Debugging with the Python Debugger - PDB
In this tutorial we’ll be having a look at the Python Debugger, what it is and how you can use it to effectively debug your Python applications.
Knowing how to debug your code is an incredibly important tool in any programmers arsenal. Thankfully Python comes with the excellent PDB or Python DeBugger which allows us to step through our code and effectively work out what has gone wrong.
The official documentation for the Python Debugger can be found here: Python Debugger
The PDB is classed as an interactive source code debugger which can be used in a similar fashion to how you would use the REPL.
This tutorial is also available in video format should you wish to enjoy the content through that format:
Starting the Python Debugger
When it comes to starting the Python debugger we have 2 main options. We can
either invoke the PDB from the start of our projects execution and step through
it from the beginning by calling
python3 -m pdb main.py, or we can add
import pdb; pdb.set_trace() above the particular section of code we wish to
debug in our Python application in the same way that you would typically set
For smaller programs the first option is fine but for massive Python systems I would tend to recommend going down the second route as this would execute everything up until we reach that particular section of code and then it would open up the PDB for us to work with.
Becoming relatively proficient with the Python debugger requires knowledge of just a few of the many commands available.
These commands can be entered either as their shorthand versions
s or there
For a full list of debugger commands have a look at the official documentation: Debugger Commands
- n(ext) - Continues execution of program until the next line is reached, or whether the current function returns.
- c(cont(inue)) - Continues execution of our program until it reaches the
- r(eturn) - Continues execution until the current function returns
- s(tep) - Executes the current line and stops at the first possible occasion.
- w(here) - Prints out a stack trace.
- l(ist) - Lists the source code for the current file. With no arguments this equates to the 11 lines surrounding current line.
- q(uit) - Quits the current debugging session.
With these 5 commands and a combination of your standard Python functions such
repr() and so on, you can effectively navigate through your
Python programs and come to a decent understanding as to when and where in your
program something has went wrong.
Let’s have a look at how we can use this to effectively step over a very simple Python program.
def compute(x, y): return x + y def main(): print("This is my program") import pdb; pdb.set_trace() x = compute(2, 3) print(x) if __name__ == '__main__': main()
We’ve set a breakpoint on the second line of our
main() function. This means
that when we run the above program you should see execution stops at the point
import pdb; pdb.set_trace() is hit and the pdb interactive command line
$ python3.6 simple.py This is my program > /location/to/simple.py(7)main() -> x = compute(2, 3) (Pdb)
The final line is where we’ll be inputting our commands. Run the
command to go to the next line and then explore our
(Pdb) n > /location/to/simple.py(8)main() -> print(x) (Pdb) x 5 (Pdb) repr(x) '5' (Pdb)
In this tutorial we covered how you can get up and running using the python
pdb to walk through various sections of your Python code and try
to gain real insight as to what it is doing. This insight will hopefully help
you make more informed decisions as to what could have went wrong within your
Hopefully you found this tutorial useful, if you did or you require further assistance then please let me know in the comments section below or by tweeting me : @Elliot_F.