How to step through Python code to help debug issues?

Question

In Java/C# you can easily step through code to trace what might be going wrong, and IDE\'s make this process very user friendly.

Can you trace through python code in

Answer 1

ipdb (IPython debugger)

ipdb adds IPython functionality to pdb, offering the following HUGE improvements:

• tab completion
• show more context lines
• syntax highlight

Much like pdg, ipdb is still far from perfect and completely rudimentary if compared to GDB, but it is already a huge improvement over pdb.

Usage is analogous to pdb, just install it with:

python3 -m pip install --user ipdb

and then add to the line you want to step debug from:

__import__('ipdb').set_trace(context=21)

You likely want to add a shortcut for that from your editor, e.g. for Vim snipmate I have:

snippet ipd
    __import__('ipdb').set_trace(context=21)

so I can type just ipd and it expands to the breakpoint. Then removing it is easy with dd since everything is contained in a single line.

context=21 increases the number of context lines as explained at: How can I make ipdb show more lines of context while debugging?

Alternatively, you can also debug programs from the start with:

ipdb3 main.py

but you generally don't want to do that because:

• you would have to go through all function and class definitions as Python reads those lines
• I don't know how to set the context size there without hacking ipdb. Patch to allow it: https://github.com/gotcha/ipdb/pull/155

Or alternatively, as in raw pdb 3.2+ you can set some breakpoints from the command line:

ipdb3 -c 'b 12' -c 'b myfunc' ~/test/a.py

although -c c is broken for some reason: https://github.com/gotcha/ipdb/issues/156

python -m module debugging has been asked at: How to debug a Python module run with python -m from the command line? and since Python 3.7 can be done with:

python -m pdb -m my_module

Serious missing features of both pdb and ipdb compared to GDB:

• persistent command history across sessions: Save command history in pdb

ipdb specific annoyances:

• multithreading does not work well if you don't hack some settings...
  • ipdb, multiple threads and autoreloading programs causing ProgrammingError
  • https://github.com/gotcha/ipdb/issues/51

Tested in Ubuntu 16.04, ipdb==0.11, Python 3.5.2.