> ## Documentation Index
> Fetch the complete documentation index at: https://genai.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# First Python file

> Write and run your first Python program

## Create a new file

With your `python-for-ai` project open in VS Code:

1. Click the "New File" button in the Explorer (looks like a page with a +)
2. Name it `hello.py`
3. Press Enter

<Warning>
  Make sure to include the `.py` extension! This tells VS Code and Python that this is a Python file.
</Warning>

## Understanding file extensions

File extensions tell your computer and VS Code what type of file you're working with. There are many files type, here are some examples:

* `.txt` - Plain text file, no special formatting
* `.md` - Markdown file, for documentation
* `.py` - Python file, contains Python code

When you save a file with `.py`, VS Code:

* Adds syntax highlighting (colors your code)
* Enables Python-specific features
* Shows Python in the bottom-right corner

## Write your first program

In your `hello.py` file, type:

```python theme={null}
print("Hello, World!")
print("I'm learning Python for AI")
```

<Note>
  The `print()` function displays text or values on your screen - we'll use it throughout this course to see what our code is doing.
</Note>

Notice how VS Code colors your code:

* `print` is highlighted as a function
* Text in quotes is shown in a different color
* This helps you spot errors quickly

## Select Python interpreter

Before running your code, make sure VS Code is using the right Python:

1. Look at the bottom-right corner of VS Code
2. You should see something like "Python 3.x.x"
3. If not, click the area that says "Select Python Interpreter"
4. Choose the Python version you installed earlier

<Note>
  We're using the main Python installation for now. Later in the course, you'll learn about virtual environments for more advanced project management.
</Note>

## Run your program

There are three ways to run your Python code:

**Method 1: Run button** (easiest)

* Click the triangle "Run" button in the top-right corner
* Or select the down arrow next to it and select "Run Python File"

**Method 2: Right-click menu**

* Right-click anywhere in your code
* Select "Run Python File in Terminal"

**Method 3: Keyboard shortcut**

* Press `Ctrl + F5` (Windows/Linux) or `Cmd + F5` (macOS)

## Understanding the output

When you run your code, the terminal opens at the bottom. You'll see something like this (yours will look slightly different):

```bash theme={null}
cd /path/to/your/project
/path/to/python /path/to/your/project/hello.py
Hello, World!
I'm learning Python for AI
```

What's happening here:

1. **First line**: VS Code navigates to your project folder
2. **Second line**: VS Code runs Python with the full path to your file
3. **Following lines**: Your program's output!

Don't worry about the long paths - VS Code handles all of this automatically. The important part is seeing your output: "Hello, World!"

## What's happening behind the scenes?

When you click the "Run" button in VS Code, it's actually running a command in the terminal for you. Let's understand what's really happening:

### The basic command

At its core, running a Python file is simple:

```bash theme={null}
python hello.py
```

This tells your computer:

1. **python** - Find the Python program
2. **hello.py** - Open this file and execute the code inside

### How Python executes your code

Here's what happens when you run `python hello.py`:

1. **Python reads your file** - Opens `hello.py` and reads the text
2. **Checks syntax correctness** - Before translating or running anything, Python checks the code for any syntax errors. If it finds an error, it stops immediately, displays an error message, and does not run any code. If there are no errors, it proceeds to the next step.
3. **Python interprets the code** - Converts your code into instructions (bytecode) that the computer can understand
4. **Python executes line by line** - Runs each instruction from top to bottom, left to right, displaying any output (like `print()` statements) in your terminal

<Note>
  Python always executes code **top to bottom** (one line after another) and **left to right** (within each line). This means the order of your code matters!
</Note>

```python theme={null}
# Step 1: Python checks if the entire file is syntactically correct.
# Step 2: Once it passes this syntax phase, it interprets and executes line by line:

print("Hello, World!")  # Line 1: Python reads, interprets, and executes this first
                        # You see: Hello, World!

print("I'm learning Python")  # Line 2: Follows the same process next
                              # You see: I'm learning Python
```

This execution order means:

* Line 2 can't run before Line 1
* What you write first happens first
* Later in the course, you'll see how to control this flow with conditions and loops

### What happens on a syntax error?

To understand how the syntax check prevents execution, look at this example:

```python theme={null}
print("This line is correct")
print("This line is also correct")
print("This line has a syntax error  # Oops! Missing closing quotes
```

If you run this program, Python's syntax check (Step 1) fails immediately on line 3. Because the entire file must be syntactically correct before interpretation starts:

* **Nothing is executed.**
* Python will **not** run Line 1 or Line 2.
* The program stops immediately and displays a `SyntaxError` in the terminal, without showing any print outputs.

### Running manually from the terminal

You can run Python files directly in the terminal without using VS Code's button:

```bash theme={null}
# Make sure you're in the same folder as your file
python hello.py
```

Or with the full path:

```bash theme={null}
# You can run from anywhere if you use the full path
python /Users/yourname/python-for-ai/hello.py
```

<Note>
  VS Code's "Run" button is just a convenient way to run `python your_file.py` in the terminal. The underlying process is exactly the same!
</Note>

### Python vs python3

On some systems (especially Mac/Linux), you might need to use `python3` instead of `python`:

```bash theme={null}
python3 hello.py
```

This ensures you're using Python 3.x instead of the older Python 2.x. VS Code usually handles this automatically.

### Why this matters

Understanding this is important because:

* You can run Python files from any terminal, not just VS Code
* You'll see these commands in tutorials and documentation
* Later, you'll pass arguments to your programs: `python script.py --option value`
* This is how Python works everywhere: on servers, in production, in automation scripts

<Tip>
  Think of VS Code as a fancy wrapper around the terminal. When you click "Run," you're really just typing `python hello.py` into the terminal. VS Code makes it easier, but the fundamentals stay the same.
</Tip>

## The integrated terminal

The terminal at the bottom is the same as your system's terminal, but inside VS Code. Here's how to work with it:

### Opening and closing the terminal

* **Open**: View > Terminal (or press `` Ctrl + ` `` on Windows/Linux, `` Cmd + ` `` on macOS)
* **Hide**: Click the X or press the same shortcut again
* **New terminal**: Click the + button or Terminal > New Terminal

### Terminal basics

* You can type commands directly
* Try typing `python --version` and press Enter
* Type `cls` (Windows) or `clear` (Mac/Linux) to clear the screen
* Use the up arrow to recall previous commands

<Tip>
  The integrated terminal means you never have to leave VS Code. You can write code, run it, install packages, and manage files all in one place.
</Tip>

## Experiment

Try changing your code and running it again:

```python theme={null}
print("Hello, World!")
print("I'm learning Python for AI")
print("My name is [Your Name]")
print("Today is a great day to code!")
```

Each `print()` statement creates a new line of output.

## Save your work

Remember to save your file:

* Press `Ctrl + S` (Windows/Linux) or `Cmd + S` (macOS)
* Or use File > Save

VS Code shows a dot next to unsaved files in the tab.

## What you've learned

Congratulations! You've:

* Created your first Python file
* Written Python code
* Selected the Python interpreter
* Run a Python program
* Understood the terminal output

This is the foundation for everything else you'll learn in Python.

## Practice and Exercises

To reinforce what you've learned, practice with these interactive notebooks:

<CardGroup cols={2}>
  <Card title="Follow-Along Practice" icon="laptop-code">
    Practice the print examples discussed in this lesson.

    [💻 VS Code](vscode://file/Users/sivaprasad/Downloads/python%20material/python4ai/public/notebooks/basics_exercises/First_Python_File_Practice.ipynb) | [🚀 Colab](https://colab.research.google.com/github/prasad230776/python4ai/blob/master/public/notebooks/basics_exercises/First_Python_File_Practice.ipynb) | <a href="/public/notebooks/basics_exercises/First_Python_File_Practice.ipynb" download>📥 Download</a>
  </Card>

  <Card title="Practice Exercises" icon="pen-to-square">
    Test your skills with exercises and view step-by-step solutions.

    [💻 VS Code](vscode://file/Users/sivaprasad/Downloads/python%20material/python4ai/public/notebooks/basics_exercises/First_Python_File_Exercises.ipynb) | [🚀 Colab](https://colab.research.google.com/github/prasad230776/python4ai/blob/master/public/notebooks/basics_exercises/First_Python_File_Exercises.ipynb) | <a href="/public/notebooks/basics_exercises/First_Python_File_Exercises.ipynb" download>📥 Download</a>
  </Card>
</CardGroup>

<Card title="Course resources" icon="download" href="/getting-started/course-resources">
  Download all templates, files, and cheat sheets
</Card>