Booleans, comparisons, and if/elif/else

What you'll learn

• Compare values with ==, !=, <, >, <=, and >= to get a bool
• Combine conditions with and, or, and not
• Understand truthiness: which values count as True in a condition
• Branch a program with if, elif, and else

Concept

Every decision in a program comes down to a bool: True or False. Comparisons produce them. age >= 18 is not a question Python answers with words; it is an expression whose value is one of those two objects. Once you see comparisons as values, you can store them, print them, and pass them around like any other value.

Three words combine conditions. and is True only if both sides are, or is True if either side is, and not flips a value. Python reads these lazily: in a and b, if a is already False, b is never evaluated. That matters when b would fail, such as dividing by a number that might be zero.

Truthiness is the rule that lets any value stand in for a bool. Empty things are falsy: 0, 0.0, "", an empty list, and None. Everything else is truthy. So if name: means "if the name is not empty", which reads well and is exactly how Python developers write it. Be careful only with 0, which is a real value that happens to be falsy.

The if statement runs a block when its condition is truthy. elif adds another condition that is checked only if the ones above were false, and else catches everything left. Python goes down the chain in order and runs the first block whose condition holds, then skips the rest, so put the most specific test first.

Code

Comparisons are values. Store one, print it, and check its type. The last two lines show that an int and a float compare equal when they represent the same number, and that comparisons chain the way they do in math.

# comparisons.py
age = 29
is_adult = age >= 18

print(is_adult)
print(type(is_adult))
print(3 == 3.0)
print(18 <= age < 65)

True
<class 'bool'>
True
True

Combining conditions with and, or, and not. The last line relies on lazy evaluation: because count != 0 is False, the division is never attempted.

# combine.py
age = 16
has_permission = True
count = 0

print(age >= 18 and has_permission)
print(age >= 18 or has_permission)
print(not has_permission)
print(count != 0 and 100 / count > 1)

False
True
False
False

Truthiness in action. bool() shows how each value would be judged by an if. The for line is a loop you will meet properly in two lessons; for now read it as "for each value in this list".

# truthiness.py
for value in [0, 1, -3, 0.0, "", "hi", [], [0], None]:
    print(repr(value).ljust(6), bool(value))

0      False
1      True
-3     True
0.0    False
''     False
'hi'   True
[]     False
[0]    True
None   False

An if/elif/else chain. Because the branches are checked top to bottom, 92 matches the first test and never reaches the others.

# grade.py
score = 92

if score >= 90:
    grade = "A"
elif score >= 75:
    grade = "B"
elif score >= 60:
    grade = "C"
else:
    grade = "F"

print(f"{score} -> {grade}")

92 -> A

Try it yourself

Create ticket.py with an age variable. Print "free" for ages under 5, "child" for 5 to 12, "adult" for 13 to 64, and "senior" for 65 and above. Then add a is_member boolean and use and to print a "member discount" line only for paying adults who are members. Test the boundaries: 4, 5, 12, 13, 64, 65.

Common mistakes

• Using = where you mean ==. if age = 18: is a SyntaxError; one sign assigns, two compare.
• Writing if is_adult == True:. It works, but if is_adult: says the same thing and reads better; booleans are already conditions.
• Ordering elif tests wrongly. If score >= 60 comes before score >= 90, a 92 gets a C, because the first true branch wins.

What's next

• Comparisons produce bool values you can store and combine with and, or, and not.
• Empty and zero values are falsy, everything else is truthy, so if name: means "if not empty".
• if/elif/else runs the first true branch and skips the rest, so order the tests from most specific to least.

Your programs can choose a path. Next they will learn to repeat one, with while loops that run until a condition changes.