Conditionals

Conditional blocks are blocks of code that run only if a defined condition exists. A conditional block starts with a conditional expression that checks whether a condition is True and is followed by a statement. For instance, the conditional block is if pant_size = 2: print(pant_size). The conditional expression is if pant_size = 2:. The condition the expression is checking is whether pant_size equals 2. The statement is print(pant_size). The line of code prints pant_size only if its value is 2. Otherwise, the Python interpreter does nothing but move onto the next line.

Because the Python interpreter reads your .py files from top to bottom in sequential order, you can use conditional statements to control the flow of your application. Use conditional statements to do things like skip over lines of code, repeat blocks of code, or choose between different code blocks.

The chapter discusses how to use if, else, and elif to write conditional expressions that check the truthiness of a Python object or expression. If the conditional expression is met, the Python interpreter reads and executes the corresponding block of Python code (the expression's statement). If the conditional expression is not met, the Python interpreter does not read or execute the corresponding statement. The chapter also dicusses the pass statement.

if

An if expression evaluates whether a Python object or expression is truthy. If the expression is truthy, the Python interpreter reads the the if expression's corresponding statement. If the expression is falsy, the Python interpreter does not read the corresponding statement.

An if expression consists of if, an expression, and a colon (:). A statement must follow an if expression. A statement can be any valid Python code. The statement can be on the same line as the expression.

# example of if and statement on same line
if True: print(True)

>>> True

It is more common, however, for the statement to appear on the line following the if expression. When on the subsequent line, the statement must be indented.

# example of statement on line following if expression
if True: 
    print(True)
    
>>> True

If your if expression does not have a statement, the Python interpreter will experience a Syntax error SyntaxError: unexpected EOF while parsing.

# example of syntax error
if True: 
   
>>> SyntaxError: unexpected EOF while parsing

Indentation matters! If the if expression's statement appears on the following line but is not indented, the Python interpreter has an Indentation error IndentationError: expected an indented block.

# example of indentation error
if True: 
print(True)
    
>>> IndentationError: expected an indented block

Indentation matters because that is how the Python interpreter knows which lines of code make up a block of code that belong together. Lines are considered in the same block of code when they have the same indentation. Here the error means that the if and print statements are in the same block of code when they really shouldn't be. If the print statement immediately follows the if statement, then the print statement should be indented.

So far this discussion has used statements that are only one line. A one-line block of code. Here is an example of an indented block of code consisting of more than one line.

# example of multi-line statement block of code
if True: 
    pants = 12
    shirts = 10
    inventory = pants + shirts
    price = 9.99
    total_sale = inventory * price
    print(total_sale)
    
>>> 219.78

You can nest blocks of code. The example below has two blocks of code.

# example of statement on line following if expression
if True: 
    # first block of code
    pants = 12
    shirts = 10
    inventory = pants + shirts
    if inventory > 20:
            # second block of code
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale)
    
>>> 219.78

The first block is four lines. The first three lines in the statement define pants, shirts, and inventory. The fourth checks if inventory is greater than 20. If True, the second block runs. It consistes of three lines. The first two lines define price and total_sale. The third line prints total_sale. Notice that what separates the two blocks of code from one another is the indentation.

You can use multiple if statements in a single block of code. When if statements appear in the same block, the Python interpreter runs the statement for the first one whose condition is met. Subsequent if statements in the same block are not read. The examples below each have three if statements. The top one checks if inventory is greater than 20. The next checks if inventory is less than or equal to 20. The last checks if inventory equals 0. Reading from top to bottom, the Python interpreter will run the corresponding statement for the first if statement that is True. If none are True, none are run.

# example of multiple if statements
if True: 
    pants = 12
    shirts = 10
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    if inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    if inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 219.78 We have plenty more!

The code above prints 219.78 We have plenty more! because inventory is greater than 20.

if True: 
    pants = 2
    shirts = 1
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    if inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    if inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 17.97 Running low.

The code above prints 17.97 Running low. because inventory is less than and not equal to 20.

if True: 
    pants = 0
    shirts = 0
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    if inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    if inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 0 None in inventory.

The code above prints 0 None in inventory. because inventory is equal to 0.

if True: 
    pants = -1
    shirts = 0
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    if inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    if inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

The code above prints nothing because inventory is none of the if statements evaluate to True. You owe someone a pair of pants.

For reference, here are additional simple examples of if statements.

q = 5
r = 0
s = 8
t = 5

# if statement basic examples
if q:
    print("yes, q")
    
>>> yes, q

if r:
    print("yes, r")


if q > r:
    print("yes, q > r")

>>> yes, q > r

if q == r:
    print("yes, q = r")


if t is q:
    print("yes, t is q")    

>>> yes, t is q

if q and s:   
    print('q and s are both truthy') 
    print('q is ' + str(q))
    print('s is ' + str(s))  
    
    if q > s:                      
        print('q is greater than s')

    print('phew, all this math; need a break')

    if q < s:                      
        print('q is less than s')    
print('Not part of if block')


>>> q and s are both truthy
>>> q is 5
>>> s is 8

>>> phew, all this math; need a break

>>> q is less than s

>>> 'Not part of if block'

elif and else

Like if expressions, elif and if each evaluate whether a Python object or expression is truthy. elif and else statements are optional when using an if expression and you cannot use either of them without a preceding if expression. In other words, only if you use an if expression can you use an elif or else expression, but you don't have to use either just by using an if. The if / elif / else blocks run until the interpreter reaches the first truthy block. The interpreter then skips over the remaining corresponding blocks. If none of the conditional expressions are truthy, none of the statements run unless there is an else statement, in which case it runs.

Think of it as though an if statement can have, but doesn't have to have, corresponding elif and else statements. Similar to how you saw above with multiple if statements, use combinations of if, elif, and else to control how the Python interpreter reads your code. Essentially, write your code so that it "makes decisions" on which block or blocks to run based on truthiness of the conditions set.

When using elif statements, any elif expressions must come after an if or elif. If there is a corresponding else to the if, the elif statements must come before the else. So any and all elif blocks come after the if but before any else.

Like if statements, elif expressions have a condition and a statement. If the condition is truthy, the statement runs. You can have as many elif statements in a row as the memory allows. But be sure that the rest of us humans can understand your code. It is not required that you have any elif statements.

It is also not required that an if expression have a corresponding else statement. If it does, the else must appear after the if and after any and all elif expressions. The else expression has a statement but no explicit condition. Its condition is implicit. Its statement runs if all the corresponding if and elif expressions are falsy. You can have only one else for each if.

Here are some commone patterns:

if only
if / else
if / elif / else
if / elif . . . elif /else where . . . represents additional elif expressions

Here are examples.

if True: 
    pants = 12
    shirts = 10
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    elif inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    else inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 219.78 We have plenty more!

The code above prints 219.78 We have plenty more! because the if condition is True.

if True: 
    pants = 2
    shirts = 1
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    elif inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    else inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 17.97 Running low.

The code above prints 17.97 Running low. because the if condition is False and elif condition is True.

if True: 
    pants = 0
    shirts = 0
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    elif inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    else inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 0 None in inventory.

The code above prints 0 None in inventory. because the if and elif conditions are False.

if True: 
    pants = -1
    shirts = 0
    inventory = pants +  shirts
    if inventory > 20:
	    price = 9.99
	    total_sale = inventory * price
	    print(total_sale, "We have plenty more!")
    elif inventory <= 20 and inventory > 0:
	    price = 5.99
	    total_sale = inventory * price
	    print(total_sale, "Running low.")
    else inventory == 0:
	    price = 0
	    total_sale = inventory * price
	    print(total_sale, "None in inventory.")

>>> 0 None in inventory.

The code above has its inventory at -1. Like the if example above, you owe someone a pair of pants! Instead of printing nothing like above where none of the if expressions accounted for negative inventory, the code above prints 0 None in inventory.. This difference occurs becuase else acts as a catchall, whereas the if conditions in the previous example have explicit conditions. The else expression's condition for running is implicit. That condition is whether all the conditions above it are falsy. Here they are all falsy, so it prints 0 None in inventory..

Here are additional basic examples of elif and else blocks.

# elif and else statement basic examples
if q:
    print("yes, q")
else:
    print("no q")

>>> "yes, q"

if r:
    print("yes, r")
else:
    print("no, r")

>>> "no, r"

if r:
    print("yes, r")
elif q or r:
    print("q or r")
else:
    print("no, q or r")

>>> "q or r"

if r:
    print("yes, r")
elif r == 10:
    print("r equals 10")
elif q or r:
    print("q or r")
else:
    print("no, q or r")

>>> "q or r"

Conditional Expressions

Conditional expressions look like if expressions but act like operators. Here is what a conditional expression looks like.

# define a variable
inventory_exists = True

# example conditional expression
print(True) if inventory_exists else print(False)

To understand how Python interprets this line, you have to find the if. The first step is Python checks the if condition. Here if inventory_exists equals True. If that condition exists, Python executes the line to the left of the if. Here the line prints True. If the condition does not exist, Python executes the line to the right of the else. Here the line prints False.

This is a boilerplate representation of how the condition expression works.

<when_condition_true> if <condition> else <when_condition_false>

Here are a few examples.

# conditional expressions basic examples
print("q > r") if q > r else print("q is not greater than r")
print("r > q") if r > q else print("r is not greater than q")

>>> q > r
>>> r is not greater than q

Although conditional expressions look similar to if statements, they act differently. They do not control program flow like if, elif, and else. Rather, they help Python decide which part of the line to run, but otherwise Python continues onto execute the next line.

Conditional expressions allow you to write more efficient code. Here is an if / else statement.

# define variables
q = 5
r = 0

# an example of if / else
if r < q:
    r += q
    print(r)
else:
    r
    print(r)

>>> 5

Here is the if / else statement written as a conditional expression.

# define variables
q = 5
r = 0

# same as above but written as a conditional expression
r += q if r < q else r
print(r)

>>> 5

Be thoughtful about when and how you use conditional expressions. Although they make your code more efficient in terms of number of lines, they can be difficult to understand sometimes.

Pass

Python’s pass statement tells the interpreter to skip ahead to the next line of code. It is commonly used in development when you want to put a placeholder block of code, yet you don’t want your code to break. Or you may find an actual use for skipping a block of code in your app's flow.

Here is an example of an if expression that doesn't have a statement and an example that uses pass.

# define variables
q = 5
r = 0

# error results "expected an indented block"
if r < q:

print("Passed!") 

>>> print("Passed!") 
        ^
>>> IndentationError: expected an indented block


# no error because of pass
if r < q:
    pass

print("Passed!") 

>>> Passed!

The example that does not have a statement for the if expression returns an error IndentationError: expected an indented block. It therefore does not print Passed!. The example that uses pass has no error. It prints Passed!.

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