The Seven Python Book

System Manual // Version 7.0 // Classified

// Table of Contents

Preface
Part One — First Steps
Part Two — The Tools of Expression
Part Three — Structuring Data
Part Four — Objects and Errors
Part Five — Files and Modules
Part Six — Practical Python
Part Seven — Going Further
Appendix — Cheat Sheet

Preface

This book teaches Python through seven progressive parts, each building on the last. It assumes no prior programming experience but rewards patience. By the end, you will be able to write real programs, understand the language deeply, and know where to go next.

Work through the parts in order. Type the code yourself — do not copy-paste. Muscle memory matters. When something confuses you, do not skip it; confusion is the feeling of learning.

"The only way to learn a language is to speak it badly until you speak it well."

Part One — First Steps

1. What Python Is

Python is a programming language created by Guido van Rossum in 1991. It is named after Monty Python, not the snake. It emphasizes readability: code should look like pseudocode that happens to run.

Python is interpreted — a program reads and executes it line by line. It is dynamically typed — you do not declare variable types. It is high-level — you do not manage memory manually.

You will find Python in web servers, data science, automation, games, machine learning, and education.

2. Installing Python

Go to python.org and download the latest stable version (3.12 or newer). On Windows, check “Add Python to PATH.” Verify:

python --version
# Python 3.12.3

You also get pip, the package installer. For serious work, install VS Code with the Python extension.

3. Your First Program

Create hello.py:

print("Hello, world!")

Run with python hello.py. print() is a function; the text is an argument; the quotes mark a string.

print("Python is fun.")
print(2 + 2)
print("2 + 2 =", 2 + 2)

4. Comments and Style

# This is a comment. Python ignores it.

"""
Multi-line string, often used as a docstring.
"""

def area(width, height):
    """Return the area of a rectangle."""
    return width * height

Style rules from PEP 8: four spaces per indent; snake_case for variables and functions; PascalCase for classes; lines under 79 characters.

5. Variables and Types

name = "Ada"
age = 36
height = 1.68
is_programmer = True

Type	Example	Meaning
`int`	`42`	whole number
`float`	`3.14`	decimal number
`str`	`"hello"`	text
`bool`	`True`/`False`	truth value
`NoneType`	`None`	absence of value

print(type(42))       # <class 'int'>
print(int("42"))      # 42
print(str(42))        # "42"
print(float("3.5"))   # 3.5

Create variables for your name, age, and favorite language, then print a sentence. Convert "100" to an integer and add 50.

Part Two — The Tools of Expression

6. Operators

print(7 + 3)    # 10
print(7 - 3)    # 4
print(7 * 3)    # 21
print(7 / 3)    # 2.333...  (true division)
print(7 // 3)   # 2         (floor division)
print(7 % 3)    # 1         (remainder)
print(7 ** 3)   # 343       (exponent)

Comparison: ==, !=, >, <, >=, <=. Logical: and, or, not. Assignment shortcuts: x += 5, x -= 3, x *= 2.

7. Strings in Depth

s = "Python"
print(s[0])      # P
print(s[-1])     # n
print(s[0:3])    # Pyt
print(s[::-1])   # nohtyP

text = "  Hello, World!  "
print(text.strip())                    # "Hello, World!"
print(text.lower())                    # "  hello, world!  "
print("apple,banana".split(","))       # ['apple', 'banana']
print("-".join(["a", "b", "c"]))       # a-b-c

name = "Ada"
age = 36
print(f"{name} is {age} years old.")
print(f"Pi to 2 decimals: {3.14159:.2f}")

Take " Data Science " and print it stripped, lowercased, and reversed. Ask for a name with input() and print "Hello, NAME!".

8. Conditionals

age = int(input("How old are you? "))

if age < 13:
    print("Child")
elif age < 20:
    print("Teenager")
elif age < 65:
    print("Adult")
else:
    print("Senior")

Ternary: status = "adult" if age >= 18 else "minor". Empty values ("", [], 0, None) are falsy.

9. Loops

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

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

count = 0
while count < 3:
    print("count is", count)
    count += 1

for n in range(10):
    if n == 5:
        break
    if n % 2 == 0:
        continue
    print(n)   # 1 3

Print all even numbers from 1 to 20. Use while to sum the numbers from 1 to 100.

Part Three — Structuring Data

10. Lists

fruits = ["apple", "banana", "cherry"]
fruits.append("date")
fruits[0] = "apricot"

nums = [3, 1, 4, 1, 5]
nums.sort()
nums.reverse()
nums.remove(4)
nums.pop()
nums.insert(0, 99)

letters = ["a", "b", "c", "d", "e"]
print(letters[1:4])    # ['b', 'c', 'd']
print(letters[::2])    # ['a', 'c', 'e']

squares = [x * x for x in range(5)]
evens = [x for x in range(10) if x % 2 == 0]

11. Tuples and Sets

point = (3, 4)
x, y = point     # unpacking

a = {1, 2, 3}
b = {3, 4, 5}
print(a & b)    # {3}       intersection
print(a | b)    # {1,2,3,4,5} union
print(a - b)    # {1, 2}    difference

12. Dictionaries

person = {
    "name": "Ada",
    "age": 36,
    "languages": ["Python", "C"]
}
print(person["name"])                # Ada
person["age"] = 37
print(person.get("job", "unknown"))  # unknown

for key, value in person.items():
    print(f"{key}: {value}")

squares = {x: x * x for x in range(5)}

Build a dict of three countries and their capitals. Given a list of words, count each word's frequency.

13. Functions

def greet(name):
    """Return a greeting for name."""
    return f"Hello, {name}!"

def power(base, exponent=2):
    return base ** exponent

print(power(3))       # 9
print(power(3, 3))    # 27

def total(*args):
    return sum(args)

def describe(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}={value}")

Write is_even(n). Write average(numbers).

Part Four — Objects and Errors

14. Classes and Objects

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def bark(self):
        return f"{self.name} says woof!"

    def __str__(self):
        return f"Dog({self.name}, {self.age})"

rex = Dog("Rex", 3)
print(rex.bark())   # Rex says woof!
print(rex)          # Dog(Rex, 3)

class Puppy(Dog):
    def bark(self):
        return f"{self.name} says yip!"

15. Errors and Exceptions

try:
    n = int(input("Number: "))
    print(10 / n)
except ValueError:
    print("Not a number.")
except ZeroDivisionError:
    print("Cannot divide by zero.")
else:
    print("No errors.")
finally:
    print("Always runs.")

def withdraw(amount):
    if amount < 0:
        raise ValueError("Amount must be positive")
    return amount

Common exceptions: ValueError, TypeError, KeyError, IndexError, FileNotFoundError.

Write a function that safely converts a string to int, returning None on failure.

Part Five — Files and Modules

16. File I/O

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")

with open("notes.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip())

with open("notes.txt", "a", encoding="utf-8") as f:
    f.write("Appended line\n")

17. Modules and Packages

# math_utils.py
def add(a, b):
    return a + b

# main.py
import math_utils
print(math_utils.add(2, 3))

from math_utils import add
print(add(2, 3))

import math
print(math.sqrt(16))       # 4.0

import random
print(random.randint(1, 10))

import datetime
print(datetime.date.today())

Install third-party packages with pip install requests.

18. JSON and CSV

import json

data = {"name": "Ada", "age": 36}
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

with open("data.json", "r") as f:
    loaded = json.load(f)
    print(loaded["name"])

import csv

with open("data.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "age"])
    writer.writerow(["Ada", 36])

Save a list of dicts to JSON, reload, print it. Read a CSV and compute an average.

Part Six — Practical Python

19. Virtual Environments

python -m venv venv
# Windows:
venv\Scripts\activate
# macOS/Linux:
source venv/bin/activate

pip install requests
pip freeze > requirements.txt
pip install -r requirements.txt

20. Regex

import re

text = "Call 555-1234 or 555-5678"
matches = re.findall(r"\d{3}-\d{4}", text)
print(matches)   # ['555-1234', '555-5678']

replaced = re.sub(r"\d", "*", "abc123")
print(replaced)   # abc***

if re.match(r"^\w+@\w+\.\w+$", "a@b.com"):
    print("valid email")

Pattern cheat: . any char, \d digit, \w word char, \s space; + one+, * zero+, ? optional; ^ start, $ end.

21. A Command-Line Tool

# wordcount.py
import sys

def main():
    if len(sys.argv) != 2:
        print("Usage: python wordcount.py FILE")
        return
    with open(sys.argv[1], encoding="utf-8") as f:
        text = f.read()
    words = text.split()
    print(f"Words: {len(words)}")

if __name__ == "__main__":
    main()

22. Project — Todo List

import json
import os

FILE = "todo.json"

def load():
    if not os.path.exists(FILE):
        return []
    with open(FILE) as f:
        return json.load(f)

def save(items):
    with open(FILE, "w") as f:
        json.dump(items, f, indent=2)

def show(items):
    for i, item in enumerate(items, 1):
        status = "✓" if item["done"] else " "
        print(f"[{status}] {i}. {item['text']}")

def main():
    items = load()
    while True:
        print()
        show(items)
        cmd = input("add/show/done/quit > ").strip()
        if cmd == "add":
            items.append({"text": input("Text: "), "done": False})
        elif cmd == "done":
            n = int(input("Number: ")) - 1
            if 0 <= n < len(items):
                items[n]["done"] = True
        elif cmd == "quit":
            break
        save(items)

if __name__ == "__main__":
    main()

Extend the todo app with delete, edit, and due dates.

Part Seven — Going Further

23. Testing

# test_math.py
import unittest
from math_utils import add

class TestAdd(unittest.TestCase):
    def test_positive(self):
        self.assertEqual(add(2, 3), 5)

    def test_negative(self):
        self.assertEqual(add(-1, 1), 0)

if __name__ == "__main__":
    unittest.main()

For bigger projects, use pytest: pip install pytest, then run pytest.

24. OOP Patterns

from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float

class Temperature:
    def __init__(self, celsius):
        self._c = celsius

    @property
    def celsius(self):
        return self._c

    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("Below absolute zero")
        self._c = value

25. The Wider Python World

Web: Flask, FastAPI, Django. Data: pandas, numpy, polars. Visualization: matplotlib, seaborn, plotly. ML: scikit-learn, PyTorch, TensorFlow. Automation: Playwright, Selenium, schedule. CLI: click, typer, rich. Async: asyncio, httpx.

Pick a domain and go deep. Breadth without depth is a hobby; depth without breadth is a trap.

26. Practice Plan

Weeks 1–2: Parts 1–3, small scripts daily. Weeks 3–4: Parts 4–5, build the todo project. Weeks 5–6: Part 6, write a CLI tool for a real problem. Ongoing: Exercism, LeetCode, Advent of Code. Build one medium project and put it on GitHub.

Appendix — Cheat Sheet

# Variables and types
x = 10
s = "hello"
b = True

# Strings
s.upper(), s.lower(), s.strip(), s.split(",")
f"{x} and {s}"

# Lists
lst = [1, 2, 3]
lst.append(4), lst.pop(), lst[0], lst[-1], lst[1:3]

# Dicts
d = {"k": "v"}
d["k"], d.get("missing", default), d.items()

# Loops
for i in range(5): ...
while cond: ...
[x for x in lst if x > 2]

# Files
with open("f.txt") as fh: data = fh.read()

# JSON
import json
json.load(fh), json.dump(obj, fh)

# Classes
class C:
    def __init__(self): ...
    def method(self): ...

Afterword

You have held in your hands the map of a language. But maps are not travel. The only way to know Python is to write it — badly at first, then less badly, then well. Every expert you admire wrote broken code and ran it and fixed it and wrote more.

Seven parts. Countless programs. Begin.