pdb-instructions.md

Python Debugger (pdb) Usage Instructions

Table of Contents

1. Introduction to pdb
2. Basic pdb Commands
3. Using pdb in a Project
4. Using pdb in a FastAPI Project

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Introduction to pdb

The Python Debugger (pdb) is a built-in interactive debugger for Python programs. It provides a powerful way to step through your code, inspect variables, set breakpoints, and execute commands to better understand what is happening in your code at runtime.

When to Use pdb

• You want to pause execution at a certain point and inspect what's going on.
• You want to debug a particular function or piece of logic.
• You need to check the value of variables and the flow of execution.

Basic pdb Commands

Here are the most common pdb commands you'll use:

Command	Description
b [lineno]	Set a breakpoint at the specified line number.
b [file:lineno]	Set a breakpoint at a line number in another file.
b	List all breakpoints.
cl or clear [bpnumber]	Clear the breakpoint by its number or clear all breakpoints.
n or next	Execute the current line and move to the next line.
s or step	Step into a function.
r or return	Continue execution until the current function returns.
l or list	List the code around the current line (or a specific location).
p [expression]	Print the value of the given expression or variable.
c or continue	Continue execution until the next breakpoint.
q or quit	Quit the debugger and stop program execution.
h or help [command]	Get help for a specific command, or show all commands if none is specified.
! [statement]	Execute a Python statement.

Using pdb in a Project

Project Structure Example

your_project/
│
├── main_folder/
│   └── main.py
│
└── bank_folder/
    └── bank.py

Setting Breakpoints

You can add breakpoints in your code by using pdb.set_trace().

Example in main.py:

import pdb
from bank_folder.bank import some_function_in_bank

def main():
    pdb.set_trace()  # Breakpoint in main.py
    result = some_function_in_bank()
    print(result)

if __name__ == '__main__':
    main()

Example in bank.py:

import pdb

def some_function_in_bank():
    pdb.set_trace()  # Breakpoint in bank.py
    return "This is from bank.py"

Running the Debugger

To run pdb, execute your script with the -m pdb flag:

python -m pdb main_folder/main.py

The debugger will start, and execution will stop at the first breakpoint.

Example Workflow

• l: List code to see where you are.
• n: Step to the next line of code.
• p variable: Print the value of a variable.
• s: Step into a function.
• c: Continue until the next breakpoint.

debug test with pytest:

pytest --pdb