Basic Debugging with pdb in Python0%

Basic Debugging with pdb in Python

Intermediate8 min read•Updated: 2026-10-01

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:

1def calculate_discount(price: float, discount_percent: float) -> float:
2    discount_amount = price * (discount_percent / 100)
3    
4    # Execution pauses here and drops into the interactive pdb prompt!
5    breakpoint()
6    
7    final_price = price - discount_amount
8    return final_price
9 
10result = calculate_discount(1500.0, 15)
11print("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:

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

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

Command	Full Name	Description
`n`	Next	Executes the current line and advances to the next line in the current function (does not step into function calls).
`s`	Step	Steps into the function being invoked on the current line.
`c`	Continue	Resumes normal execution until the next breakpoint is hit or program ends.
`p <expr>`	Print	Evaluates and prints the value of a variable or Python expression (e.g. `p discount_amount`).
`pp <expr>`	Pretty Print	Nicely formats large dictionaries or lists using the `pprint` module.
`l`	List	Displays 11 lines of source code centered around the current execution line.
`w`	Where	Prints a stack trace, showing the call path leading to the current frame.
`b <line>`	Breakpoint	Sets a new breakpoint at a specific line number (e.g. `b 24`).
`q`	Quit	Aborts 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:

1def 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:

1(Pdb) l
2  3         subtotal = sum(i["price"] for i in items)
3  4         if subtotal > 1000:
4  5             rate = 0.15
5  6         else:
6  7             rate = 0.05
7  8  ->     breakpoint()
8  9         discount = subtotal * rate
9 10         return subtotal - discount
10 
11(Pdb) p subtotal
121250.0
13 
14(Pdb) p rate
150.15
16 
17(Pdb) n
18> /workspace/app.py(9)process_cart()
19-> discount = subtotal * rate
20 
21(Pdb) n
22> /workspace/app.py(10)process_cart()
23-> return subtotal - discount
24 
25(Pdb) p discount
26187.5
27 
28(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:

1python -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:

1# On Linux/macOS
2export PYTHONBREAKPOINT=0
3 
4# On Windows PowerShell
5$env:PYTHONBREAKPOINT="0"

⚠️ Common Intermediate Mistakes & Gotchas

1. Leaving `breakpoint()` in Production Code

Forgetting to remove breakpoint() or import pdb; pdb.set_trace() will freeze background production servers waiting for user terminal input!

2. Confusing `n` (Next) and `s` (Step)

n (Next): Executes the current line and advances to the next line in the current function (steps over function calls).
s (Step): Steps into the function called on the current line.

💻 Try It Yourself: Interactive PDB Inspection Practice

Scenario

Run a small script that sets a programmatic breakpoint using breakpoint(). Inspect variables x, y, and their accumulated product.

Complete Solution

1def calculate_compound(principal, rate, years):
2    total = principal
3    for year in range(1, years + 1):
4        interest = total * (rate / 100)
5        total += interest
6        # In interactive terminal: breakpoint()
7        # Allows inspecting year, interest, and total with 'p year', 'c'
8    return total
9 
10result = calculate_compound(10000, 10, 3)
11print(f"Final Amount after 3 years: ₹{result:,.2f}")

Expected Output

1Final Amount after 3 years: ₹13,310.00

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

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

Question 2

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

Question 3

Which pdb command prints the current value of a variable?

Question 4

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

Question 5

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

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Assertions (9: Debugging and Logging).

👉 Continue to Next Lesson: Assertions →

Next Lesson

Assertions

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Logging Levels	Assertions

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

PreviousPrevNext

MSK Institute • Shikohabad

Loading Page Content...

Preparing courses, live labs, and learning resources.सामग्री लोड हो रही है, कृपया प्रतीक्षा करें...

Connecting serverPlease wait...

💻 Practical Labs🚀 Free Demo📜 Verified Certificates

Assertions

Basic Debugging with pdb in Python

Intermediate8 min read•Updated: 2026-10-01

Study Materials Cheatsheet

Basic Debugging with pdb in Python

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:

1def calculate_discount(price: float, discount_percent: float) -> float:
2    discount_amount = price * (discount_percent / 100)
3    
4    # Execution pauses here and drops into the interactive pdb prompt!
5    breakpoint()
6    
7    final_price = price - discount_amount
8    return final_price
9 
10result = calculate_discount(1500.0, 15)
11print("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:

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

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

Command	Full Name	Description
`n`	Next	Executes the current line and advances to the next line in the current function (does not step into function calls).
`s`	Step	Steps into the function being invoked on the current line.
`c`	Continue	Resumes normal execution until the next breakpoint is hit or program ends.
`p <expr>`	Print	Evaluates and prints the value of a variable or Python expression (e.g. `p discount_amount`).
`pp <expr>`	Pretty Print	Nicely formats large dictionaries or lists using the `pprint` module.
`l`	List	Displays 11 lines of source code centered around the current execution line.
`w`	Where	Prints a stack trace, showing the call path leading to the current frame.
`b <line>`	Breakpoint	Sets a new breakpoint at a specific line number (e.g. `b 24`).
`q`	Quit	Aborts 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:

1def 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:

1(Pdb) l
2  3         subtotal = sum(i["price"] for i in items)
3  4         if subtotal > 1000:
4  5             rate = 0.15
5  6         else:
6  7             rate = 0.05
7  8  ->     breakpoint()
8  9         discount = subtotal * rate
9 10         return subtotal - discount
10 
11(Pdb) p subtotal
121250.0
13 
14(Pdb) p rate
150.15
16 
17(Pdb) n
18> /workspace/app.py(9)process_cart()
19-> discount = subtotal * rate
20 
21(Pdb) n
22> /workspace/app.py(10)process_cart()
23-> return subtotal - discount
24 
25(Pdb) p discount
26187.5
27 
28(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:

1python -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:

1# On Linux/macOS
2export PYTHONBREAKPOINT=0
3 
4# On Windows PowerShell
5$env:PYTHONBREAKPOINT="0"

⚠️ Common Intermediate Mistakes & Gotchas

1. Leaving `breakpoint()` in Production Code

Forgetting to remove breakpoint() or import pdb; pdb.set_trace() will freeze background production servers waiting for user terminal input!

2. Confusing `n` (Next) and `s` (Step)

n (Next): Executes the current line and advances to the next line in the current function (steps over function calls).
s (Step): Steps into the function called on the current line.

💻 Try It Yourself: Interactive PDB Inspection Practice

Scenario

Run a small script that sets a programmatic breakpoint using breakpoint(). Inspect variables x, y, and their accumulated product.

Complete Solution

1def calculate_compound(principal, rate, years):
2    total = principal
3    for year in range(1, years + 1):
4        interest = total * (rate / 100)
5        total += interest
6        # In interactive terminal: breakpoint()
7        # Allows inspecting year, interest, and total with 'p year', 'c'
8    return total
9 
10result = calculate_compound(10000, 10, 3)
11print(f"Final Amount after 3 years: ₹{result:,.2f}")

Expected Output

1Final Amount after 3 years: ₹13,310.00

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

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

Question 2

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

Question 3

Which pdb command prints the current value of a variable?

Question 4

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

Question 5

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

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Assertions (9: Debugging and Logging).

👉 Continue to Next Lesson: Assertions →

Next Lesson

Assertions

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Logging Levels	Assertions

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

Basic Debugging with pdb in Python

1. Setting Breakpoints: breakpoint()

2. Essential pdb Navigation Commands

3. Interactive Debugging Session Walkthrough

Navigating the Interactive Prompt:

4. Starting pdb from the Command Line

5. Disabling Breakpoints Globally

⚠️ Common Intermediate Mistakes & Gotchas

1. Leaving breakpoint() in Production Code

2. Confusing n (Next) and s (Step)

💻 Try It Yourself: Interactive PDB Inspection Practice

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Assertions

Related Lessons

Practice Quiz

Loading Page Content...

Basic Debugging with pdb in Python

1. Setting Breakpoints: breakpoint()

2. Essential pdb Navigation Commands

3. Interactive Debugging Session Walkthrough

Navigating the Interactive Prompt:

4. Starting pdb from the Command Line

5. Disabling Breakpoints Globally

⚠️ Common Intermediate Mistakes & Gotchas

1. Leaving breakpoint() in Production Code

2. Confusing n (Next) and s (Step)

💻 Try It Yourself: Interactive PDB Inspection Practice

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Assertions

Related Lessons

Practice Quiz

1. Setting Breakpoints: `breakpoint()`

2. Essential `pdb` Navigation Commands

4. Starting `pdb` from the Command Line

1. Leaving `breakpoint()` in Production Code

2. Confusing `n` (Next) and `s` (Step)

1. Setting Breakpoints: `breakpoint()`

2. Essential `pdb` Navigation Commands

4. Starting `pdb` from the Command Line

1. Leaving `breakpoint()` in Production Code

2. Confusing `n` (Next) and `s` (Step)