[Python] Debugging

Debugging is an important thing to fix a problem of code.

Debugger

pdb

pdb is a command-line tool to debug Python code.

import pdb
pdb.set_trace()

ipdb

ipdb is the same kind of tool.

In addition, ipdb can debug IPython as well.

But, this is not a built-in module.

$ pip install ipdb

import ipdb
ipdb.set_trace()

pudb

pudb is a more useful debugger than pdb and ipdb.

But, this is not a built-in module like ipdb.

Also, it looks like a GUI, but it's a command-line tool.

$ pip install pudb

import pudb
pudb.set_trace()

Usage of Debugger

pdb

If you use this tool, simply input the command as following:

• ? / h / help : Help
• w / where : Print the current stack trace
• s / step : Run the current line
• n / next : Continue execution until the next line
• r / return : Continue execution until the current function returns
• a / args : Print the argument list of the current function
• l / list : List source code for the current file
• j # / jump # : Jump to the line
• unt # / until # : Continue execution until the line
• p <expression> : Evaluate the expression and print its value
• pp <expression> : Like the p, except the value of the expression is pretty-printed using the pprint module
• q / quit : Quit from the debugger

ipdb

Its command is the same as pdb.

pudb

If you press ? key, you can see the GUI help menu.

You can even use a mouse.

• ? / F1 : Help
• n : Step over
• s : Step into
• c : Continue
• r / f : Finish current function
• t : Run to cursor
• e : Show traceback
• b : Set/Clear breakpoint
• m : Open module
• L : show (file/line) location / Go to line
• / : Search
• q : Quit

Breakpoint

breakpoint() can be used in Python v3.7 or higher.

When this built-in function is called where debugging is desired, the debugger is automatically called at that location.

x = 3
y = 4
breakpoint()
z = x + y

If you don't want to use breakpoint(), set the environment variable PYTHONBREAKPOINT to 0.

# On MAC
$ export PYTHONBREAKPOINT=0

If not, set it as an empty string.

# On MAC
$ export PYTHONBREAKPOINT=

Conclusion

There is a web-based debugger as well.

But, I think the pudb is a good option.

Debugging skill is very important, so try to get used to it.