sudo apt update
sudo apt install python3
python3 --version

python --version        # Windows
python3 --version       # macOS and Linux

>>> 2 + 3
5
>>> "hello".upper()
'HELLO'
>>> name = "Ada"
>>> print("Hello,", name)
Hello, Ada

cd C:\Users\YourName\projects\     # Windows navigation
python hello.py                    # Windows execution

cd /home/yourname/projects/        # macOS/Linux navigation
python3 hello.py                   # macOS/Linux execution

python_course/
    chapter_01/
        hello.py
        practice.py
    chapter_03/
        expressions.py
        variables.py
    projects/
        unit_converter.py
        capstone/
            main.py
            data.csv

cd          # Show current directory (Windows)
pwd         # Show current directory (macOS/Linux)

cd python_course          # Move into a subfolder
cd chapter_03             # Move further into a subfolder
cd ..                     # Move up one level
cd ../projects            # Move up one, then into projects
cd ~                      # Home directory (macOS/Linux)
cd C:\Users\Name\projects # Full Windows path

dir             # Windows
ls              # macOS/Linux
ls -l           # Detailed listing (macOS/Linux)

python filename.py               # Windows
python3 filename.py              # macOS/Linux
python3 chapter_03/values.py    # File in a subfolder

score = 95
player_name = "Alex"
is_active = True

x = 10
x = x + 1          # update x
a, b = 3, 7         # assign multiple values at once

# This is a single-line comment
total = price + tax   # inline comment at end of a line

# Integers and floats
count = 42
price = 9.99

# Arithmetic operators
10 + 3      # 13    (addition)
10 - 3      # 7     (subtraction)
10 * 3      # 30    (multiplication)
10 / 3      # 3.333 (division — always returns float)
10 // 3     # 3     (floor division — discard remainder)
10 % 3      # 1     (modulo — remainder only)
10 ** 3     # 1000  (exponentiation)

# Conversion
int(3.9)    # 3   (truncates, does not round)
float(5)    # 5.0
round(3.7)  # 4

name = "Alice"
word = "Python"

len("hello")            # 5
word[0]                 # 'P'
word[-1]                # 'n'
word[0:3]               # 'Pyt'

"Hello" + " " + "World" # concatenation
"ha" * 3                # 'hahaha'

# f-strings
f"Hello, {name}! Score: {score}."

# Common methods
"hello".upper()             # 'HELLO'
"HELLO".lower()             # 'hello'
"  hello  ".strip()         # 'hello'
"hello world".split()       # ['hello', 'world']
", ".join(["a", "b", "c"])  # 'a, b, c'
"hello".replace("l", "r")   # 'herro'
"hello".find("ll")          # 2
"hello".count("l")          # 2

True
False

5 == 5      # True    (equality)
5 != 3      # True    (not equal)
5 > 3       # True
5 < 3       # False
5 >= 5      # True
5 <= 4      # False

True and False   # False
True or False    # True
not True         # False

print("Hello, world!")
print("Score:", 95)
print(f"Name: {name}, Age: {age}")
print()                             # blank line

name = input("Enter your name: ")  # always returns str
age = int(input("Enter your age: "))
price = float(input("Enter a price: "))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

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

# for loop with range
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
    print(i)

# for loop over a list
for name in names:
    print(name)

break       # exit loop immediately
continue    # skip to next iteration

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

greet("Alice")

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

result = add(3, 4)   # result is 7

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

def square(n):
    """Return the square of n."""    # docstring
    return n * n

numbers = [1, 2, 3, 4, 5]

numbers[0]              # 1
numbers[-1]             # 5
numbers[1:4]            # [2, 3, 4]
numbers[0] = 99         # update element

numbers.append(6)
numbers.insert(0, 0)
numbers.remove(3)
numbers.pop()
numbers.sort()
numbers.reverse()
numbers.count(2)
numbers.index(4)

3 in numbers            # True or False

for n in numbers:
    print(n)

student = {"name": "Alex", "grade": 90}

student["name"]                         # "Alex"
student.get("grade")                    # 90
student.get("missing", 0)               # 0 (default)
student["email"] = "[email protected]"  # add entry
student["grade"] = 95                   # update entry
del student["email"]                    # remove entry

"name" in student                       # True (key check)

for key, value in student.items():
    print(key, value)

student.keys()
student.values()
len(student)

with open("notes.txt", "r") as f:
    content = f.read()

with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())

with open("output.txt", "w") as f:
    f.write("Hello, file!\n")

with open("log.txt", "a") as f:
    f.write("New entry.\n")

class Student:
    def __init__(self, name, grade):
        self.name = name
        self.grade = grade

    def is_passing(self):
        return self.grade >= 60

    def __str__(self):
        return f"{self.name} ({self.grade})"

s1 = Student("Ana", 88)
print(s1.name)          # Ana
print(s1.is_passing())  # True
print(s1)               # Ana (88)

Traceback (most recent call last):
  File "calculator.py", line 12, in <module>
    result = divide(10, 0)
  File "calculator.py", line 5, in divide
    return a / b
ZeroDivisionError: division by zero

# Unclear
x = 92
y = x * 0.9

# Clear
exam_score = 92
discounted_price = exam_score * 0.9

# Variables — noun phrases (what it is)
student_count = 30
passing_grades = []

# Functions — verb phrases (what it does)
def calculate_average(scores): ...
def find_maximum(numbers): ...

# Good — explains the decision
# Multiply by 0.9 to apply a 10% discount
final_price = base_price * 0.9

# Unnecessary — restates the obvious
# Set x to 5
x = 5

def calculate_average(numbers):
    """Return the average of a list of numbers.

    Returns 0 if the list is empty.
    """
    if len(numbers) == 0:
        return 0
    return sum(numbers) / len(numbers)

# Repetitive
print(f"Student: {s1_name}, Grade: {s1_grade}")
print(f"Student: {s2_name}, Grade: {s2_grade}")

# Better — extract a function
def print_student(name, grade):
    print(f"Student: {name}, Grade: {grade}")

print_student(s1_name, s1_grade)
print_student(s2_name, s2_grade)

# Unnecessary
if is_active == True:
    ...

# Better
if is_active:
    ...

my_project/
    main.py
    student.py
    gradebook.py
    report.py
    data/
        students.csv
    tests/
        test_gradebook.py

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

greet("Alice", "Hello")      # "Alice", "Hello" = arguments

# Printing inside — result cannot be reused
def add_and_print(a, b):
    print(a + b)

x = add_and_print(3, 4)   # prints 7, but x is None

# Returning — result can be reused
def add(a, b):
    return a + b

x = add(3, 4)         # x is 7
y = add(x, 10)        # y is 17
print(x + y)          # prints 24

# Fragile — depends on global state
total = 0
def add_to_total(n):
    global total
    total += n

# Better — explicit and testable
def add(current_total, n):
    return current_total + n

total = add(total, 5)

def is_valid_score(score):
    """Return True if score is between 0 and 100."""
    return 0 <= score <= 100

def letter_grade(score):
    """Return the letter grade for a score."""
    if not is_valid_score(score):
        return "Invalid"
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

print(letter_grade(95))    # A
print(letter_grade(83))    # B
print(letter_grade(60))    # D
print(letter_grade(59))    # F
print(letter_grade(-5))    # Invalid
print(letter_grade(105))   # Invalid

s = "hello"
len(s)                  # 5
s[0]                    # 'h'
s[-1]                   # 'o'
s[1:4]                  # 'ell'
s.upper()               # 'HELLO'
s.strip()               # removes whitespace
s.split(",")            # splits on delimiter
", ".join(["a","b"])    # joins list
s.replace("l","r")      # 'herro'
s.find("ll")            # 2
s.count("l")            # 2

lst = [10, 20, 30, 40, 50]
lst[0]                  # 10
lst[-1]                 # 50
lst[1:4]                # [20, 30, 40]
lst[0] = 99             # update in place
lst.append(60)
lst.insert(0, 5)
lst.remove(30)
lst.pop()
lst.sort()
lst.reverse()
lst.count(10)
lst.index(20)
20 in lst               # True

point = (3, 7)
point[0]                # 3
len(point)              # 2
x, y = point            # unpacking
single = (42,)          # trailing comma required for single element

colors = {"red", "blue", "green"}
empty = set()           # NOT {} — that creates an empty dict

colors.add("yellow")
colors.remove("red")
colors.discard("pink")  # no error if not present

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
a | b                   # union: {1, 2, 3, 4, 5, 6}
a & b                   # intersection: {3, 4}
a - b                   # difference: {1, 2}

person = {"name": "Sam", "age": 25}
person["name"]                      # "Sam"
person.get("phone", "Unknown")      # safe lookup
person["email"] = "[email protected]"    # add
person["age"] = 26                  # update
del person["age"]                   # remove

"name" in person                    # True

for key, value in person.items():
    print(key, "->", value)

# List of dictionaries (collection of records)
students = [
    {"name": "Ana", "grade": 91},
    {"name": "Ben", "grade": 78}
]
students[0]["name"]     # "Ana"
for s in students:
    print(s["name"], s["grade"])

# Dictionary of lists (grouped values)
schedule = {
    "Monday":    ["Math", "Science"],
    "Tuesday":   ["English", "History"]
}
schedule["Monday"][0]   # "Math"

# Read entire file
with open("notes.txt", "r") as f:
    content = f.read()

# Read one line at a time
with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())

# Read all lines into a list
with open("notes.txt", "r") as f:
    lines = f.readlines()

# Write (creates or overwrites)
with open("output.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")

# Append to end
with open("log.txt", "a") as f:
    f.write("New entry.\n")

name,score,grade
Ana,91,A
Ben,78,C
Cal,85,B

import csv

# Read with csv.reader (rows as lists)
with open("students.csv", "r", newline="") as f:
    reader = csv.reader(f)
    header = next(reader)
    for row in reader:
        print(row)

# Read with csv.DictReader (rows as dictionaries)
with open("students.csv", "r", newline="") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

# Write with csv.writer
with open("results.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "score", "grade"])
    writer.writerow(["Ana", 91, "A"])

# Write with csv.DictWriter
students = [{"name": "Ana", "score": 91}]
with open("results.csv", "w", newline="") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "score"])
    writer.writeheader()
    for row in students:
        writer.writerow(row)

import math

math.sqrt(16)           # 4.0
math.floor(3.9)         # 3
math.ceil(3.1)          # 4
math.pi                 # 3.14159...
math.e                  # 2.71828...
math.log(100, 10)       # 2.0
math.pow(2, 10)         # 1024.0
math.factorial(5)       # 120
math.gcd(12, 8)         # 4

import random

random.random()                  # float in [0.0, 1.0)
random.randint(1, 10)            # integer in [1, 10] inclusive
random.uniform(1.0, 5.0)        # float in [1.0, 5.0]
random.choice(["a", "b", "c"])  # random item from list
random.shuffle(my_list)          # shuffle in place
random.sample(my_list, 3)       # 3 unique random items
random.seed(42)                  # set seed for reproducibility

from datetime import date, datetime, timedelta

today = date.today()
now = datetime.now()

today.strftime("%B %d, %Y")        # "November 15, 2024"
now.strftime("%H:%M")              # "14:32"
datetime.strptime("2024-01-01", "%Y-%m-%d")

yesterday = today - timedelta(days=1)
two_weeks = today + timedelta(weeks=2)

from pathlib import Path

p = Path("data/students.csv")
p.exists()          # True if file exists
p.name              # "students.csv"
p.stem              # "students"
p.suffix            # ".csv"
p.parent            # Path("data")

p.read_text()
p.write_text("content")

for f in Path("data").glob("*.txt"):
    print(f)

import os

os.getcwd()                          # current working directory
os.listdir(".")                      # files in current directory
os.path.exists("data.txt")           # True if path exists
os.path.join("data", "file.txt")     # "data/file.txt"
os.path.basename("/a/b/c.txt")       # "c.txt"

import sys

sys.argv            # command-line arguments list
sys.exit()          # terminate the program
sys.exit(1)         # terminate with error code

# Example: python greet.py Alice
if len(sys.argv) < 2:
    print("Usage: python greet.py <name>")
    sys.exit(1)
name = sys.argv[1]
print(f"Hello, {name}!")

import json

data = {"name": "Ana", "score": 91}
json_string = json.dumps(data)         # to string
parsed = json.loads(json_string)       # from string
parsed["name"]                         # "Ana"

with open("data.json", "w") as f:      # write to file
    json.dump(data, f, indent=4)

with open("data.json", "r") as f:      # read from file
    loaded = json.load(f)

import statistics

scores = [78, 85, 91, 63, 88]

statistics.mean(scores)     # average
statistics.median(scores)   # middle value
statistics.mode(scores)     # most common value
statistics.stdev(scores)    # sample standard deviation

class Student:
    def __init__(self, name, grade):  # initializer
        self.name = name              # instance attribute
        self.grade = grade

    def is_passing(self):             # method
        return self.grade >= 60

    def apply_bonus(self, points):
        self.grade += points

    def __str__(self):                # string representation
        return f"{self.name} ({self.grade})"

s = Student("Ben", 55)
print(s.is_passing())    # False
s.apply_bonus(10)
print(s.grade)           # 65
print(s.is_passing())    # True
print(s)                 # Ben (65)

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1

    def reset(self):
        self.count = 0

c = Counter()
c.increment()
c.increment()
print(c.count)     # 2
c.reset()
print(c.count)     # 0

students = [
    Student("Ana", 91),
    Student("Ben", 78),
    Student("Cal", 55)
]

for s in students:
    status = "passing" if s.is_passing() else "failing"
    print(f"{s.name}: {status}")

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

assert add(2, 3) == 5
assert add(0, 0) == 0
assert add(-1, 1) == 0

print("All tests passed.")

import unittest

def multiply(a, b):
    return a * b

class TestMultiply(unittest.TestCase):

    def test_positive(self):
        self.assertEqual(multiply(3, 4), 12)

    def test_zero(self):
        self.assertEqual(multiply(0, 5), 0)

    def test_negative(self):
        self.assertEqual(multiply(-2, 3), -6)

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

# Before — duplicated logic
if score1 >= 90: print(f"{name1}: A")
elif score1 >= 80: print(f"{name1}: B")
else: print(f"{name1}: F")

if score2 >= 90: print(f"{name2}: A")
elif score2 >= 80: print(f"{name2}: B")
else: print(f"{name2}: F")

# After — extracted function
def print_grade(name, score):
    if score >= 90: print(f"{name}: A")
    elif score >= 80: print(f"{name}: B")
    else: print(f"{name}: F")

print_grade(name1, score1)
print_grade(name2, score2)

Program: Flashcard Quiz Tool
Author: Student Name
Date: November 2024

Description:
    Reads questions from a text file and quizzes the user.
    Tracks the score and reports results at the end.

How to run:
    python flashcard.py questions.txt

File format (questions.txt):
    One question per line, question and answer separated by |
    Example:
    What is 7 * 8?|56
    Capital of France?|Paris