Control Flow & Functions

Master if/elif/else, for and while loops, and write reusable functions with arguments, return values, and default parameters.

Easy 50m 3 tasks

Prerequisites: Python Basics & Environment Setup

Learning Objectives

→ Use if/elif/else for conditional branching
→ Iterate with for loops and while loops
→ Use break, continue, and else clauses on loops
→ Define functions with positional and keyword arguments
→ Return values and understand scope

Conditionals

port = 443
if port == 80:
    print("HTTP")
elif port == 443:
    print("HTTPS")
elif port == 22:
    print("SSH")
else:
    print(f"Unknown service on port {port}")

Comparison operators: ==, !=, <, >, <=, >=
Logical: and, or, not

if port > 1024 and port < 65535:
    print("Ephemeral port range")

For Loops

ports = [22, 80, 443, 8080]
for p in ports:
    print(f"Checking port {p}...")

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

# enumerate() gives index + value
for idx, p in enumerate(ports, start=1):
    print(f"{idx}. Port {p}")

While Loops

attempts = 0
max_retries = 3
while attempts < max_retries:
    response = input("Password: ")
    if response == "secret":
        print("Access granted")
        break
    attempts += 1
else:
    print("Too many failed attempts")

The else clause on a loop runs when the loop finishes without hitting break.

Loop Control

for port in range(1, 1025):
    if port == 25:
        continue    # skip SMTP
    if port > 100:
        break       # stop scanning
    print(port)

Functions

def scan_port(host, port, timeout=1):
    """Returns True if port is open."""
    import socket
    s = socket.socket()
    s.settimeout(timeout)
    try:
        s.connect((host, port))
        return True
    except (socket.timeout, ConnectionRefusedError):
        return False
    finally:
        s.close()

result = scan_port("127.0.0.1", 22)
print("Open" if result else "Closed")

Key features:
- def keyword, snake_case name
- Default argument: timeout=1
- Docstring (first string inside function)
- Returns a value

Multiple Return Values

def parse_endpoint(endpoint):
    host, _, port = endpoint.rpartition(":")
    return host, int(port)

host, port = parse_endpoint("192.168.1.1:8080")
print(host, port)   # 192.168.1.1  8080

Scope

threshold = 1000    # global

def check_port(p):
    label = "high" if p > threshold else "low"  # reads global threshold
    return label

# Use 'global' keyword to modify a global from inside a function (avoid if possible)

Lambda Functions

ports = [443, 22, 80, 8080, 3306]
ports.sort(key=lambda p: p)           # sort ascending
high = list(filter(lambda p: p > 1024, ports))   # [8080, 3306]

Recursion Example

def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

print(factorial(5))   # 120

Mini Project – Port Range Scanner

import socket

def is_open(host, port, timeout=0.5):
    try:
        with socket.create_connection((host, port), timeout=timeout):
            return True
    except OSError:
        return False

def scan_range(host, start, end):
    open_ports = []
    for port in range(start, end + 1):
        if is_open(host, port):
            open_ports.append(port)
            print(f"  OPEN → {port}")
    return open_ports

if __name__ == "__main__":
    host = input("Host: ")
    start = int(input("Start port: "))
    end   = int(input("End port: "))
    results = scan_range(host, start, end)
    print(f"\nFound {len(results)} open ports.")