Essentially, there are two ways of debugging Python source code. You
can either change the code for debugging purposes or start the program
with a proper debugger. In either case, you should start by making an
educated guess on where the problem is hidden.
In the case of immersive debugging you can either add print
function calls or use a debugger's set_trace method.
print StatementsOnce you've identified a potentially problematic location, add
print(locals(), "\n")
before and after the statements you want to investigate. Frequently,
you'll add the statements as the first line of a function and right before
its return statement.
Then, execute the program with input that
is known to produce unexpected results. If the function arguments are wrong,
your problem is higher up the call stack. If the return values are incorrect,
the function is responsible for the problem. If neither is unexpected, you need
to keep looking for the problem elsewhere. As an alternative to using
print(.), you could also use a logging function configured to
log output when in debugging mode. This has the advantage of making useful
changes persistent instead of adding and removing print statements.
TODO: write notes about logging and add a link there (if students desire a session wrt logging).
For those preferring not to use print(.), a debugger is a
great way of finding bugs in source code. The immersive approach of
using a debugger is to add
import ipdb # ... # this is where you expect things to go wrong ipdb.set_trace()
to your code. Once you execute your program and the
set_trace() call is reached, the program execution is halted
and an interactive debugger is started. In case you don't have ipdb
installed you can replace ipdb with pdb.
Python can be debugged with a number of different debugging tools. The
commandline-debugger available with any Python 3 installation is pdb3.
However, I'd prefer ipdb3. To use it, execute the program from the
commandline like you'd normally would. However, replace
python3 with your commanline debugger:
ipdb3 filename.py [arg] ...
To get help for ipdb, use its built-in help system
ipdb> h
A useful hint for the command-line debugger is that pressing the enter key
on an otherwise empty command-line will repeat the last command. If you don't
like the command-line debugger but don't have an IDE providing an interface to
pdb, you could use pudb3. It is a nice console based GUI
debugger. It has the advantages of
being small, fast and swiftly usable over ssh connections. It is quite
easy to learn its basic usage. Once installed, it can be used like any
console based debugger
pudb3 filename.py [arg] ...
Finally, you could use a debugger made available by your favorite Python IDE. However, don't underestimate the effort of learning to properly use an IDE. This means that you should not switch to an IDE when being in a hurry to complete a small task. On the plus side, it will almost always pay off to learn a proper IDE. Excellent open source alternatives include KDevelop and Eclipse, each with a plugin for Python 3 development.
sudo apt install python3-ipdb
sudo apt install python3-pudb
In case your package manager's version of pudb is too old (there are
quite many bugs in older versions) install it via pip
sudo pip3 install pudb
sudo apt install kdevelop kdevelop-python