Basic Debugging with pdb0%
Assertions

Basic Debugging with pdb

Beginner12 min read•Updated: Jul 11, 2026
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Basic Debugging with pdb in Python

When complex logical bugs appear, inserting dozens of temporary print() statements is slow and inefficient. Python provides a powerful interactive command-line debugging tool: the pdb (Python Debugger) module. With pdb, you can pause execution at any point, inspect variables in memory, step through code line by line, and analyze call stacks in real time.

1. Setting Breakpoints: breakpoint()

Since Python 3.7, the standard way to pause execution and enter the debugger is by calling the built-in breakpoint() function:

Python
def calculate_discount(price: float, discount_percent: float) -> float:
discount_amount = price * (discount_percent / 100)
# Execution pauses here and drops into the interactive pdb prompt!
breakpoint()
final_price = price - discount_amount
return final_price
 
result = calculate_discount(1500.0, 15)
print("Final:", result)

(In Python 3.6 and earlier, developers wrote import pdb; pdb.set_trace()).

When Python executes breakpoint(), execution pauses in your terminal and renders the interactive debugger prompt:

Output
> /workspace/app.py(6)calculate_discount()
-> final_price = price - discount_amount
(Pdb)

2. Essential pdb Navigation Commands

Once inside the (Pdb) prompt, you control the Python runtime using single-letter commands:

CommandFull NameDescription
nNextExecutes the current line and advances to the next line in the current function (does not step into function calls).
sStepSteps into the function being invoked on the current line.
cContinueResumes normal execution until the next breakpoint is hit or program ends.
p <expr>PrintEvaluates and prints the value of a variable or Python expression (e.g. p discount_amount).
pp <expr>Pretty PrintNicely formats large dictionaries or lists using the pprint module.
lListDisplays 11 lines of source code centered around the current execution line.
wWherePrints a stack trace, showing the call path leading to the current frame.
b <line>BreakpointSets a new breakpoint at a specific line number (e.g. b 24).
qQuitAborts the debugger and terminates the Python process immediately.

3. Interactive Debugging Session Walkthrough

Consider a function with a subtle logical bug where discounts on high-value orders calculate incorrectly:

Python
def process_cart(items):
subtotal = sum(i["price"] for i in items)
if subtotal > 1000:
rate = 0.15
else:
rate = 0.05
breakpoint() # Drop into debugger to inspect calculation
discount = subtotal * rate
return subtotal - discount

Navigating the Interactive Prompt:

Output
(Pdb) l
3 subtotal = sum(i["price"] for i in items)
4 if subtotal > 1000:
5 rate = 0.15
6 else:
7 rate = 0.05
8 -> breakpoint()
9 discount = subtotal * rate
10 return subtotal - discount
 
(Pdb) p subtotal
1250.0
 
(Pdb) p rate
0.15
 
(Pdb) n
> /workspace/app.py(9)process_cart()
-> discount = subtotal * rate
 
(Pdb) n
> /workspace/app.py(10)process_cart()
-> return subtotal - discount
 
(Pdb) p discount
187.5
 
(Pdb) c

4. Starting pdb from the Command Line

You don't need to modify your source code to use pdb. You can launch any Python script directly inside the debugger from your terminal:

Bash / Terminal
python -m pdb script_to_debug.py

Execution will pause at the very first line of the file, allowing you to set breakpoints dynamically using b <lineno> and step through execution from the beginning.

5. Disabling Breakpoints Globally

When running tests or running in production environments, you can disable all breakpoint() calls across your entire application without modifying code by setting the environment variable:

Bash / Terminal
# On Linux/macOS
export PYTHONBREAKPOINT=0
 
# On Windows PowerShell
$env:PYTHONBREAKPOINT="0"

Multiple Choice Questions

1. Which built-in function introduced in Python 3.7 drops execution directly into the interactive debugger?

A. debug() B. breakpoint() C. inspect() D. pause() Answer: B Explanation: Python 3.7 introduced the built-in breakpoint() function, which delegates to sys.breakpointhook() (by default calling pdb.set_trace()).

2. In pdb, what is the difference between the n (next) and s (step) commands?

A. n advances by 10 lines, while s advances by 1 line B. n executes the current line and moves to the next line in the current function, while s steps inside any function call on that line C. s skips the line entirely D. There is no difference Answer: B Explanation: n (next) steps over function calls in the current frame, whereas s (step) steps into the body of the called function.

3. Which pdb command prints the current value of a variable?

A. v <name> B. p <name> C. show <name> D. echo <name> Answer: B Explanation: The p <expr> (print) command evaluates and displays the expression or variable in the current frame.

4. How can you resume normal program execution until the next breakpoint is hit or the program finishes?

A. Command r B. Command c C. Command run D. Command go Answer: B Explanation: The c (continue) command tells pdb to resume continuous execution until another breakpoint is hit or the program exits.

5. How can you disable all breakpoint() calls across a Python environment without deleting code?

A. Set environment variable PYTHONBREAKPOINT=0 B. Delete the pdb.py file from Python installation C. Pass --no-break flag to Python D. Wrap all functions in try/except Answer: A Explanation: Setting PYTHONBREAKPOINT=0 disables the built-in breakpoint() hook globally.

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