beginner⏱50 minLección 2 de 10

Python Setup & First Program

Learn to install Python, use the REPL, write your first Hello World program, and run Python scripts from the command line.

Python Setup & First Program

Welcome to your first hands-on Python lesson! In this lesson, you'll install Python, write your first program, and learn different ways to run Python code.

Installing Python

Python is available for all major operating systems. Let's get you set up.

Checking if Python is Already Installed

Many systems come with Python pre-installed. Open your terminal or command prompt and type:

bash

python3 --version
# or on Windows:
python --version

Expected output:

Python 3.11.4

ℹ️Note

This course requires Python 3.8 or higher. If you have an older version or Python is not installed, follow the installation steps below.

Installing on Windows

💡 Tip: Use Ctrl + Mouse Wheel to zoom, or click the fullscreen icon

Step-by-step:

Visit python.org/downloads
Click "Download Python 3.x.x" (latest version)
Run the downloaded installer
IMPORTANT: Check "Add Python to PATH" at the bottom
Click "Install Now"
Verify by opening Command Prompt and typing python --version

Installing on macOS

bash

# Option 1: Using Homebrew (recommended)
brew install python3

# Option 2: Download from python.org
# Visit https://www.python.org/downloads/macos/

Verify installation:

bash

python3 --version

Installing on Linux (Ubuntu/Debian)

bash

# Update package list
sudo apt update

# Install Python 3
sudo apt install python3 python3-pip

# Verify installation
python3 --version

💡Tip

On Linux, you may already have Python installed. Use python3 instead of python to ensure you're using Python 3.

The Python REPL

REPL stands for Read-Eval-Print Loop. It's an interactive environment where you can type Python code and see results immediately.

Starting the REPL

bash

# Windows
python

# macOS/Linux
python3

You'll see something like:

Python 3.11.4 (main, Jun  1 2024, 00:00:00) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Using the REPL

python

# The >>> is the prompt. Type code after it.
>>> 2 + 2
4

>>> "Hello, World!"
'Hello, World!'

>>> name = "Alice"
>>> name
'Alice'

>>> len(name)
5

>>> type(42)
<class 'int'>

>>> type("hello")
<class 'str'>

💡 Tip: Use Ctrl + Mouse Wheel to zoom, or click the fullscreen icon

Exiting the REPL

python

# Method 1: Use exit()
>>> exit()

# Method 2: Use Ctrl+D (macOS/Linux) or Ctrl+Z then Enter (Windows)

💡Tip

The REPL is perfect for testing small code snippets, exploring Python features, and doing quick calculations. Use it frequently while learning!

Your First Python Program: Hello World

The tradition in programming is to start with a "Hello, World!" program. Let's create yours.

Using a Text Editor

Create a new file called hello.py with the following content:

python

# hello.py - My first Python program
print("Hello, World!")

That's it! One line of code. Let's break it down:

Part	Purpose
`#`	Starts a comment (ignored by Python)
`print()`	A built-in function that displays output
`"Hello, World!"`	A string (text) to display

Running Your Script

bash

# Navigate to the folder where hello.py is saved
cd path/to/your/folder

# Run the script
python3 hello.py

Output:

Hello, World!

💡 Tip: Use Ctrl + Mouse Wheel to zoom, or click the fullscreen icon

Running Python Scripts

There are multiple ways to run Python code. Let's explore them all.

Method 1: Direct Script Execution

bash

python3 script.py

Method 2: Running as a Module

bash

python3 -m script

Method 3: Making a Script Executable (Linux/macOS)

python

#!/usr/bin/env python3
# hello.py

print("Hello, World!")

bash

# Make the file executable
chmod +x hello.py

# Run it directly
./hello.py

Method 4: Using an IDE

Popular Python IDEs and editors:

IDE/Editor	Type	Best For
VS Code	Free editor	General purpose
PyCharm	Free/Paid IDE	Professional development
Thonny	Free IDE	Beginners
Jupyter Notebook	Web-based	Data science, exploration
IDLE	Bundled with Python	Simple testing

ℹ️Note

For this course, any text editor works. VS Code with the Python extension is recommended for its simplicity and powerful features.

Python Comments

Comments are notes in your code that Python ignores. They help explain what your code does.

Single-Line Comments

python

# This is a single-line comment
print("Hello")  # This is an inline comment

Multi-Line Comments

python

# This is a comment
# that spans multiple lines
# explaining complex logic
print("Hello")

# Or use triple quotes (technically a string, but often used as multi-line comment)
"""
This is a multi-line comment
using triple quotes.
It's often used for documentation.
"""
print("Hello")

Working with Input and Output

Let's make our program interactive by taking user input.

The input() Function

python

# greeting.py
name = input("What is your name? ")
print(f"Hello, {name}! Welcome to Python!")

Running this script:

$ python3 greeting.py
What is your name? Alice
Hello, Alice! Welcome to Python!

⚠️Warning

The input() function always returns a string. If you need a number, you must convert it using int() or float().

Complete Example: Simple Calculator

python

# simple_calculator.py
# A simple calculator that adds two numbers

# Get user input
print("=== Simple Addition Calculator ===")
num1 = float(input("Enter the first number: "))
num2 = float(input("Enter the second number: "))

# Perform calculation
result = num1 + num2

# Display result
print(f"{num1} + {num2} = {result}")

Sample output:

=== Simple Addition Calculator ===
Enter the first number: 15.5
Enter the second number: 7.3
15.5 + 7.3 = 22.8

Understanding File Extensions

Python files use the .py extension. Here's what different Python file types mean:

Extension	Description
`.py`	Python source code file
`.pyc`	Compiled Python bytecode (auto-generated)
`.pyw`	Python script without console window (Windows)
`.pyi`	Python stub file for type hints

Python Version Management

As you progress, you might need multiple Python versions. Here are tools to manage them:

💡 Tip: Use Ctrl + Mouse Wheel to zoom, or click the fullscreen icon

Creating a Virtual Environment

bash

# Create a virtual environment
python3 -m venv myenv

# Activate it (Linux/macOS)
source myenv/bin/activate

# Activate it (Windows)
myenv\Scripts\activate

# Now python points to the environment's Python
python --version

# Deactivate when done
deactivate

💡Tip

Always use virtual environments for your projects. They keep dependencies isolated and prevent conflicts between projects.

Real-World Example: System Information Script

Let's create a useful script that displays system information:

python

# system_info.py
import platform
import sys
import os

def display_system_info():
    """Display basic system information."""
    print("=" * 40)
    print("       SYSTEM INFORMATION")
    print("=" * 40)
    print(f"Python Version: {sys.version}")
    print(f"Operating System: {platform.system()} {platform.release()}")
    print(f"Machine: {platform.machine()}")
    print(f"Processor: {platform.processor()}")
    print(f"Current Directory: {os.getcwd()}")
    print(f"Python Path: {sys.executable}")
    print("=" * 40)

if __name__ == "__main__":
    display_system_info()

Sample output:

========================================
       SYSTEM INFORMATION
========================================
Python Version: 3.11.4 (main, Jun  1 2024, 00:00:00) [GCC 11.3.0]
Operating System: Linux 5.15.0-91-generic
Machine: x86_64
Processor: x86_64
Current Directory: /home/user/projects
Python Path: /usr/bin/python3
========================================

Practice Exercises

Exercise 1: Installation Check

Verify Python is installed correctly by running python3 --version and pip3 --version in your terminal.

Exercise 2: REPL Exploration

Open the Python REPL and try the following:

Calculate 17 * 23
Find the length of the string "Python Programming"
Check the type of 3.14
Use help(print) to see the documentation for the print function

Exercise 3: Personalized Greeting

Create a script called greeting.py that asks for the user's name and age, then prints a personalized greeting including the year they were born.

Exercise 4: Temperature Converter

Write a script that asks for a temperature in Celsius and converts it to Fahrenheit using the formula: F = C * 9/5 + 32

Exercise 5: Multi-line Output

Create a script that prints a simple ASCII art or pattern using multiple print statements:

  *
 ***
*****
 ***
  *

Exercise 6: Virtual Environment

Create a virtual environment, activate it, verify it's active, and then deactivate it.

Exercise 7: Comments Practice

Write a script with at least 5 lines of code and add meaningful comments explaining each step.

Exercise 8: Error Exploration

In the REPL, try these and observe the error messages:

print("hello" (missing closing parenthesis)
2 + "two" (adding number and string)
prnt("hello") (typo in function name)

Summary

In this lesson, you learned:

How to install Python on Windows, macOS, and Linux
How to use the Python REPL for interactive coding
How to write and run your first Python script
Different methods to execute Python code
How to use comments to document your code
How to get user input with input()
How to create and use virtual environments
How to build practical scripts

You're now ready to start writing Python programs! The next lesson will dive into variables and data types.

Progreso20%

How Computers Work

Variables & Data Types

Contenido del Curso

1. How Computers Work 2. Python Setup & First Program 3. Variables & Data Types 4. Operators & Expressions 5. Control Flow: Conditionals 6. Control Flow: Loops 7. Functions 8. Data Structures: Lists & Tuples 9. Data Structures: Dictionaries & Sets 10. Imperative Algorithm Projects