Beginner

Control Flow

Learn how to make decisions with conditionals, repeat actions with loops, and write concise code with comprehensions.

if / elif / else Statements

Conditionals let your program make decisions based on conditions:

Python
age = 18

if age < 13:
    print("Child")
elif age < 18:
    print("Teenager")
elif age < 65:
    print("Adult")
else:
    print("Senior")
# Output: Adult
💡
Indentation matters! Python uses indentation (4 spaces by convention) to define code blocks. Incorrect indentation causes IndentationError.

Comparison Operators

OperatorMeaningExample
==Equal to5 == 5 → True
!=Not equal to5 != 3 → True
<Less than3 < 5 → True
>Greater than5 > 3 → True
<=Less than or equal5 <= 5 → True
>=Greater than or equal5 >= 3 → True

Logical Operators

Python
x = 15

# and - both must be True
if x > 10 and x < 20:
    print("Between 10 and 20")

# or - at least one must be True
if x < 5 or x > 10:
    print("Outside 5-10 range")

# not - inverts the boolean
is_raining = False
if not is_raining:
    print("Go outside!")

# Chained comparison (Pythonic)
if 10 < x < 20:
    print("Between 10 and 20")

for Loops

Iterate over sequences (lists, strings, ranges, etc.):

Python
# Loop over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Loop with index using enumerate
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# Loop over a string
for char in "Python":
    print(char, end=" ")  # P y t h o n

# Loop over a dictionary
scores = {"Alice": 95, "Bob": 87}
for name, score in scores.items():
    print(f"{name}: {score}")

range() Function

Python
# range(stop)
for i in range(5):
    print(i, end=" ")  # 0 1 2 3 4

# range(start, stop)
for i in range(2, 6):
    print(i, end=" ")  # 2 3 4 5

# range(start, stop, step)
for i in range(0, 20, 5):
    print(i, end=" ")  # 0 5 10 15

# Count backwards
for i in range(5, 0, -1):
    print(i, end=" ")  # 5 4 3 2 1

while Loops

Python
# Basic while loop
count = 0
while count < 5:
    print(count, end=" ")
    count += 1  # 0 1 2 3 4

# User input loop
while True:
    answer = input("Type 'quit' to exit: ")
    if answer == "quit":
        break

break, continue, pass

Python
# break - exit the loop entirely
for i in range(10):
    if i == 5:
        break
    print(i, end=" ")  # 0 1 2 3 4

# continue - skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")  # 1 3 5 7 9

# pass - placeholder (do nothing)
for i in range(5):
    pass  # TODO: implement later

List Comprehensions

A concise, Pythonic way to create lists from loops:

Python
# Basic comprehension
squares = [x**2 for x in range(10)]
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# With condition (filter)
evens = [x for x in range(20) if x % 2 == 0]
# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# With transformation
names = ["alice", "bob", "charlie"]
capitalized = [name.capitalize() for name in names]
# ['Alice', 'Bob', 'Charlie']

# Ternary in comprehension
labels = ["even" if x % 2 == 0 else "odd" for x in range(5)]
# ['even', 'odd', 'even', 'odd', 'even']

Nested Loops

Python
# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}", end="  ")
    print()  # New line

# Nested list comprehension (flatten a matrix)
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [num for row in matrix for num in row]
# [1, 2, 3, 4, 5, 6, 7, 8, 9]
✅
Pythonic tip: Prefer list comprehensions over manual for-loop appending when the logic is simple. But if a comprehension becomes hard to read, switch back to a regular loop.