What is Control Flow in Python?

Control flow in Python determines the order in which statements are executed. It allows programs to make decisions, repeat tasks, and branch logic based on conditions. The key components of control flow in Python are:

  1. Conditional Statements (if, elif, else)
  2. Loops (for, while)
  3. Control Flow Tools (break, continue, pass)

Conditional Statements in Python

Overview:

Conditional statements allow the program to execute specific blocks of code based on conditions.

Syntax:

if condition:
    # Code block 1
elif another_condition:
    # Code block 2
else:
    # Code block 3

Example:

age = 20

if age < 18:
    print("You are a minor.")
elif age == 18:
    print("You just became an adult!")
else:
    print("You are an adult.")

Output

You are an adult.

Loops in Python

Loops repeat a block of code as long as a condition is true. Python offers two types of loops: for and while.

A. for Loop

The for loop iterates over a sequence (e.g., list, string, range).

Syntax:

for item in sequence:
    # Code block

Example:

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

for fruit in fruits:
    print(f"I like {fruit}.")

Output:

I like apple.
I like banana.
I like cherry.

Using range() in a for Loop:

for i in range(5):
    print(i)

Output:

0
1
2
3
4

B. while Loop

The while loop executes a block of code as long as a condition is true.

Syntax:

while condition:
    # Code block

Example

count = 0

while count < 5:
    print(f"Count is {count}")
    count += 1

Output

Count is 0
Count is 1
Count is 2
Count is 3
Count is 4

Control Flow Tools

Python provides tools to manage loops and conditions effectively.

A. break Statement

  • Exits the loop prematurely.

Example:

 
for i in range(10):
    if i == 5:
        break
    print(i)

Output:

0
1
2
3
4

B. continue Statement

  • Skips the rest of the code in the loop for the current iteration.

Example:

for i in range(3):
    if i == 1:
        pass  # Placeholder
    print(i)

Output:

for i in range(3):
    if i == 1:
        pass  # Placeholder
    print(i)

Best Practices for Control Flow in Python

Use Descriptive Conditions:

  • Write clear and meaningful conditions to improve readability.
if age > 18:  # Clear condition

  • Avoid Deep Nesting:

    • Refactor deeply nested if statements into simpler logic or functions.

  • Use break and continue Sparingly:

    • Overusing them can make code harder to follow.

  • Leverage else with Loops:

    • Use else after loops to run code if no break was encountered.
for i in range(5):
    if i == 3:
        break
else:
    print("No break encountered.")

  • Test Edge Cases:

    • Test your control flow logic for all possible conditions.

  • Comment Complex Logic:

    • Add comments to explain the intent of complex conditions or loops.

Practice Exercise

Task: FizzBuzz Problem

Write a program that prints numbers from 1 to 20. For multiples of 3, print “Fizz”; for multiples of 5, print “Buzz”; for multiples of both, print “FizzBuzz.”

Solution:

for i in range(1, 21):
    if i % 3 == 0 and i % 5 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

Output

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz