Python Control Flow: Conditionals and Loops

Control flow statements determine the order in which a program's instructions execute — whether a block runs at all, and how many times it repeats. Python provides conditional branching with if/elif/else, and two looping constructs: for loops (for iterating over a known sequence) and while loops (for repeating while a condition holds).

Because Python has no switch statement in most of its history, if/elif chains carry more weight than in some other languages, and Python's for loop is fundamentally different from a C-style counting loop — it iterates directly over the items of any iterable rather than manually managing an index.

Why it matters

Without conditions and loops a program can only run the same statements once, in order. Deciding what to do and repeating work are what make it useful, and every later topic, from processing a list to handling web requests, is built from if, for and while.

if / elif / else

An if statement evaluates a condition; if it is truthy, its indented block runs. Any number of elif ("else if") branches can follow to check additional conditions in order, and an optional final else branch runs only if none of the previous conditions matched. Only the first matching branch executes — Python does not fall through to later branches the way some other languages' switch statements can.

for Loops and range()

A for loop in Python iterates directly over the elements of an iterable — a string, list, tuple, dictionary, or any other iterable object — assigning each element to the loop variable in turn. To loop a specific number of times or generate a numeric sequence, Python provides the built-in range() function: range(5) produces 0 through 4, range(2, 10) produces 2 through 9, and range(0, 10, 2) produces 0, 2, 4, 6, 8 (start, stop, step).

while Loops and break / continue

A while loop repeats its block as long as its condition remains truthy, and is best suited to situations where the number of iterations is not known in advance. Inside any loop, break immediately exits the loop entirely, while continue skips the rest of the current iteration and moves on to the next one.

The Walrus Operator

Python 3.8 introduced the walrus operator, :=, which assigns a value to a name as part of a larger expression, rather than requiring a separate assignment statement first. It is most often used to avoid computing or calling something twice, for example inside a while loop's condition: while (line := input()) != "quit": assigns and tests line in one step.

Examples

score = 82

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print("Grade:", grade)

for i in range(0, 10, 2):
    if i == 6:
        continue
    print(i)

Grade: B
0
2
4
8

count = 0
while True:
    count += 1
    if count > 3:
        break
    print("count is", count)

numbers = [1, 2, 3, 4, 5]
total = 0
i = 0
while (n := numbers[i]) < 5:
    total += n
    i += 1
print("total before hitting 5:", total)

count is 1
count is 2
count is 3
total before hitting 5: 10

Exercise

Using one loop over the numbers 1 to 20, work out the sum of the even numbers and count how many numbers are divisible by 3. Print both.

Expected output: 110 then 6

even_sum = 0
divisible_by_three = 0

# TODO: loop over the numbers 1 to 20 and update both variables

print(even_sum)
print(divisible_by_three)

even_sum = 0
divisible_by_three = 0

for number in range(1, 21):
    if number % 2 == 0:
        even_sum += number
    if number % 3 == 0:
        divisible_by_three += 1

print(even_sum)
print(divisible_by_three)

Quick quiz

Interview questions