Control Flow

if/elif/else

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'Grade: {grade}')  # Grade: B

Ternary Operator

# Conditional expression
age = 20
status = 'adult' if age >= 18 else 'minor'

# Nested ternary (avoid!)
result = 'positive' if x > 0 else 'zero' if x == 0 else 'negative'

Match-Case (Python 3.10+)

def http_status(code):
    match code:
        case 200:
            return 'OK'
        case 404:
            return 'Not Found'
        case 500:
            return 'Server Error'
        case _:
            return 'Unknown'

# Pattern matching with guards
match command:
    case ['quit']:
        exit()
    case ['go', direction] if direction in ['north', 'south', 'east', 'west']:
        move(direction)
    case ['get', item]:
        pick_up(item)

Common Patterns

# Truthy/falsy checking
if items:           # Instead of: if len(items) > 0:
    process(items)

if not name:        # Instead of: if name == '':
    name = 'Anonymous'

# Chained comparisons
if 0 < x < 100:
    valid = True

# Guard clauses
def process(data):
    if not data:
        return None
    if not isinstance(data, list):
        return None
    # Main logic here

Basic For Loop

# Iterating over a list
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

# Using range
for i in range(5):     # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10):  # 2, 3, ..., 9
    print(i)

for i in range(0, 10, 2):  # 0, 2, 4, 6, 8
    print(i)

for i in range(10, 0, -1):  # 10, 9, ..., 1
    print(i)

Enumerate

# Get index and value
fruits = ['apple', 'banana', 'cherry']

for index, fruit in enumerate(fruits):
    print(f'{index}: {fruit}')

# Custom start index
for index, fruit in enumerate(fruits, start=1):
    print(f'{index}. {fruit}')

Iterating Over Different Types

# Strings
for char in 'Hello':
    print(char)  # H, e, l, l, o

# Dictionaries
person = {'name': 'Alice', 'age': 30}
for key, value in person.items():
    print(f'{key}: {value}')

# Only keys
for key in person:
    print(key)

# Sets
unique = {1, 2, 3}
for num in unique:
    print(num)

List Comprehension (Preview)

# Instead of:
squares = []
for x in range(10):
    squares.append(x ** 2)

# Write:
squares = [x ** 2 for x in range(10)]

# With condition
evens = [x for x in range(10) if x % 2 == 0]

Loop Control

# break - exit loop
for i in range(100):
    if i == 5:
        break  # Stop at 5

# continue - skip iteration
for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)  # 1, 3, 5, 7, 9

# pass - do nothing
for i in range(10):
    if i % 2 == 0:
        pass  # TODO: handle later

Basic While Loop

count = 0
while count < 5:
    print(count)
    count += 1

# Output: 0, 1, 2, 3, 4

While with User Input

while True:
    user_input = input('Enter command: ')
    if user_input == 'quit':
        break
    print(f'Processing: {user_input}')

Avoid Infinite Loops

# ⚠️ Dangerous: no termination condition
while True:
    pass  # Program hangs!

# ✅ Safe: always have a way out
while True:
    user_input = input('Enter: ')
    if user_input == 'quit':
        break

For-Else and While-Else

# else clause runs if loop completes without 'break'
def find_item(items, target):
    for item in items:
        if item == target:
            print(f'Found {target}')
            break
    else:
        print(f'{target} not found')

find_item([1, 2, 3], 4)  # 4 not found
find_item([1, 2, 3], 2)  # Found 2

Nested Loops

# Matrix traversal
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in matrix:
    for item in row:
        print(item, end=' ')
    print()

# Output:
# 1 2 3
# 4 5 6
# 7 8 9

Loop Performance Tips

# ✅ Use enumerate instead of range(len())
for i, item in enumerate(items):  # Pythonic
    pass

for i in range(len(items)):  # Less Pythonic
    pass

# ✅ Use zip for parallel iteration
names = ['Alice', 'Bob']
ages = [30, 25]
for name, age in zip(names, ages):
    print(f'{name} is {age}')

# ✅ Use any() and all()
numbers = [1, 3, 5, 7]
all_odd = all(n % 2 == 1 for n in numbers)  # True
any_even = any(n % 2 == 0 for n in numbers)  # False