user_name =input("What's your name: ")
if user_name =="len(str)":
    print ("Hi", user_name)
elif user_name =="":
    print("Sorry please enter your name, invalid input make sure your name is ")

user_name = input("What's your name: ")
if user_name:
    print ("Hi", user_name)
else:
    print("Sorry please enter your name, invalid input make sure your name is ")

user_name =input("What's your name: ")

while not user_name:
   user_name = input("Please enter a name: ")

if user_name:
   print("Welcome: " + user_name)

What's your name:
Please enter a name: Mark
Welcome: Mark