Python

This is a quick overview combined with practice problems. Things might appear in a reversed order sometimes - we'll introduce concepts by solving problems and covering tools as needed.

Resources

Free Books:

• Python 101
• Think Python
• A Practical Introduction to Python Programming

If you want to buy:

• Intro to Python for Computer Science and Data Science
• Python Crash Course

Your First Program

print("Hello, World!")

The print() Function

Optional arguments: sep and end

sep (separator) - what goes between values:

print("A", "B", "C")              # A B C (default: space)
print("A", "B", "C", sep="-")     # A-B-C
print(1, 2, 3, sep=" | ")         # 1 | 2 | 3

end - what prints after the line:

print("Hello")
print("World")
# Output:
# Hello
# World

print("Hello", end=" ")
print("World")
# Output: Hello World

Escape Characters

Practice

Variables and Assignment

A variable stores a value in memory so you can use it later.

x = 7
y = 3
total = x + y
print(total)  # 11

Multiple assignment:

x, y, z = 1, 2, 3

Variable Naming Rules

• Must start with letter or underscore
• Can contain letters, numbers, underscores
• Cannot start with number
• Cannot contain spaces
• Cannot use Python keywords (for, if, class, etc.)
• Case sensitive: age, Age, AGE are different

Assignment Operators

Reading Input

name = input("What's your name? ")
print(f"Hello, {name}!")

Converting input:

age = int(input("How old are you? "))
price = float(input("Enter price: $"))

Practice

Basic Data Types

Strings

Text inside quotes:

name = "Mahmoud"
message = 'Hello'

Can use single or double quotes. Strings can contain letters, numbers, spaces, symbols.

Numbers

• int → Whole numbers: 7, 0, -100
• float → Decimals: 3.14, 0.5, -2.7

Boolean

True or false values:

print(5 > 3)        # True
print(2 == 10)      # False
print("a" in "cat") # True

Logical Operators

Practice

Type Checking and Casting

print(type("hello"))  # <class 'str'>
print(type(10))       # <class 'int'>
print(type(3.5))      # <class 'float'>
print(type(True))     # <class 'bool'>

Type casting:

int("10")      # 10
float(5)       # 5.0
str(3.14)      # "3.14"
bool(0)        # False
bool(5)        # True
list("hi")     # ['h', 'i']

Sequences

Strings

Strings are sequences of characters.

Indexing

Indexes start from 0:

name = "Python"
print(name[0])   # P
print(name[3])   # h

String Operations

# Concatenation
"Hello" + " " + "World"  # "Hello World"

# Multiplication
"ha" * 3                 # "hahaha"

# Length
len("Python")            # 6

# Methods
text = "hello"
text.upper()             # "HELLO"
text.replace("h", "j")   # "jello"

Common String Methods

Practice

Lists

Lists can contain different types and are mutable (changeable).

numbers = [1, 2, 3]
mixed = [1, "hello", True]

List Operations

# Accessing
colors = ["red", "blue", "green"]
print(colors[1])  # "blue"

# Modifying (lists ARE mutable!)
colors[1] = "yellow"

# Adding
colors.append("black")          # Add at end
colors.insert(1, "white")       # Add at position

# Removing
del colors[1]                   # Remove by index
value = colors.pop()            # Remove last
colors.remove("red")            # Remove by value

# Sorting
numbers = [3, 1, 2]
numbers.sort()                  # Permanent
sorted(numbers)                 # Temporary

# Other operations
numbers.reverse()               # Reverse in place
len(numbers)                    # Length

Practice

Slicing

Extract portions of sequences: [start:stop:step]

Basic Slicing

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

numbers[2:5]      # [2, 3, 4]
numbers[:3]       # [0, 1, 2] - from beginning
numbers[5:]       # [5, 6, 7, 8, 9] - to end
numbers[:]        # Copy everything
numbers[::2]      # [0, 2, 4, 6, 8] - every 2nd element

Negative Indices

Count from the end: -1 is last, -2 is second-to-last

numbers[-1]       # 9 - last element
numbers[-3:]      # [7, 8, 9] - last 3 elements
numbers[:-2]      # [0, 1, 2, 3, 4, 5, 6, 7] - all except last 2
numbers[::-1]     # Reverse!

Practice

Control Flow

If Statements

age = 18

if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teen")
else:
    print("Child")

Practice

Loops

For Loops

# Loop through list
for fruit in ["apple", "banana"]:
    print(fruit)

# With index
for i, fruit in enumerate(["apple", "banana"]):
    print(f"{i}: {fruit}")

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

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

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

While Loops

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

Control Statements

Practice

String & List Exercises

F-Strings (String Formatting)

Modern, clean way to format strings:

name = 'Ahmed'
age = 45
txt = f"My name is {name}, I am {age}"

Number Formatting

pi = 3.14159265359

f'{pi:.2f}'              # '3.14' - 2 decimals
f'{10:03d}'              # '010' - pad with zeros
f'{12345678:,d}'         # '12,345,678' - commas
f'{42:>10d}'             # '        42' - right align
f'{1234.5:>10,.2f}'      # '  1,234.50' - combined

Functions in F-Strings

name = "alice"
f"Hello, {name.upper()}!"        # 'Hello, ALICE!'

numbers = [3, 1, 4]
f"Sum: {sum(numbers)}"           # 'Sum: 8'

String Methods

split() and join()

# Split
text = "one,two,three"
words = text.split(',')          # ['one', 'two', 'three']
text.split()                     # Split on any whitespace

# Join
words = ['one', 'two', 'three']
', '.join(words)                 # 'one, two, three'
''.join(['H','e','l','l','o'])   # 'Hello'

partition()

Splits at first occurrence:

email = "user@example.com"
username, _, domain = email.partition('@')
# username = 'user', domain = 'example.com'

Character Checks

'123'.isdigit()          # True - all digits
'Hello123'.isalnum()     # True - letters and numbers
'hello'.isalpha()        # True - only letters
'hello'.islower()        # True - all lowercase
'HELLO'.isupper()        # True - all uppercase

Two Sum Problem

# Input: nums = [2, 7, 11, 15], target = 9
# Output: [0, 1]  (because 2 + 7 = 9)

Brute Force Solution (O(n²))

nums = [2, 7, 11, 15]
target = 9

for i in range(len(nums)):
    for j in range(i + 1, len(nums)):
        if nums[i] + nums[j] == target:
            print([i, j])

Unpacking with * and **

Unpacking Iterables (*)

# Basic unpacking
numbers = [1, 2, 3]
a, b, c = numbers

# Catch remaining items
first, *middle, last = [1, 2, 3, 4, 5]
# first = 1, middle = [2, 3, 4], last = 5

# In function calls
def add(a, b, c):
    return a + b + c

numbers = [1, 2, 3]
add(*numbers)  # Same as add(1, 2, 3)

# Combining lists
list1 = [1, 2]
list2 = [3, 4]
combined = [*list1, *list2]  # [1, 2, 3, 4]

Unpacking Dictionaries (**)

# Merge dictionaries
defaults = {'color': 'blue', 'size': 'M'}
custom = {'size': 'L'}
final = {**defaults, **custom}
# {'color': 'blue', 'size': 'L'}

# In function calls
def create_user(name, age, city):
    print(f"{name}, {age}, {city}")

data = {'name': 'Bob', 'age': 30, 'city': 'NYC'}
create_user(**data)