Conditionals: if, elif, else - Python Mastery

Making Decisions in Code

Conditionals let your program take different paths based on data. Python uses indentation — no curly braces. The colon at the end of the condition is mandatory.

temperature = 35

# Basic if
if temperature > 30:
    print("It's hot outside")
    print("Stay hydrated")

# if/else
if temperature > 30:
    print("Hot")
else:
    print("Not hot")

# if/elif/else — multiple conditions
if temperature >= 35:
    print("Very hot — danger zone")
elif temperature >= 25:
    print("Warm — comfortable")
elif temperature >= 15:
    print("Cool — grab a jacket")
elif temperature >= 0:
    print("Cold — dress warmly")
else:
    print("Freezing — stay inside")

Indentation is Syntax

Python uses indentation (4 spaces) to define code blocks. Unlike other languages, this isn't just style — it's how Python knows what's inside an if/else. Mixing spaces and tabs causes IndentationError. Use spaces, 4 per level.

Comparison and Logical Operators

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	True
`!=`	Not equal	`5 != 3`	True
`>`	Greater than	`5 > 3`	True
`<`	Less than	`3 < 5`	True
`>=`	Greater or equal	`5 >= 5`	True
`<=`	Less or equal	`4 <= 5`	True
`is`	Same object (identity)	`x is None`	depends
`in`	Membership test	`"a" in "cat"`	True
`not in`	Not member	`5 not in [1,2]`	True

age = 25
has_id = True

# and — BOTH must be true
if age >= 18 and has_id:
    print("Entry allowed")

# or — AT LEAST ONE must be true
if age < 12 or age >= 65:
    print("Discounted ticket")

# not — inverts boolean
if not has_id:
    print("You need ID")

# Chained comparisons — Python's unique feature!
score = 75
if 60 <= score < 80:
    print("Grade: B")    # More readable than: 60 <= score and score < 80

# Short-circuit evaluation
# 'and' stops at first False
# 'or' stops at first True
x = None
if x is not None and x > 0:  # Safe — stops at 'x is not None' if x is None
    print(x)

Ternary Operator

A compact one-liner for simple if/else assignments:

# Syntax: value_if_true if condition else value_if_false
age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # "adult"

# Useful in list comprehensions and assignments
numbers = [1, -2, 3, -4, 5]
abs_nums = [n if n >= 0 else -n for n in numbers]
# [1, 2, 3, 4, 5]

# Can be nested (but avoid for readability)
grade = "A" if score >= 90 else "B" if score >= 80 else "C"

Pattern Matching (Python 3.10+)

The match statement — Python's answer to switch/case — with powerful pattern matching:

command = "quit"

match command:
    case "quit" | "exit" | "q":
        print("Goodbye!")
    case "help":
        print("Available commands: quit, help, status")
    case "status":
        print("All systems running")
    case _:
        print(f"Unknown command: '{command}'")

# Pattern matching on structure (powerful!)
point = (0, 5)
match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"On x-axis at {x}")
    case (0, y):
        print(f"On y-axis at {y}")
    case (x, y):
        print(f"Point at ({x}, {y})")

# Match with guard conditions
value = 42
match value:
    case n if n < 0:
        print("Negative")
    case 0:
        print("Zero")
    case n if n > 0:
        print(f"Positive: {n}")

Common Patterns

# Guard clause — return early to avoid nesting
def process_order(order):
    if order is None:
        return "No order provided"
    if not order.get("items"):
        return "Order has no items"
    if order.get("total", 0) <= 0:
        return "Invalid total"
    
    # Happy path — no deep nesting
    return f"Processing order: {order}"

# Boolean as index (Pythonic trick)
value = 42
signs = ["negative", "non-negative"]
print(signs[value >= 0])  # "non-negative"

# Walrus operator := (Python 3.8+) — assign and test in one expression
import re
text = "Phone: 555-1234"
if match := re.search(r'\d{3}-\d{4}', text):
    print(f"Found: {match.group()}")  # Found: 555-1234

Key Takeaways

Indentation is mandatory: 4 spaces per level — it's syntax, not style
Chained comparisons: 0 < x < 10 works natively and reads naturally
Short-circuit evaluation: use for safety checks (x is not None and x.method())
Ternary for simple cases: x if condition else y — don't overuse
Guard clauses over nesting: return early to keep code flat and readable

Practice Exercises

Write a grade calculator: score 0-100 → letter grade (A: 90+, B: 80+, C: 70+, D: 60+, F: below 60).
Write code to check if a year is a leap year. Rules: divisible by 4, but not 100, unless also by 400. (2000 = leap, 1900 = not, 2024 = leap)
Write a basic login check: username "admin" AND password "secret123" → success; else print "Invalid credentials".
Rewrite the grade calculator using a match statement (Python 3.10+).

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