Your First Python Program and the REPL

1. Introduction to Code Execution Modes in Python

In software engineering, understanding how code is supplied to an execution engine is fundamental. Python provides two primary operational modes for executing code:

1. Interactive Mode (The REPL): An interactive command-line environment that reads, evaluates, and prints Python code instantly line-by-line.
2. Script Mode (Source Files): Writing Python code in persistent files (ending with a .py extension) and running the complete script through the Python interpreter.

While interactive mode is excellent for quick testing, API exploration, and debugging, script mode is where production software, web applications, data pipelines, and automation scripts are built and preserved.

2. Deep Dive: What is the Python REPL?

The acronym REPL stands for Read - Eval - Print - Loop. It represents an interactive shell environment that processes code in a continuous four-stage lifecycle:

Launching and Interacting with the REPL

To start the Python REPL on any system:

1. Open Terminal (macOS/Linux) or Command Prompt / PowerShell (Windows).
2. Type python (or python3 on macOS/Linux) and press Enter.
3. You will see system configuration details followed by the primary prompt symbol: >>>.

Python 3.12.0 (main, Oct 10 2023, 15:12:10) 
[GCC 11.2.0 on linux] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Executing Expressions in the REPL

In the REPL, you do not always need an explicit print() function to see the result of mathematical calculations or variable evaluations:

>>> 5 + 10
15
>>> name = "Python Engineer"
>>> name
'Python Engineer'
>>> len(name)
15

3. Interactive Practice: Live Code Execution Examples

Below are interactive compiler-style code blocks reflecting actual Python execution inside script environments.

Practice Box 1: Simple Output and String Printing

print("Hello, World!")
print("Welcome to Professional Python Development!")

Hello, World!
Welcome to Professional Python Development!

Practice Box 2: Working with Arithmetic and Variables

item_price = 120
quantity = 3
total = item_price * quantity
print("Total:", total)

Total: 360

Practice Box 3: Formatted Strings and Dynamic Calculations

hours_worked = 40
hourly_rate = 25.50
gross_pay = hours_worked * hourly_rate
print(f"Gross Pay: ${gross_pay:.2f}")

Gross Pay: $1020.00

4. Writing Your First Persistent Script File

While the REPL is efficient for isolated testing, code disappears as soon as the session closes. To write reusable programs, save code into text files with a .py extension using VS Code.

Step-by-Step Script Workflow in VS Code

1. Open VS Code and ensure your workspace folder (e.g., python_mastery_course/lesson_03/) is loaded in the Explorer sidebar.
2. Create a new file and name it main.py.
3. Enter the following standard Python program code:

# main.py - First Python Script
user_name = "Developer"
course_title = "Python Mastery"

print("--- System Initialization ---")
print(f"User: {user_name}")
print(f"Course: {course_title}")
print("Status: Ready to write code.")

4. Save the file (Ctrl + S / Cmd + S).
5. Open the VS Code integrated terminal (Ctrl + `).
6. Run the script by typing:

   python main.py

5. Anatomy of the `print()` Built-In Function

The print() function is Python's standard primary output mechanism. It accepts multiple parameters that control how data displays in the console.

Signature of the `print()` Function:

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

Key Arguments Explained:

• `*objects`: One or more values, variables, or expressions separated by commas.
• `sep` (Separator): Specifies the character inserted between multiple objects. The default is a single space (' ').
• `end` (End character): Specifies what is printed at the end of the line. The default is a newline character ('\n').

Advanced `print()` Code Demonstrations

# Custom separators
print("2026", "07", "25", sep="-")

# Custom end characters (preventing newlines)
print("Processing", end="... ")
print("Done!")

2026-07-25
Processing... Done!

6. Understanding Early Execution Errors

Beginning developers encounter execution errors. Recognizing error types early builds confidence and improves debugging speed.

1. SyntaxError

Occurs when the code violates Python's grammar rules during the compilation phase before execution starts.

# Invalid Code (Missing closing string quote):
print("Hello World)

# System Error Raised:
SyntaxError: unterminated string literal

2. NameError

Occurs at runtime when trying to evaluate a variable or function name that has not been defined.

# Invalid Code (Variable misspelled):
user_score = 100
print(usr_score)

# System Error Raised:
NameError: name 'usr_score' is not defined. Did you mean: 'user_score'?

7. Frequently Asked Interview Questions with Answers

8. Homework & Practical Assignment

# Broken code for diagnostic task
UserScore = 50
print("User score is:", userscore)

9. File & Workspace Directory Structure

python_mastery_course/
│
├── lesson_01/
│   └── homework_submission.md
│
├── lesson_02/
│   └── environment_setup_verification.txt
│
└── lesson_03/
    ├── hello.py
    ├── bill.py
    ├── calculator.py
    ├── print_formats.py
    └── receipt.py

10. What We Will Learn Next

```