Request for assessing my code (beginner of python3 and programming)

Question

Ok I've started programming 3 days ago. I decided to choose python3 as my language because its pretty easy to learn yet reasonably powerful. I've used a variety of resources on the net, most of which were originally made for python2. I've used my new found skills to make 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. Heres the code:

shape = ("triangle","square","rectangle","circle")                  # This creates a tuple containing the string names of all the shapes to be calculated . 
constants = (3.14)                                                  # This stores the value of pi. Later I will add more constants to it and make it a tuple.
start_up = input("Do you want to calculate a shape's area?: ")      # Fairly self explanatory.

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.
   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")

Edit: Ok thanks for all the helpful comments. Here's my revised code. I've tried to take on board the advice of everyone to the best of my ability.

while True:
    startup = None
    while startup not in ("yes", "no"):
        if startup is not None:
            print("Sorry but I don't know what you are saying.")
        startup = input("Do you want to calculate a shape's area?: ").lower()
    if startup == "no":
        print("Goodbye then!")
        input("Press enter to close")
        break
    elif startup == "yes":
        print("Cool!")
        shape = None
        while shape not in ("triangle", "square", "rectangle", "circle"):
            if shape is not None:
                print("""sorry but I don't know that shape.
That is if it even is a shape. -_-""")
            shape = input("What shape?: ").lower()
            if shape == "triangle":
                print("Ok you need to give me the height and the base length")
                h = float(input("height = ")); b = float(input("base = "))
                area = h * 1/2 * b
            elif shape == "square":
                print("Ok I just need the length of a side from you")
                l = float(input("length = "))
                area = l ** 2
            elif shape == "rectangle":
                print("give me the length and width")
                l = float(input("length = ")); w = float(input("width = "))
                area = l * w
            elif shape == "circle":
                print("I just need the radius")
                r = float(input("radius = "))
                area = 2 * 3.14 * r
        print("The area = "); print(area)

Note that there is one huge problem in this code that I still haven't resolved. As Justin had pointed out, if someone types in an invalid value or even nothing at all for the area formulas, the program will fail. I've tried to fix this but I can't. Any help here would be great. Also I tried to substitue 3.14 with math.pi like Cygal suggested but it seemed to make my program no longer work. I'm pretty sure I'm just using it wrong.

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.