Python Debugging Tips

Python debugging tips

PDB

You can use the pdb module, insert pdb.set_trace() anywhere and it will function as a breakpoint.

>>> import pdb
>>> a="a string"
>>> pdb.set_trace()
--Return--
> <stdin>(1)<module>()->None
(Pdb) p a
'a string'
(Pdb)

To continue execution use c (or cont or continue).

It is possible to execute arbitrary Python expressions using pdb. For example, if you find a mistake, you can correct the code, then type a type expression to have the same effect in the running code

ipdb is a version of pdb for IPython. It allows the use of pdb with all the IPython features including tab completion.

It is also possible to set pdb to automatically run on an uncaught exception.

Pydb was written to be an enhanced version of Pdb. Benefits?

Tips for debugging list comprehensions?

If it's complicated enough that it's not obvious at first glance, unpack it into multiple steps and/or for loops. It's clearly too complicated, and making it more explicit is the easiest way to go about debugging it. Added bonus: you can now step through with the debugger or add print statements!

What are good ways to make my Python code run first time?

If you're having problems with syntax, you could try an editor with syntax highlighting. Until you get the feel for a language, simple errors won't just pop out at you.

The simplest form of debugging is just to insert some print statements. A more advanced (and extensible) way to do this would be to use the logging module from the std lib.

The interactive interpreter is a wonderful tool for working with python code, and IPython is a great improvement over the built-in REPL (Read Eval Print Loop).

If you actually want to step through your code, the python debugger is called pdb, which can be called from the command line, or embedded in your code.

If you're used to a fully integrated IDE, I would recommend using Eclipse with pydev, and PyCharm has a great commercial offering, with autocomplete, quick access to docs, and numerous shortcuts, among many other interesting features.

How to step through Python code to help debug issues?

Yes! There's a Python debugger called pdb just for doing that!

You can launch a Python program through pdb by using pdb myscript.py or python -m pdb myscript.py.

There are a few commands you can then issue, which are documented on the pdb page.

Some useful ones to remember are:

• b: set a breakpoint
• c: continue debugging until you hit a breakpoint
• s: step through the code
• n: to go to next line of code
• l: list source code for the current file (default: 11 lines including the line being executed)
• u: navigate up a stack frame
• d: navigate down a stack frame
• p: to print the value of an expression in the current context

If you don't want to use a command line debugger, some IDEs like Pydev, Wing IDE or PyCharm have a GUI debugger. Wing and PyCharm are commercial products, but Wing has a free "Personal" edition, and PyCharm has a free community edition.

Python debugging tips

PDB

You can use the pdb module, insert pdb.set_trace() anywhere and it will function as a breakpoint.

>>> import pdb
>>> a="a string"
>>> pdb.set_trace()
--Return--
> <stdin>(1)<module>()->None
(Pdb) p a
'a string'
(Pdb)

To continue execution use c (or cont or continue).

It is possible to execute arbitrary Python expressions using pdb. For example, if you find a mistake, you can correct the code, then type a type expression to have the same effect in the running code

ipdb is a version of pdb for IPython. It allows the use of pdb with all the IPython features including tab completion.

It is also possible to set pdb to automatically run on an uncaught exception.

Pydb was written to be an enhanced version of Pdb. Benefits?

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