Steps followed:
1. take length of side of square as input from user
2. validate user input by converting to float
3. calculate perimeter of a square with formula 4 * side
4. print perimeter of a square

Program

#python program to calculate perimeter of a square

#take side of square as input from user

side=input("Enter length of side of square: ")

try:

    side=float(side)

    #calculate perimeter of a square

    perimeter = 4 * side

    #print perimeter of a square

    print("---output---")

    print("perimeter of a square with side %0.2f is %0.2f"%(side,perimeter))

except:

    print("---error---")

    print("please enter a valid length")

Output

Enter length of side of square: 16

---output---

perimeter of a square with side 16.00 is 64.00

 

Enter length of side of square: side

---error---

please enter a valid length

► Python program to calculate perimeter of a square