saving it to csv, so you can define the index column by using that name
df.index.name = "index"
df.to_csv("fdi_out_vdem.csv")
df = pd.read_csv("C:/Users/asus/Desktop/classen/fdi_out_vdem.csv", index_col="index")

Answer 2

You can either pass the parameter index_col=[0] to pandas.read_csv :
fdi_out_vdem = pd.read_csv("C:/Users/asus/Desktop/classen/fdi_out_vdem.csv", index_col=[0])
Or even better, get rid of the index at the beginning when calling pandas.DataFrame.to_csv:
fdi_out_vdem.to_csv("fdi_out_vdem.csv", index=False)