Debugging Python

I have talked about Python a number of times in the past, but one thing that I haven't really touched much upon is debugging Python. There are two distinct approaches you might take, depending on whether you're debugging Python code, or whether you're debugging something in a native library. Today I want to talk about the former.

The pdb debugger is the package to use, but the linked reference goes into quite a bit of detail on how to access the debugger package and not so much on using it in practice. So let's start with a hypothetical exception you might run into.

You run a command like python3 blargh.py and then you get an error of this sort. What to do next?

Traceback (most recent call last):
  File "/Users/foo/blargh.py", line 10, in <module>
    main()
  File "/Users/foo/blargh.py", line 20, in main
    raise e
  File "/Users/foo/blargh.py", line 30, in foo
    l = blargh()
  File "/Users/foo/blargh.py", line 40, in blargh
    raise ValueError("oh no")
ValueError: oh no


To debug this, you can take the following approach. Note that the commands are shortcuts that resembly gdb/lldb, and you can get help with h.

• Start the program under debugger: python3 -m pdb blargh.py
• Set a breakpoint near the failure - otherwise you'll be stepping through a lot of initial code: b blargh
• Continue until you hit the breakpoint: c
• Now, use p EXPR to evaluate an expression, n to step to the next statement, and s to step inside.
• If you lose track of where you are, bt will print out the backtrace.
• x exits the debugger when you're done.

Happy Python debugging!

Tags:  debugging python

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