Python Lesson 1 - Setup, Basics, and Strings

What you will learn

In this lesson you will learn:

Python versions and why they matter
Why Java can run the “same code” on multiple operating systems (JVM) and how that compares to Python
How to install Python on Linux, macOS, and Windows
IDEs: VS Code and PyCharm, and how to open new/existing projects
Python basics:
- arithmetic operators
- comments and printing
- strings and escaping
- variables and naming styles
- data types and casting
- input from the user
- string slicing
- common string methods

1) Python versions (what students must know)

Python has major versions:

Python 2 (old, end-of-life)
Python 3 (current)

Most modern learning and production code uses Python 3.

Check your Python version

On Linux/macOS:

python3 --version

On Windows (PowerShell):

py --version
python --version

2) JVM vs Python: “write once run anywhere”

2.1 How Java does it

Java code is compiled into bytecode (.class) that runs on the JVM (Java Virtual Machine).

Each OS has a JVM implementation
Your Java bytecode runs on top of the JVM

That is why the same Java program can run on Windows, Linux, and macOS.

2.2 How Python does it

Python code is usually executed by a Python interpreter.

Each OS has a Python interpreter build
Your .py files run on the interpreter

Python also compiles to bytecode internally (.pyc), but the main point for beginners:

You need Python installed (or packaged) on the system where you run your code

3) Installing Python

3.1 Linux

Many Linux distributions come with Python 3 already.

Ubuntu/Debian:

sudo apt update
sudo apt install -y python3 python3-pip
python3 --version
pip3 --version

3.2 macOS

You can use:

Official installer from python.org
Homebrew

Homebrew example:

brew install python
python3 --version

3.3 Windows

Recommended:

Install Python from python.org
During install, check: Add Python to PATH

Then verify:

python --version
py --version

4) IDEs: VS Code and PyCharm (projects)

4.1 VS Code

Install the Python extension
Open a folder:
- File -> Open Folder
Create a new file: main.py
Run:
- the Run button
- or terminal: python3 main.py

4.2 PyCharm

Create project:
- New Project
- Choose Python interpreter (system or virtual environment)
Open existing project:
- Open
- Select the folder

5) The code we learned (explained)

5.1 Arithmetic operators

# + : add
print(1 + 1)

# - : subtract
print(1 - 6)

# * : multiply
print(10 * 4)

# / : divide (always returns float)
print(10 / 3)

# // : floor division (whole number result)
print(10 // 3)

# % : modulo (remainder)
print(10 % 3)

# ** : power
print(2 ** 3)

5.2 Comments

# This is a comment. Python ignores it.

6) Printing strings

6.1 Single, double, and multi-line strings

print('hello world!')
print("hello world!")

print('''hello
world!''')

print("""hello
world!""")

6.2 Quotes inside strings (escaping)

This is wrong:

# print('i'm')  # wrong

Correct ways:

print("i'm")
print('i"m')
print('i\'"m')

7) Special characters: new lines, tabs, raw strings

7.1 New line (\n)

print('hello\ncat')
print('hello')
print('cat')

7.2 Windows paths (escaping backslashes)

print('C:\\USERS\\neria\\tomer')
print('hodi\thodi')

7.3 Raw strings (r’…’)

Raw strings treat backslashes as normal characters:

print(r'C:\\USERS\\neria\\tomer')

8) Variables and naming styles

8.1 Variables

eli_age = 34
student3_name = 'eytan'
guy_height = 1.82
is_on = False

def1 = 'hello'

8.2 Naming styles

# PascalCase
SaharLastName = "sahar"

# camelCase
saharLastName = "sahar"

# snake_case (recommended in Python)
sahar_last_name = "sahar"

# CONSTANT_CASE (used for constants)
SAHAR_LAST_NAME = "sahar"

9) Basic data types

Common types:

int (1, -22, 0)
float (1.0, 0.66, -7.5)
bool (True / False)
str (“text”)

Examples:

up = False
print(type(up))

name = 'donAld tRump'
print(type(name))

10) Casting (type conversion)

10.1 int to float

a = 10
b = 1.5
print(type(a))
print(type(b))
print(float(a))

10.2 division results

a = 100
b = 13
ans = a // b
print(type(ans))
print(ans)

10.3 Converting strings to numbers

f = '55'
print(int(f))

# int('3.5') is invalid
# int('hodi') is invalid

11) String concatenation

String concatenation means joining strings together to form a new string.

11.1 Using `+` (string + string)

When you use + with two strings, Python joins them.

print('11' + '22')
print('hothaifa ' + 'zoubi')

domain = '@hr.com'
print('guy' + domain)

Important notes:

str + str produces str
str + int is an error (you must convert first)

Example:

age = 20

# print('Age: ' + age)  # error
print('Age: ' + str(age))

11.2 Adding spaces correctly

If you want a space between words, you must include it:

first = 'hen'
last = 'keter'
full = first + ' ' + last
print(full)

11.3 Better printing: `print(a, b)` and f-strings

Instead of concatenating, you can let print() add spaces:

first = 'hen'
last = 'keter'
print(first, last)

For variables inside strings, f-strings are usually the cleanest:

first = 'hen'
last = 'keter'
age = 20
print(f'{first} {last} is {age} years old')

12) User input

input() always returns a string.

first_name = input('enter your name: ')
last_name = input('enter your last name: ')
full_name = first_name + ' ' + last_name
print(full_name)

If you want a number, cast it:

current_year = 2026
birth_year = int(input('Enter your birth year: '))
age = current_year - birth_year
print(f'{age=}')

13) String slicing (indexes)

13.1 Indexing (single character)

In Python, strings are sequences.

Indexes start at 0
The last character can be accessed with -1

Example string:

name = 'hen kEteR'

Index map (for understanding):

h  e  n     k  E  t  e  R
0  1  2  3  4  5  6  7  8
-9 -8 -7 -6 -5 -4 -3 -2 -1

name = 'hen kEteR'

print(name[0])
print(name[-1])
print(len(name))

print(name[0:3])
print(name[4:9])
print(name[4:])
print(name[:4])
print(name[:])

13.2 Slicing rules (start:stop)

Slicing uses this format:

text[start:stop]

Rules:

start is included
stop is excluded

So:

name[0:3] returns characters at index 0,1,2

13.3 Slicing with step (start:stop:step)

You can add a step:

text[start:stop:step]