Use pass for an empty block

#Credits to NPTEL MOOC, Programming, Data Structures & Algorithms in 
#Python by Madhavan Mukund, Chennai Mathematical Institute 

while(True):  
  try:
    userdata = input("Enter a number: ")
    usernum = int(userdata)
  except ValueError:
    print("Not a number. Try again")
  except NameError:
    pass
  else:
    break  

Check wheather a number is composite or not

#Credits to NPTEL MOOC, Programming, Data Structures & Algorithms in 
#Python by Madhavan Mukund, Chennai Mathematical Institute 

def composite(n):
      for i in range(2,n):
        if n%i == 0:
          return(True)
      return(False)