import os
un = os.environ.get("USERNAME")
hd = os.getenv("HOMEDRIVE")
hp = os.getenv("HOMEPATH")
dt = hd + hp + "\\\\Desktop"
print("username",un)
print("HOMEDRIVE",hd)
print("HOMEPATH",hp)
print("Desktop",dt)

結果

[ユーザー名] HOMEDRIVE C: HOMEPATH \Users\ [ユーザー名] Desktop C:\Users\ [ユーザー名] \Desktop