Python for Beginners: A Complete Getting Started Guide

python --version

brew install python
python3 --version

python3 --version

print("Hello, World!")

python hello.py

name = "Alex"
age = 30
height = 5.11
is_admin = True

print(name)    # Alex
print(age)     # 30

print(type("Hello"))  # <class 'str'>
print(type(42))       # <class 'int'>
print(type(3.14))     # <class 'float'>

first_name = "Jane"
last_name = "Smith"

# Combine strings (concatenation)
full_name = first_name + " " + last_name
print(full_name)  # Jane Smith

# f-strings (the modern, preferred way to insert variables into text)
greeting = f"Hello, {first_name}! You are {30} years old."
print(greeting)  # Hello, Jane! You are 30 years old.

# Useful string methods
message = "  Hello, World!  "
print(message.strip())       # "Hello, World!" (removes whitespace)
print(message.lower())       # "  hello, world!  "
print(message.upper())       # "  HELLO, WORLD!  "
print(message.replace("World", "Python"))  # "  Hello, Python!  "

# Check length
print(len("Hello"))  # 5

# Check if text contains something
print("World" in message)  # True

# Basic arithmetic
print(10 + 3)   # 13 (addition)
print(10 - 3)   # 7 (subtraction)
print(10 * 3)   # 30 (multiplication)
print(10 / 3)   # 3.333... (division — always returns a float)
print(10 // 3)  # 3 (integer division — rounds down)
print(10 % 3)   # 1 (modulo — the remainder)
print(10 ** 3)  # 1000 (exponent — 10 to the power of 3)

# Converting between types
price = "49.99"
price_number = float(price)  # Convert string to decimal number
quantity = int("5")          # Convert string to whole number
label = str(100)             # Convert number to string

# Create a list
fruits = ["apple", "banana", "cherry"]
numbers = [10, 20, 30, 40, 50]
mixed = ["hello", 42, True, 3.14]  # lists can hold different types

# Access items by position (index starts at 0)
print(fruits[0])   # "apple" (first item)
print(fruits[1])   # "banana" (second item)
print(fruits[-1])  # "cherry" (last item)

# Modify a list
fruits.append("date")         # Add to the end
fruits.insert(1, "avocado")   # Insert at position 1
fruits.remove("banana")       # Remove by value
popped = fruits.pop()         # Remove and return the last item

# Check length
print(len(fruits))  # Number of items

# Check if something is in the list
print("apple" in fruits)  # True

# Sort a list
numbers.sort()               # Sort in place (ascending)
numbers.sort(reverse=True)   # Sort descending

# Slice a list (get a sub-list)
print(numbers[1:3])   # Items at index 1 and 2
print(numbers[:3])    # First 3 items
print(numbers[2:])    # Everything from index 2 onwards

# Create a dictionary
user = {
    "name": "Alex",
    "email": "[email protected]",
    "age": 30,
    "is_admin": True,
}

# Access values by key
print(user["name"])          # "Alex"
print(user.get("phone"))    # None (no error if key missing)
print(user.get("phone", "N/A"))  # "N/A" (default if missing)

# Add or update
user["phone"] = "555-1234"  # Add new key
user["age"] = 31            # Update existing key

# Remove
del user["is_admin"]

# Loop through a dictionary
for key, value in user.items():
    print(f"{key}: {value}")

# Check if a key exists
print("email" in user)  # True

age = 25

if age < 13:
    print("Child")
elif age < 18:
    print("Teenager")
elif age < 65:
    print("Adult")
else:
    print("Senior")
# Output: Adult

age = 25
has_id = True

if age >= 18 and has_id:
    print("Access granted")

if age < 18 or not has_id:
    print("Access denied")

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

# Loop through a range of numbers
for i in range(5):       # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):    # 1, 2, 3, 4, 5
    print(i)

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

# Loop through a dictionary
user = {"name": "Alex", "age": 30}
for key, value in user.items():
    print(f"{key} = {value}")

# Loop with index
fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

count = 0
while count < 5:
    print(f"Count is {count}")
    count += 1  # This is shorthand for count = count + 1

# Breaking out of a loop
while True:
    user_input = input("Type 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You typed: {user_input}")

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

# List comprehension (same result, one line)
squares = [x ** 2 for x in range(10)]

# With a condition
even_squares = [x ** 2 for x in range(10) if x % 2 == 0]
# [0, 4, 16, 36, 64]

# Define a function
def greet(name):
    """Say hello to someone."""
    print(f"Hello, {name}!")

# Call it
greet("Alex")   # Hello, Alex!
greet("Sarah")  # Hello, Sarah!

def add(a, b):
    return a + b

result = add(10, 20)
print(result)  # 30

def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Alex")              # Hello, Alex!
greet("Alex", "Good morning")  # Good morning, Alex!

def get_user():
    return "Alex", "[email protected]", 30

name, email, age = get_user()
print(name)   # Alex
print(email)  # [email protected]

def celsius_to_fahrenheit(celsius):
    """Convert Celsius to Fahrenheit."""
    return (celsius * 9/5) + 32

def fahrenheit_to_celsius(fahrenheit):
    """Convert Fahrenheit to Celsius."""
    return (fahrenheit - 32) * 5/9

# Use them
print(celsius_to_fahrenheit(100))  # 212.0
print(fahrenheit_to_celsius(72))   # 22.22...

# Read the entire file at once
with open("data.txt", "r") as file:
    content = file.read()
    print(content)

# Read line by line (better for large files)
with open("data.txt", "r") as file:
    for line in file:
        print(line.strip())  # .strip() removes trailing newline

# Write (creates file or overwrites existing)
with open("output.txt", "w") as file:
    file.write("Line 1\n")
    file.write("Line 2\n")

# Append (adds to end of existing file)
with open("output.txt", "a") as file:
    file.write("Line 3\n")

import csv

with open("people.csv", "r") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(f"{row['name']} - {row['email']}")

import csv

people = [
    {"name": "Alex", "email": "[email protected]", "age": 30},
    {"name": "Sarah", "email": "[email protected]", "age": 25},
]

with open("output.csv", "w", newline="") as file:
    writer = csv.DictWriter(file, fieldnames=["name", "email", "age"])
    writer.writeheader()
    writer.writerows(people)

# Basic try/except
try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number.")
except ZeroDivisionError:
    print("You can't divide by zero.")
except Exception as e:
    print(f"Something went wrong: {e}")

# Install a package
pip install requests

# Install a specific version
pip install pandas==2.2.0

# Install multiple packages from a file
pip install -r requirements.txt

# List installed packages
pip list

# Uninstall a package
pip uninstall requests

import json
import os

CONTACTS_FILE = "contacts.json"


def load_contacts():
    """Load contacts from the JSON file."""
    if not os.path.exists(CONTACTS_FILE):
        return []
    with open(CONTACTS_FILE, "r") as f:
        return json.load(f)


def save_contacts(contacts):
    """Save contacts to the JSON file."""
    with open(CONTACTS_FILE, "w") as f:
        json.dump(contacts, f, indent=2)


def add_contact(contacts):
    """Add a new contact."""
    name = input("Name: ").strip()
    email = input("Email: ").strip()
    phone = input("Phone: ").strip()

    if not name:
        print("Name is required.")
        return

    contacts.append({"name": name, "email": email, "phone": phone})
    save_contacts(contacts)
    print(f"Added {name}.")


def search_contacts(contacts):
    """Search contacts by name."""
    query = input("Search: ").strip().lower()
    results = [c for c in contacts if query in c["name"].lower()]

    if not results:
        print("No contacts found.")
        return

    for c in results:
        print(f"  {c['name']} | {c['email']} | {c['phone']}")


def list_contacts(contacts):
    """List all contacts."""
    if not contacts:
        print("No contacts yet.")
        return

    print(f"\n{'Name':<20} {'Email':<30} {'Phone':<15}")
    print("-" * 65)
    for c in contacts:
        print(f"{c['name']:<20} {c['email']:<30} {c['phone']:<15}")


def main():
    """Main menu loop."""
    contacts = load_contacts()
    print("Contact Manager")
    print("Commands: add, search, list, quit\n")

    while True:
        command = input("> ").strip().lower()

        if command == "add":
            add_contact(contacts)
        elif command == "search":
            search_contacts(contacts)
        elif command == "list":
            list_contacts(contacts)
        elif command == "quit":
            print("Goodbye!")
            break
        else:
            print("Unknown command. Try: add, search, list, quit")


if __name__ == "__main__":
    main()

python contact_manager.py

name = "Alex"          # str
age = 30               # int
price = 9.99           # float
is_active = True       # bool
items = [1, 2, 3]      # list
user = {"name": "Alex"}  # dict

# String formatting
f"Hello, {name}!"

# List operations
items.append(4)        # Add to end
items.pop()            # Remove last
len(items)             # Length
item in items          # Check membership

# Dictionary operations
user["key"]            # Get value (error if missing)
user.get("key", None)  # Get value (default if missing)
user["key"] = "value"  # Set value
del user["key"]        # Delete key

# If/else
if condition:
    ...
elif other_condition:
    ...
else:
    ...

# For loop
for item in collection:
    ...

# While loop
while condition:
    ...

def function_name(param1, param2="default"):
    """Description of what this function does."""
    return result