How to Debug Python Code With PDB

Edited 2 weeks ago by ExtremeHow Editorial Team

PythonDebuggingPDBTroubleshootingCommand LineWindowsMacLinuxProgrammingDevelopment

Python is a versatile and widely used programming language, praised for its readability and simplicity. However, like any other programming language, Python code can contain bugs. Debugging is an essential skill for any developer, as it allows them to identify and fix problems in their code. In Python, one of the most popular tools for debugging is PDB, the Python Debugger. This article will explain in detail how to debug Python code using PDB.

Understanding PDB (Python Debugger)

PDB stands for Python Debugger, it is a built-in debugging tool that comes with Python. It allows developers to look at their code, inspect variables, and identify where things are going wrong. PDB is a command-line tool that may seem daunting at first, but once you get the hang of it, it becomes incredibly powerful and useful.

Basic commands in PDB

Before getting into debugging, it is important to understand some basic commands in PDB. Here are some important commands you will use:

• l (list): This command lists the source code around the current execution point.
• n (next): The 'next' command moves to the next line of code.
• c (continue): Continue execution until a breakpoint is reached.
• b (break): Set a breakpoint. You can specify a line number or a function name.
• s(step): The step in the function call.
• p (print): Print the value of an expression.
• q (quit): Exit the debugger and terminate the program.

Starting PDB

To start using PDB, you can invoke it from the command line while running your Python script. Let's say you have a script named example.py. You can start the debugger like this:

python -m pdb example.py

This command runs your script under PDB, and you will be able to start debugging from the first line of your script.

Setting a breakpoint

Breakpoints are very important when debugging with PDB. They allow you to stop the execution of your code at a specific point. Once the program stops, you can inspect the environment, variables, and flow of execution.

You can set a breakpoint at a specific line number as follows:

b 10

Here, the debugger will stop execution on line 10 of your script. You can also set a breakpoint on the function:

b my_function

This command will stop the execution each time my_function is called.

Stepping forward through the code

Moving forward through code is a common task in debugging. It allows you to execute the program line by line and observe the flow of execution. You can use the 'n' command to move to the next line in your code:

n

If you're interested in entering the function call, use the 's' command:

s

The difference between 'n' and 's' is important: while 'n' moves to the next line in the current scope, 's' moves to any function called in the current position.

Inspecting variables

Inspecting variables is another important part of debugging. You can check the value of variables with the 'p' command:

p variable_name

For more complex expressions, simply type this expression:

p my_variable + 10

This will evaluate my_variable + 10 and print the result.

Continuous execution

Once you've finished inspecting or stepping through your code, you may want to continue normal execution. To do this, use the 'c' command:

c

It will continue execution until another breakpoint is hit or the program terminates.

Example of use of PDB

Let's consider a simple example of how PDB can be used. Let's say we have the following script, which has a bug:

def add_numbers(a, b): return a + b def main(): result = add_numbers(3, "4") print("The result is:", result) if __name__ == "__main__": main()

In this script, there is a bug in the main function. add_numbers function is called with a string instead of an integer, which will generate TypeError.

To debug this, let's run the script with PDB:

python -m pdb script_name.py

The debugger starts, and you can set a breakpoint at the point of interest, perhaps where add_numbers function is called:

b main c

The program runs until it reaches a breakpoint on main function. From here, you can go ahead and inspect the problematic line:

ss pa pb

These commands will help you identify that the inputs to add_numbers function are of the wrong type. You will see that b is actually a string, leading to TypeError.

Leaving the PDB

Once you have finished debugging, you can exit the PDB using the 'q' command:

q

This will exit the debugger and end the program.

Advanced features

In addition to the basic commands, PDB provides advanced features that can enhance your debugging experience:

• Conditional break: You can set a breakpoint that triggers only if a certain condition is true. For example:

b 10, a == 5

• Post-mortem debugging: To analyze a crash after it has occurred, you can use:

import pdb import traceback try: main() except Exception: traceback.print_exc() pdb.post_mortem()

• Set the trace in code: You can also add pdb.set_trace() directly in your code where you want to stop the debugger:

import pdb def buggy_function(): a = 10 b = "5" pdb.set_trace() # Initiate debugging here return a + b buggy_function()

Conclusion

Learning to use the PDB can greatly enhance your ability to debug and understand Python programs. Although it may take some time to become familiar with all the commands, the reward is a deeper and more comprehensive insight into your codebase. Remember that debugging is not just about finding and fixing errors, but an opportunity to improve your understanding of the logic and flow of your application. With the PDB, you have a powerful tool to efficiently inspect, evaluate, and improve your Python code.

Debugging is a vital part of software development, and mastering tools like PDB can significantly increase your productivity and code quality. Through practice and experience, you will find yourself debugging faster and more effectively.

  Share This Article

If you find anything wrong with the article content, you can request an update