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14 - If, elif, and else

We will use the concepts of True and False to drive our scripts ability to make decisions. The following exercise will help introduce program decisions and conditionals.

Note: This will be the first time we see indented blocks of code. Whenever you see an indented line of code, follow the Python standard of always indenting 4 spaces (do not use tabs). Your text editor can and should be configured to insert 4 spaces instead of a tab character \t when the "Tab" key is pressed. Using 4 spaces is a convention used in Python that will cause you trouble should you choose to ignore it because all production code in Python has this feature. Furthermore, you are trying to learn Python aren't you?

# if_else.py

# let's start with some variables
# this will tell us to print an error
# message if necessary
error = False

# this is how much each farmer 
# needs in revenue to break even
break_even = 1000.00

# these are farming data
berry_farmer = "Barry Farmer"
wheat_farmer = "Ray Wheatley"

number_of_berries = 3294
bushels_of_wheat  = 523.45

berry_hired_hands = 34
wheat_hired_hands = 23

price_per_bushel = 2.50
price_per_100_berries = 15.23

# now let's get the farmer we
# want to know about
farmer_name = input("Enter the name of the farmer\n\t--> ")

if farmer_name == berry_farmer:
    produce = "Berries"
    amount  = number_of_berries / 100
    hands   = berry_hired_hands
    pricing = price_per_100_berries

elif farmer_name == wheat_farmer:
    produce = "Wheat"
    amount  = bushels_of_wheat
    hands   = wheat_hired_hands
    pricing = price_per_bushel

else:
    error = True

if not error:
    revenue = amount * pricing
    payment_per_hand = revenue / hands

    if revenue < break_even:
        message = "Farm is below break-even point. Sorry"
    else:
        message = "Farm is above break-even point. Congratulations!"

    print(f"""
        --- Farm Report for {farmer_name} ---
    Product for sale               : {produce:>11}
    Price per unit                 : ${pricing:10.2f}
    Total revenue                  : ${revenue:10.2f}
    Amount owed to each hired hand : ${payment_per_hand:10.2f}

        {message}
    """)

else:
    print(f"Farmer name \"{farmer_name}\" does not exist in record!")

Here is what should happen

Run this program 3 times as shown below. You should see similar results.

$ python if_else.py
Enter the name of the farmer
    --> Barry Farmer

        --- Farm Report for Barry Farmer ---
    Product for sale               :     Berries
    Price per unit                 : $     15.23
    Total revenue                  : $    501.68
    Amount owed to each hired hand : $     14.76

        Farm is below break-even point. Sorry

$ python if_else.py
Enter the name of the farmer
    --> Ray Wheatley

        --- Farm Report for Ray Wheatley ---
    Product for sale               :       Wheat
    Price per unit                 : $      2.50
    Total revenue                  : $   1308.62
    Amount owed to each hired hand : $     56.90

        Farm is above break-even point. Congratulations!

$ python if_else.py
Enter the name of the farmer
    --> Gary Hart
Farmer name "Gary Hart" does not exist in record!
$
What is happening Here?

We are using conditional logic to get data for a pair of farmers. We begin by prompting the user for the farmer's name. We then check the names of the farmers we have data for. If the name does not match any of the farmer's on record we print an error message. (This is the else part on line 63.) The error variable we set earlier in the script will be set to True on line 42 in the case that we do not find the name of any farmer in record.

Should the name given match a name in our records we prepare the output by setting appropriate variables and then formatting and printing them out in the absence of an error.

Conditionals

The section about logical operators introduced us to the idea that everything can be evaluated with a value of True or False. In this section we put that logic to work for us. The conditional or "if" statement in python has the following syntax:

if Expression1: # don't forget the colon
    do_thing_one() # indent 4 spaces

# if Expression1 evaluates to False check Expression 2
elif Expression2: 
    do_thing_two()

# and so on
elif Expression3:
    do_thing_three()

# if all previous expressions evaluate to False do the 'else' part
else:
    do_thing_four()

We will explain the syntax step by step. When Python comes to an if statement, the following is executed:

  • Python evaluates whatever is in place for Expression1 for a truth value.
  • If Expression1 evaluates to True:
  • Everything in the indented block of code under if will be executed.
  • Once the block has ended (by un-indenting), the rest of the elif and else blocks of code are skipped.
  • If Expression1 evaluates to False, the block of code uner Expression1 is skipped and Expression2 is checked for its truth value
  • IfExpression2 evaluates to True :
  • Everything in the indented block of code under elif will be executed.
  • Once the block has ended (by un-indenting), the rest of the elif and else blocks of code are skipped.
  • If Expression2 evaluates to False, Expression3 is checked for its truth value and so on...
  • This pattern continues for all the elif expressions.
  • Finally, if none of the 'if' or elif expressions evaluate to True, the code under the else is executed.

Python's conditional syntax allows as many elif statements as the programmer wishes. However, the elif and else parts are not necessary. You may choose to have a conditional block without an else or without an elif part. We will discuss good practices for conditional statements in the section about errors.

Indented Blocks

We have introduced an important syntax in Python. That is the indented block. This one feature that Python uses to make code more readable and to avoid cluttering the code page with unnecessary symbols or words. As you saw below each if, elif or else statement, the code is executed as normal. An indented block is just a way of Python separating a chunk of code and executing it in a different context, in this case, the context is that the code is executed if the logical expression evaluates to True. There are many other contexts that we will cover later but for now, memorize the following rules about indented blocks:

  • Indented blocks must start with a colon :. A colon tells Python in effect, "This is the beginning of a new indented block"
  • Indent 4 spaces. Four spaces is an accepted convention although Python will technically allow whatever spacing you want as long as you are consistent. Any code you see written will follow the 4 space convention and that is the standard so it is best if you follow suit. If you mix spacings or are inconsistent with this Python will throw a syntax error.
  • End the block by un-indenting. The only way Python knows that the indented block has ended is when an un-indented line of code is observed.
  • Nested blocks are allowed. If you want to have an indented block inside an indented block that is fine. the same rules apply in all cases.

You may not know understand yet why these are so important but as we go along it will be obvious why they exist.

Match-Case Statements (Python Version 3.10 and later )

This section only applies to Python versions 3.10 and later. This is a new feature introduced in Python version 3.10 that allows functionality similar to a switch statement in C. However, at time of writing (December 2021), I am still using version 3.9 for my work. As soon as I upgrade to 3.10 I will add more about this feature here (as well as update other sections.)

For those that are using version 3.10 or later, here is brief primer on how this feature works:

match "x":
    case "y":
        print("y")
    case 'x':
        print('x')
    case _:
        print("Err")

# this is equivalent to

var = "x"
if var == "y":
    print("y")
elif var == 'x':
    print('x')
else:
    print("Err")

While the differences between the two examples may seem trivial, this becomes more important as your code base grows larger. This may be treated in more detail in a later version of this book. For now you do not need to use it but you should be aware that it exists.

Hone Your Skills

  1. Write a script that has several nested if and elif statements. Can you simplify the logic to not need as many nested blocks of code?

Advanced Mastery

  1. There is a problem in the above script if_else.py that can cause errors if we're not careful. Can you find the problem? How would you fix it? Answer is here.

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