Python Control Flows

Control flow determines which code runs, when it runs, and how many times it runs. Python's control flow tools — conditional statements and loops — follow the same universal concepts covered in the Programming Language Principles series, expressed in Python's clean, indentation-based syntax.

Conditional Statements

Conditional statements execute a block of code only when a specified condition is true.

if Statement

age = 20

if age >= 18:
    print("You are an adult.")

The condition does not need parentheses (though they are allowed). The colon : and indentation define the block.

if / else

age = 16

if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

if / elif / else

Python uses elif (not else if) for additional conditions:

age = 20

if age < 18:
    print("Minor")
elif age < 21:
    print("Young adult")
elif age < 65:
    print("Adult")
else:
    print("Senior")

Python evaluates conditions from top to bottom and executes the first block whose condition is True. Once a match is found, the remaining branches are skipped entirely.

Nested if statements

Conditions can be nested inside other conditions:

is_logged_in = True
is_admin = False

if is_logged_in:
    if is_admin:
        print("Welcome, admin.")
    else:
        print("Welcome, user.")
else:
    print("Please log in.")

Keep nesting shallow where possible — deeply nested conditions are hard to read and are often a signal that the logic should be restructured.

Truthy and Falsy Values

Python evaluates conditions as boolean — any value can be used in a condition, not just explicit True or False. Understanding which values are truthy and which are falsy is essential.

Falsy values in Python

False
None
0           # integer zero
0.0         # float zero
""          # empty string
[]          # empty list
{}          # empty dictionary
()          # empty tuple
set()       # empty set

Everything else is truthy. This means you can write:

name = ""

if name:
    print(f"Hello, {name}")
else:
    print("Name is empty")   # This runs — empty string is falsy

This pattern — checking if a value is non-empty without an explicit comparison — is idiomatic Python.

Comparison and Logical Operators in Conditions

Chained comparisons

Python uniquely allows chaining comparison operators — a concise and readable feature:

age = 20

if 18 <= age < 65:
    print("Working age")

This is equivalent to age >= 18 and age < 65 but reads more naturally. JavaScript does not support this syntax.

Logical operators

if age >= 18 and has_id:
    print("Entry allowed")

if is_weekend or is_holiday:
    print("Day off")

if not is_banned:
    print("Access granted")

in and not in in conditions

allowed_roles = ["admin", "editor", "moderator"]
role = "editor"

if role in allowed_roles:
    print("Access granted")

if role not in ["banned", "suspended"]:
    print("Account active")

The Ternary Operator

Python has a one-line conditional expression — the equivalent of JavaScript's ternary operator:

# Syntax: value_if_true if condition else value_if_false
status = "adult" if age >= 18 else "minor"
print(status)

Compare with JavaScript:

const status = age >= 18 ? "adult" : "minor";

Python's syntax reads in a different order — the true value comes first, then the condition, then the false value. Use it for simple, readable one-liners. For complex conditions, a full if/else block is clearer.

match Statement (Python 3.10+)

Python 3.10 introduced structural pattern matching with the match statement — similar to switch in other languages but significantly more powerful:

day = 3

match day:
    case 1:
        print("Monday")
    case 2:
        print("Tuesday")
    case 3:
        print("Wednesday")
    case 4 | 5:
        print("Thursday or Friday")    # Multiple values with |
    case _:
        print("Weekend")               # _ is the default (catch-all)

Matching against strings

command = "quit"

match command:
    case "quit":
        print("Quitting...")
    case "help":
        print("Showing help...")
    case _:
        print(f"Unknown command: {command}")

Matching with conditions (guards)

point = (1, 5)

match point:
    case (x, y) if x == y:
        print("On the diagonal")
    case (x, y):
        print(f"Point at {x}, {y}")

match is more expressive than a simple switch. Its full power — matching against complex data structures — is covered in the Python-Specific Features topic.

Loops

Loops repeat a block of code either for a specific number of iterations or while a condition holds.

for Loop

Python's for loop iterates directly over any iterable — a sequence or collection of values. You do not need an index counter unless you explicitly want one.

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

for char in "Wariz":
    print(char)    # W a r i z (one per line)

range()

To iterate a specific number of times, use range():

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

range() accepts up to three arguments:

range(5)          # 0, 1, 2, 3, 4
range(2, 8)       # 2, 3, 4, 5, 6, 7
range(0, 10, 2)   # 0, 2, 4, 6, 8 — step of 2
range(10, 0, -1)  # 10, 9, 8, ..., 1 — count down

enumerate() — loop with index

When you need both the index and the value, use enumerate() instead of manually tracking a counter:

fruits = ["apple", "banana", "cherry"]

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

zip() — loop over multiple sequences

To iterate over two sequences in parallel:

names = ["Wariz", "Ada", "Aliyu"]
scores = [95, 87, 92]

for name, score in zip(names, scores):
    print(f"{name}: {score}")
# Wariz: 95
# Ada: 87
# Aliyu: 92

while Loop

Runs as long as the condition remains True:

count = 0

while count < 5:
    print(count)
    count += 1

Note: Always ensure the condition will eventually become False. A while True loop with no exit condition runs forever — an infinite loop. Use break (covered below) to exit when needed.

while True with break

A common and intentional pattern for loops that need to run until a specific event occurs:

while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

Loop Control Statements

break — exit the loop immediately

for i in range(10):
    if i == 5:
        break
    print(i)    # 0, 1, 2, 3, 4

continue — skip the current iteration

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

else on loops

Python has a unique feature: loops can have an else clause. The else block runs only if the loop completed without hitting a break:

for i in range(5):
    if i == 10:
        break
else:
    print("Loop completed without break")  # This runs

for i in range(5):
    if i == 3:
        break
else:
    print("This will NOT run — loop was broken")

This pattern is useful when searching for something in a loop — the else block runs if the item was not found:

target = 7
numbers = [1, 3, 5, 9, 11]

for n in numbers:
    if n == target:
        print("Found it!")
        break
else:
    print("Not found")    # Runs because target was never found

pass — the Do-Nothing Statement

pass is a no-op — it does nothing. It exists because Python requires at least one statement in every code block. Use it as a placeholder when you need a block to be syntactically valid but not do anything yet:

if age >= 18:
    pass    # TODO: implement adult logic
else:
    print("Minor")

for item in items:
    pass    # Placeholder — logic to be added

Summary