Fill empty cells in column with value of other columns

I have a HC list in which every entry should have an ID, but some entries do not have an ID. I would like to fill those empty cells by combining the the first name column and the last name column. How would I go about this? I tried googling for fillna and the like but couldn’t get it to work. What I want is basically this:

If hc["ID"] == "": 
    hc["ID"] = hc["First Name"] + hc["Last Name"]

Answer

You can use loc and a boolean mask if NaN then:

hc.loc[hc["ID"].isnull(),'ID'] = hc["First Name"] + hc["Last Name"] 

otherwise for empty string:

hc.loc[hc["ID"] == '','ID'] = hc["First Name"] + hc["Last Name"]