Execution of Python Programs and Debugging Python Code

Understanding how Python programs are executed and how to debug them is fundamental to becoming a proficient Python programmer. Here’s a detailed but clear explanation of both concepts:

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1. Execution of Python Programs

Python is an interpreted language, which means that Python code is executed line-by-line by a program called the Python interpreter. There are several ways to run Python code:

a. Interactive Mode

• Type python or python3 in the command prompt or terminal.
• You can type and execute Python commands one by one.
• Good for testing small code snippets.

Example:

$ python
>>> print("Hello, World!")
Hello, World!

b. Script Mode

• Write your Python code in a .py file (e.g., program.py).
• Run it from the command line:

$ python program.py

c. Using an IDE

• Python code can be written and run directly inside an IDE like PyCharm, VS Code, Thonny, or IDLE.
• Most IDEs have a “Run” button or a shortcut (usually F5 or Ctrl+Shift+F10) to execute the script.

d. Using Jupyter Notebook

• For data science and visualization, code is written and executed in cells.
• Great for running code in parts and viewing immediate output.

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2. Debugging Python Code

Debugging is the process of identifying and fixing errors (bugs) in your code. Python offers multiple ways to debug, from built-in tools to full-featured IDE debuggers.

a. Types of Errors

1. Syntax Errors: Mistakes in code structure.
   • Example: missing colon, incorrect indentation.
2. Runtime Errors: Errors that occur while the code is running.
   • Example: dividing by zero, accessing an undefined variable.
3. Logical Errors: Code runs but gives incorrect results.
   • Example: incorrect calculations or wrong conditions.

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b. Basic Debugging Techniques

1. Print Statements

• Use print() to check variable values and flow of execution.
• Simple but effective for small scripts.

x = 10
print("Value of x:", x)

2. Using Assertions

• You can use assert statements to test assumptions in the code.

assert x > 0, "x should be positive"

3. Exception Handling

• Use try...except blocks to catch and handle errors gracefully.

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

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c. Using Python Debuggers

1. pdb – Python Debugger (Built-in)

Python comes with a built-in debugger called pdb.

import pdb

x = 10
pdb.set_trace()
y = x + 5
print(y)

• Use commands like n (next), c (continue), q (quit), and p (print variable) in the debugger console.

2. IDE Debuggers

• IDEs like PyCharm, VS Code, and Thonny have graphical debuggers.
• Features include:
  • Breakpoints
  • Step over/in/out
  • Watch variables
  • Inspect call stack

3. Logging (Alternative to Print Debugging)

• Use the logging module for more advanced tracking.

import logging

logging.basicConfig(level=logging.INFO)
logging.info("Starting the program")

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Conclusion

Running and debugging Python code is simple thanks to Python’s interactive nature and helpful tools. You can execute code in different ways — terminal, script, IDEs, or Jupyter. When debugging, start with print statements or exception handling, and move to advanced tools like pdb or IDE-based debuggers for larger projects. Mastering these techniques helps you write clean, efficient, and error-free Python programs.