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When I use Pycharm a while ago, I get used to debugging my code with its built-in GUI debugger, which works really well. But when I write code on a server without X11 support, I am forced to use Neovim for Python development. My debugging is reduced to a serize of print statement in the source code. It works well for simple programs. Once my source code becomes long enough or complex enough, it becomes inefficient to add print one by one to just play with an object.

Finally, I decided to give terminal debuggers a try. In this post, I introduce three debuggers.

Pdb
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Python ships with a native debugger called pdb.

You can debug your code with intrusive way or non-introsive way. For the intrusive way, you need to modify your source code:

import pdb

# put the following statement where you want the code to stop and step to pdb
pdb.set_trace()

Personally, I prefer the non-intrusive way to invode pdb. You can run pdb as module:

python -m pdb your_script.py

It will start debugging your code from the begining of the code.

Common commands
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Pdb provides a few commands to help you debug your code on its interacitve interface. Here, I list some of the commands and their meanings:

n: execute the next line
p: print the value of an object
s: step into a function
r: return from a function
b [num]: set a breakpoint at line [NUM]
c: continue to run the code until a break point is met
unt [NUM]: run the code until line [NUM]
whatis: print the type of object (similar to p type(some_object))
l: list the context of current line (default 11 lines)
h: show help message
q: quit the debugger

How to enable autocomplete
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By default, pdb does not provide auto-completion for variable names and object methods. To enable auto-completion, create a file .pdbrc under your HOME directory with the following settings:

import rlcompleter
import pdb

pdb.Pdb.complete=rlcompleter.Completer(locals()).complete

ipdb
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Ipdb is like pdb, but with built-in autocompletion, syntax highlighting and more commands. You can install it with pip:

pip install ipdb

Its usage is similar to pdb, so I will not waste time here. One thing worth noting is that ipdb adds a sticky command:

sticky [start end]

Toggle sticky mode. When in sticky mode, it clear the screen
and longlist the current functions, making the source
appearing always in the same position. Useful to follow the
flow control of a function when doing step-by-step execution.

If ``start`` and ``end`` are given, sticky mode is enabled and
only lines within that range (extremes included) will be
displayed.

It means that in stick mode, the current function will be shown and current line will be highlighted to make your follow the execution of program easier.

pudb
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Another terminal debugger worth trying is pudb. First, install it with pip:

pip install pudb

To invoke it, you can either use

python -m pudb my_script.py

or simply

pudb3 my_script.py

Pudb creates a GUI-like interface for debugging your code. It will show your code, current variable, stack trace, break points and a terminal window in its terminal interface.

You can use k and j to move the cursor line up and down and use b to set a break point, which will be highlighted in red. To execute next line of code, use n. To run the code until a break point, use c. For more usage, press h.

Overall, it is quite powerful, considering that it is implemented in a terminal.

Conclusion
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In this post, I introduced three Python debuggers — pdb, ipdb and pudb. They can both help us to debug the code. Personally, I think pudb is the most beautiful one I want to use.