Calculating the area of various 2D shapes

Question

I've made a simple program that can calculate the area of various 2D shapes. I'd like your expereanced eyes to asses my code. Be as harsh as you want but please be specific and offer me ways in which I can improve my coding.

shape = ("triangle","square","rectangle","circle")                  # This is a tuple containing the string names of all the supported shapes so far.
constants = (3.14)                                                  # An object containing the value of pi. I will add more constants in future and turn this into a tuple.
start_up = input("Do you want to calculate a shape's area?: ")      # Fairly self explainatory

if start_up != "yes" or "no":                                       # This is to prevent the program failing if the user answers the start_up question
   print("Sorry but I don't understand what your saying")           # with an answer outside of the simple yes/no that it requires
   start_up = input("Do you want to calculate a shape's area?: ")   # re-iterates the start up question.

   while start_up == "yes":                                            # This is the condition that obviously contains the bulk of the program.
      target_shape = input("Choose your shape: ")                       # Once again, self explainatory.

      if target_shape == shape[0]:                                      # Uses the shape tuple to bring forth the case for the first shape, the triangle.
         h = float(input("give the height: "))                          # These Two lines allow the user to input the required values to calculate the area of the triangle.
         b = float(input("give the base length: "))                     # They are using the float funtion to increase the accuracy of the answer.
         area_triangle = h * 1/2 * b                                    # The formula we all learned in school.    
         print("the area of the %s is %.2f" % (shape[0],area_triangle)) # This generates a format string which uses both the shape tuple and the area_triangle object 
                                                                # to display the answer. note the %.2f operand used to specify how many decimal places for the answer.

      if target_shape == shape[1]:                                      # Square
         l = float(input("give the length: "))                          
         area_square = l ** 2
         print("the area of the %s is %.2f" % (shape[1],area_square))

      if target_shape == shape[2]:                                     # Rectangle
         l = float(input("give the length: "))
         w = float(input("give the width: "))
         area_rectangle = l * w
         print("the area of the %s is %.2f" % (shape[2],area_rectangle))

      if target_shape == shape[3]:                                     # Circle
         r = float(input("give the radius: "))
         pi = constants                                                # In this line we are calling forth the constants object to use the value of pi.
         area_circle = 2 * pi * r
         print("the area of the %s is %.2f" %(shape[3],area_circle))

         start_up = input("Do you want to calculate another shape's area?: ") # This line allows the user the chance to do just what it says in the brackets.

      else:                                                                   # This is to safegaurd against failure in the event of the user 
        print("That shape's not in my database")                             # inputing a shape non-existant in the shape tuple.
        start_up = input("Do you want to calculate another shape's area?: ")

        if start_up == "no":                                                      #This is for when the user no longer wants to use the program.
           print("Goodbye then!")
           input("Press any key to close")

Answer 1

In addition to Justin's comments:

You want to be more DRY. You have the line:

start_up = input("Do you want to calculate a shape's area?: ")

in your code 5 times (one of which is just completely broken because regardless of the answer it's asked again) when once would suffice:

while True:
    startup = None
    while startup not in ("yes", "no"):
        if startup is not None:
            print("Sorry but I don't understand what your saying")
        startup = input("Do you want to calculate a shape's area?: ")
    if startup == "no":
        print("Goodbye then!")
        input("Press enter to close") # <-- "any key" is a lie; will require return...
        break
    # do the rest of the stuff here.

The bit that prints the area is also repeated unnecessarily; call the results for each section "area" and use that; you already insert the correct shape name.

Your indentation is also inconsistent; don't switch between 3 and 4 space indents; pick one.

The else clause at the end also only applies to the last if; any shape but circle will be calculated and the user will be told the shape isn't in the "database". replace all the ifs but the first with elif.

Answer 2

Since you're not using shape extensively I would get rid of that data structure all together. It would be easier to read inline then having to look to the margin for comments.

For example:

  if target_shape == 'square':                                     
     l = float(input("give the length: "))                          
     area_square = l ** 2
     print("the area of the square is %.2f" % (area_square))

When comparing strings if you don't want to be strict you should normalize them before compare.

For example:

'Square'.lower() == 'square'

so your code would become:

  if target_shape.lower() == 'square':                                     
     l = float(input("give the length: "))                          
     area_square = l ** 2
     print("the area of the square is %.2f" % (area_square))

You're not accounting for any errors. What if the user hits enter without giving a length?

ValueError: could not convert string to float

Answer 3

Your code is very good for a beginner! As you have noticed, there are always improvements to be done, so don't worry too much about this. Here's what I feel the other answers have missed:

constants = (3.14)

Use math.pi instead!

start_up = input("Do you want to calculate a shape's area?: ")      # Fairly self explanatory.

I understand that those comments help you as a Python beginner, but they should be avoided in real code. As the PEP 8 section on online comments says, inline comments that state the obvious are distracting. You're also breaking the 79 characters rule.