Python Control Flow — Core Concepts

Conditionals

if / elif / else

Python’s conditional structure is standard:

score = 75

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

Python evaluates conditions top-to-bottom and executes the first matching block. Subsequent conditions are skipped — even if they would also be true.

Truthiness

Python has a concept of “truthy” and “falsy” values. In any boolean context, many values evaluate to False:

• None
• 0, 0.0
• Empty sequences: "", [], (), {}
• Objects that define __bool__ returning False

Everything else is truthy. This lets you write concise conditions:

items = []
if not items:
    print("Nothing in the list")

name = "Alice"
if name:
    print(f"Hello, {name}")

Ternary Expression

Python has a one-line conditional expression (often called “ternary”):

label = "Adult" if age >= 18 else "Minor"

Use it for simple value selection. Avoid nesting it — deeply nested ternaries are unreadable.

Loops

for Loops

Python’s for iterates over any iterable — lists, strings, ranges, dictionaries, files, and anything else that implements iteration:

# Over a list
for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

# Over a string
for char in "Python":
    print(char)

# Over a range
for i in range(5):       # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2):  # 2, 4, 6, 8
    print(i)

range() generates numbers on demand without storing them all in memory — an important detail for large ranges.

Common Patterns

Enumerate — when you need both index and value:

items = ["a", "b", "c"]
for i, item in enumerate(items):
    print(f"{i}: {item}")
# 0: a
# 1: b
# 2: c

Zip — iterate over two sequences together:

names = ["Alice", "Bob"]
scores = [92, 87]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

while Loops

while repeats as long as a condition is true:

attempts = 0
while attempts < 3:
    guess = input("Guess the number: ")
    if guess == "42":
        print("Correct!")
        break
    attempts += 1

Use while when you don’t know ahead of time how many iterations you need. Use for when you’re iterating over a collection or a known range.

Loop Control

break

Exits the loop immediately:

for n in range(100):
    if n == 7:
        break   # Stop here, don't continue

continue

Skips to the next iteration:

for n in range(10):
    if n % 2 == 0:
        continue   # Skip even numbers
    print(n)       # Prints 1, 3, 5, 7, 9

else on Loops (Python-Unique)

Python lets you attach else to a loop. It runs when the loop finishes without hitting a break:

for item in items:
    if item == target:
        print("Found it!")
        break
else:
    print("Not found")   # Only runs if break never hit

This pattern cleanly handles “search and report” logic. Most Python developers don’t know about it; the ones who do use it regularly.

The Walrus Operator (:=)

Introduced in Python 3.8, := assigns a value and evaluates it in the same expression:

import re

if m := re.search(r"\d+", text):
    print(f"Found number: {m.group()}")

Without walrus:

m = re.search(r"\d+", text)
if m:
    print(f"Found number: {m.group()}")

The walrus version is more concise, particularly useful in while loops reading chunks of data:

with open("large_file.txt") as f:
    while chunk := f.read(8192):
        process(chunk)

match / case (Python 3.10+)

Python 3.10 added structural pattern matching — similar to switch statements in other languages, but more powerful:

command = "quit"

match command:
    case "quit":
        exit()
    case "go" | "move":
        move_player()
    case _:
        print(f"Unknown command: {command}")

It works on complex structures too:

match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"X-axis at {x}")
    case (0, y):
        print(f"Y-axis at {y}")
    case (x, y):
        print(f"Point at ({x}, {y})")

Common Misconception: for Loops Are Only for Lists

Python’s for works on any iterable, including database query results, file lines, network streams, and custom objects. Anything that implements __iter__ and __next__ works.

for line in open("data.txt"):   # Reads one line at a time
    process(line)

This reads one line at a time without loading the entire file into memory — critical for large files.

One Thing to Remember

Python loops (for and while) can have an else clause that runs when no break occurred — it’s one of the language’s most underused features, and it’s perfect for “search and report” logic.