Python If-Else and Loops: Control Flow Explained Simply

Quick answer: Python uses if / elif / else to make decisions and for / while to repeat work. Control depends on indentation: every block under a condition or loop is indented four spaces, and moving one character sideways changes what runs. Comparisons (==, !=, <, >, in) produce booleans that drive every branch and loop.

Part 3 of our Python series. Need the prerequisite knowledge? Core data types comes first.

Why indentation is law

In most languages braces {} group statements. Python uses indentation itself — it is not cosmetic, it is the grammar. Two statements at different depths are two different blocks. This design makes Python code readable by enforcement: correct and pretty are the same thing.

temp = 35
if temp > 32:
    print("Hot day")          # runs only while indented
    print("Drink water")

De-indenting one line ends the block. totally flat or mixed tabs/spaces raise IndentationError — the error message will tell you the line number, read it literally.

if / elif / else in practice

Conditions run top-down and stop at the first match:

grade = 74

if grade >= 90:
    letter = "A"
elif grade >= 75:
    letter = "B"              # this branch fires for 74? No — 74 < 75
elif grade >= 60:
    letter = "C"              # 74 lands here
else:
    letter = "F"

print(letter)                 # C

for loops: repeat over a collection

A for loop walks through items — lists, strings, dicts, or number ranges:

scores = [88, 92, 79]

for s in scores:              # s takes 88, then 92, then 79
    print("Score:", s)

for ch in "Py":               # strings are sequences too
    print(ch)                 # P then y

for n in range(3):            # 0, 1, 2  — range is half-open
    print(n)

for i, s in enumerate(scores):          # value AND position
    print(f"{i + 1}. {s}")

Looping a dict: .keys() by default, .items() for both halves:

prices = {"rice": 60, "oil": 140}
for item, cost in prices.items():
    print(f"{item} costs {cost}")

while loops: repeat until something becomes false

for = known collection; while = unknown count, defined condition:

attempts = 0
password = "open"

while attempts < 3:
    guess = input("Password: ")
    attempts = attempts + 1       # (attempts += 1 is the shorthand)
    if guess == password:
        print("Unlocked")
        break                     # exit the loop now
else:
    print("Locked out")           # runs only if the loop was never broken

The rare while...else fires when the loop finishes naturally, with break never happening. Security systems and search loops use it constantly.

break, continue, and the two-loop idiom

• break — leave the loop entirely.
• continue — skip to the next item.
• Nested loops: break only escapes the innermost one — a classic interview trap.

for n in range(10):
    if n % 2 == 0:
        continue            # skip even numbers
    if n > 7:
        break               # stop once past 7
    print(n)                # 1, 3, 5, 7

Complete executable example

# flow-demo.py — control flow in one runnable script
temps = [21, 29, 35, 33, 19, 30]

hot_days = []                          # collect, don't just print
for t in temps:
    if t >= 30:                        # decision
        hot_days.append(t)             # side effect inside the branch

print(f"{len(hot_days)} hot days: {hot_days}")

total, count = 0, 0                    # while with a running total
while count < len(temps):
    total += temps[count]
    count += 1

average = total / len(temps)
if average >= 30:
    verdict = "Heat wave"
elif average >= 25:
    verdict = "Warm week"
else:
    verdict = "Pleasant week"
print(f"Average {average:.1f} — {verdict}")

Line by line: the for loop filters with if and appends into a new list (the most common real-world pattern); the while replicates what sum(temps) does built-in — deliberately, so you see both; :.1f formats the float to one decimal place; the final branch picks a verdict string.

Common mistakes and edge cases

• Comparing with = instead of == — if x = 5: is a SyntaxError; = assigns, == compares.
• Forgetting the colon — every if, elif, else, for, while header ends with :.
• Off-by-one in range — range(1, 5) yields 1, 2, 3, 4 — never 5. Half-open, always.
• Mutating a list while looping over it — appending inside for t in temps grows the loop forever in pathological cases; loop over a copy (for t in temps[:]) if you must edit the original.
• Infinite while loops — every while needs a line that moves the condition toward False. If your terminal hangs, Ctrl+C kills the script.

Key takeaways and challenge

• Indentation defines blocks; four spaces, no mixing.
• if/elif/else picks one path; for iterates, while repeats on condition.
• break exits, continue skips; dict loops use .items().

Challenge: write fizzbuzz.py that loops 1–15 and prints Fizz for multiples of 3, Buzz for multiples of 5, FizzBuzz for both (test that case first: n % 3 == 0 and n % 5 == 0), otherwise the number. This is the single most famous screening exercise on Earth — it will be asked in interviews.

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