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🔀 Control Flow

Control flow determines which code runs based on conditions. It's how your program makes decisions.

What is Control Flow?

Without control flow, code runs line by line, top to bottom:

print("Line 1")
print("Line 2")
print("Line 3")
# Always prints all three lines

With control flow, you can make decisions:

if weather == "sunny":
    print("Go outside!")
else:
    print("Stay inside.")
# Only one message prints, depending on the condition

The if Statement

The most basic form of control flow.

age = 20

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

Key Points:

  • The condition must evaluate to True or False
  • The colon (:) is required after the condition
  • The code block must be indented (usually 4 spaces)

if-else Statement

Do one thing or another.

temperature = 35

if temperature > 30:
    print("It's hot!")
else:
    print("It's not too hot.")

The Flow:

       ┌─────────────────┐
       │  temperature>30 │
       └───────┬─────────┘
              / \
         True/   \False
            /     \
   ┌───────▼─┐   ┌─▼───────┐
   │ "It's   │   │ "It's   │
   │  hot!"  │   │ not..." │
   └─────────┘   └─────────┘

if-elif-else Statement

Multiple conditions, check one after another.

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is {grade}")  # B

Key Points:

  • elif is short for "else if"
  • Only the first matching condition runs
  • else catches everything that didn't match
  • else is optional

Common Mistake:

score = 95

# ❌ Wrong: All conditions checked independently
if score >= 60:
    print("D")  # This runs!
if score >= 70:
    print("C")  # This also runs!
if score >= 80:
    print("B")  # This also runs!
if score >= 90:
    print("A")  # This also runs!

# ✅ Correct: Use elif for mutually exclusive conditions
if score >= 90:
    print("A")  # Only this runs
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")

Nested Conditions

Conditions inside conditions.

has_ticket = True
age = 15

if has_ticket:
    if age >= 18:
        print("Welcome to the movie!")
    else:
        print("You need a guardian for this movie.")
else:
    print("Please buy a ticket first.")

When to Nest:

  • When one condition depends on another
  • Keep nesting shallow (2-3 levels max)
  • Consider using and / or to flatten

Flattened Version:

if not has_ticket:
    print("Please buy a ticket first.")
elif age < 18:
    print("You need a guardian for this movie.")
else:
    print("Welcome to the movie!")

Conditional Expressions (Ternary Operator)

A one-liner for simple if-else.

# Traditional if-else
if age >= 18:
    status = "adult"
else:
    status = "minor"

# Conditional expression (same result)
status = "adult" if age >= 18 else "minor"

Syntax:

value_if_true if condition else value_if_false

Best Used For:

# Simple assignments
max_value = a if a > b else b

# Default values
name = username if username else "Anonymous"

# Quick formatting
print(f"You have {count} item{'s' if count != 1 else ''}")

When NOT to Use:

# ❌ Too complex, hard to read
result = "A" if score >= 90 else "B" if score >= 80 else "C" if score >= 70 else "F"

# ✅ Better as regular if-elif-else
if score >= 90:
    result = "A"
elif score >= 80:
    result = "B"
# ...

Truthy and Falsy Values

In conditions, some values act as True or False:

Falsy Values (evaluate to False):

bool(False)    # False
bool(None)     # False
bool(0)        # False
bool(0.0)      # False
bool("")       # False (empty string)
bool([])       # False (empty list)
bool({})       # False (empty dict)

Truthy Values (evaluate to True):

bool(True)     # True
bool(1)        # True (any non-zero number)
bool(-1)       # True
bool("hello")  # True (non-empty string)
bool([1, 2])   # True (non-empty list)
bool({"a": 1}) # True (non-empty dict)

Practical Use:

# Check if a list has items
items = []
if items:
    print(f"You have {len(items)} items")
else:
    print("Your list is empty")

# Check if a string has content
name = ""
if name:
    print(f"Hello, {name}")
else:
    print("Hello, stranger")

Common Patterns

Guard Clauses

Check for invalid cases first, return early:

def process_order(order):
    if order is None:
        return "No order provided"

    if len(order.items) == 0:
        return "Order is empty"

    if not order.is_paid:
        return "Order not paid"

    # Main logic here (only reached if all checks pass)
    return "Processing order..."

Default Values with or

# If username is empty/None, use "Guest"
display_name = username or "Guest"

Safe Access with and

# Only access .name if user exists
name = user and user.name

Quick Reference

# Basic if
if condition:
    # code

# If-else
if condition:
    # code if true
else:
    # code if false

# If-elif-else
if condition1:
    # code
elif condition2:
    # code
else:
    # code

# Conditional expression
value = true_val if condition else false_val

# Truthy/Falsy shorthand
if my_list:  # same as: if len(my_list) > 0
    pass

if name:  # same as: if name != ""
    pass

Next Steps

Practice with examples.py, then try exercises.py!