e.g. To retrieve a username in lower case:

username = input("Enter your username:").lower() 

e.g. To retrieve a last name in title case:

lastname = input("Enter your last name:").title() 

e.g. To retrieve a postcode in upper case:

postcode = input("Enter your postcode:").upper() 

You can also remove all leading and tailing space from a user input you can use the .strip() method.

name = input("Enter your name:").strip() 

Input Validation: Presence Check

name = input("Enter your name:").strip() 

if name=="":
    print("Empty name!")
else:
    print("Thank you!")

Input Validation: Type Check – Integer?

number = input("Type a number:")

if number.isdigit():
    print("This is a number")
else:
    print("This is not a whole number")

Input Validation: Range Check

number = int(input("Type a number between 1 and 5:"))

if number>=1 and number<=5:
    print("Valid number")
else:
    print("Invalid number")

Input Validation: Lookup Check

drive = input("Can you drive?").lower()

if drive in ["yes","no"]:
    print("Valid answer")
else:
    print("Invalid answer")

Input Validation: Character Check

email = input("Type your e-mail address:")

if "@" in email:
    print("Valid e-mail address")
else:
    print("Invalid e-mail address")

postcode = input("Type your postocode:").upper()

if postcode[0] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ" and postcode[1] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ" and postcode[2] in "123456789":
    print("Valid postcode")
else:
    print("Invalid postcode")

Input Validation: Length Check

password = input("Type a password:")

if len(password)>=8:
    print("Valid password")
else:
    print("Invalid password")

Try Again! Using a While Loop:

name = input("Enter your name:")

while name=="":
    print("You must enter your name! Please try again!")
    name = input("Enter your name:")

print("Welcome " +  name)

You can investigate more advance approaches to implement validation subroutines on this blog post.