Stop Using print(): Learn the VSCode Debugger

Prerequisites

To follow this guide, you should have Python installed on your system and a basic understanding of functions and variables. You will also need Visual Studio Code as your primary IDE.

Key Libraries & Tools

• Python Extension for VSCode: This is the essential bridge between the editor and the Python interpreter, providing linting, debugging, and IntelliSense.
• Python Debugger (debugpy): The default engine used to attach to your scripts.
• Framework-Specific Configurations: VSCode includes pre-built profiles for FastAPI and Django.

Code Walkthrough

Setting Breakpoints

Breakpoints are the foundation of debugging. You set them by clicking to the left of the line number.

def process_author_scores(author_name, scores):
    # Click the margin here to set a line breakpoint
    avg = calculate_average(scores)
    print(f"{author_name}: {avg}")

Conditional Logic and Function Breakpoints

Sometimes you only care about a specific state. Right-click a breakpoint and select Edit Breakpoint to add an expression. For example, author_name == "ArjanCodes" ensures the debugger only triggers for that specific case. You can also add Function Breakpoints in the Debug panel to stop every time a specific function is called, even if you don't have the file open.

Inspecting State

Once the code pauses, use the Call Stack to see the path the interpreter took. Clicking different levels in the stack reveals the local variables specific to that function's scope.

Syntax Notes

When using Conditional Breakpoints, the syntax follows standard Python boolean logic. You can use any variable currently in scope within your expression. In the Watch Panel, you can evaluate complex expressions like len(scores) > 0 which update in real-time as you step through the code.

Practical Examples

• Loop Debugging: Use a hit count breakpoint to stop a loop only after the 100th iteration to find an edge case.
• API Development: Use the FastAPI configuration to debug incoming requests and inspect headers or body payloads mid-request.

Tips & Gotchas

• The 'Step Into' vs 'Step Over' Trap: Use Step Over (F10) to stay in the current function. Use Step Into (F11) if you want to dive into the internal logic of a called function.
• Save to Refresh: If you change code while debugging, you usually need to restart the session (Ctrl+Shift+F5) for the changes to take effect.