If / Elif / Else Logic

Control the flow of scripts based on conditions using if, elif, and else.

The if Statement

An if statement executes a block of code only if a condition is True.

• Syntax: if <condition>: followed by an indented block
• Comparison operators: ==, !=, <, >, <=, >=, in
• Combine conditions with and, or, not

server_status = "running"

if server_status == "running":
    print("Service is active.")

Truthiness

Python treats many values as truthy or falsy in conditionals.

• Falsy: False, None, 0, 0.0, '', [], {}
• Truthy: non-zero numbers, non-empty sequences/collections

servers = ["web01", "web02"]
error_message = ""
default_config = {}

if servers:
    print(f"Processing {len(servers)} servers.")
if error_message:
    print("Something went wrong:", error_message)
if not default_config:
    print("Default config not available, please provide the configuration values.")

The else statement

Use else to execute code when the if condition is false.

cpu_usage = 85.0

if cpu_usage > 90.0:
    print("ALERT: High CPU Usage")
else:
    print("CPU Usage is normal.")

The elif statement

Chain multiple checks; the first true block runs.

http_status = 503

if http_status == 200:
    print("Status OK")
elif http_status == 404:
    print("Resource not found")
elif http_status >= 500:
    print("Server error (5xx)")
else:
    print("Another status:", http_status)

Guard Clauses

Handle edge cases at the top of functions to avoid deep nesting of if conditions.

def process_data_guarded(data):
    if not data:
        print("No data provided")
    elif not isinstance(data, list):
        print(f"Invalid value type for 'data'. Provided {type(data)}; Required: list")
    else:
        print(f"Processing {len(data)} items...")
        print("Processed")

process_data_guarded(None)
process_data_guarded([])
process_data_guarded("abc")
process_data_guarded(10)
process_data_guarded([1, 2, 3])