
Posted on • Originally published atlearnbyexample.github.io
Debugging
Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. — Brian W. Kernighan
There are 2 hard problems in computer science: cache invalidation, naming things, and off-by-1 errors. — Leon Bambrick
Debuggers don't remove bugs. They only show them in slow motion. — Unknown
Above quotes chosen fromthis collection at softwareengineering.stackexchange.
General tips
Knowing how to debug your programs is crucial and should be ideally taught right from the start instead of a chapter at the end of a beginner's learning resource.Think Python is an awesome example for such a resource material.
Debugging is often a frustrating experience. Taking a break helps. It is common to find or solve issues in your dreams too (I've had my share of these, especially during college and intense work days).
If you are stuck with a problem, reduce the code as much as possible so that you are left with minimal code necessary to reproduce the issue. Talking about the problem to a friend/colleague/inanimate-objects/etc can help too — famously termed asRubber duck debugging. I have often found the issue while formulating a question to be asked on forums like stackoverflow/reddit because writing down your problem is another way to bring clarity than just having a vague idea in your mind.
Here's some awesome articles on this challenging topic:
- What does debugging a program look like?
- How to debug small programs
- Debugging guide
- Problem solving skills
Here's an interesting snippet (modified to keep it small) from a collection ofinteresting bug stories.
A jpeg parser choked whenever the CEO came into the room, because he always had a shirt with a square pattern on it, which triggered some special case of contrast and block boundary algorithms.
See alsocurated list of absurd software bug stories.
Common beginner mistakes
The previous chapter already covered syntax errors. This section will discuss more Python gotchas.
Python allows you to redefine built-in functions, modules, classes etc (seestackoverflow: metaprogramming). Unless that's your intention, do not usekeywords,built-in functions and modules as your variable name, function name, program filename, etc. Here's an example:
# normal behavior>>>str(2)'2'# unintentional use of 'str' as variable name>>>str=input("what is your name? ")whatisyourname?learnbyexample# 'str' is no longer usable as built-in function>>>str(2)Traceback(mostrecentcalllast):File"<stdin>",line1,in<module>TypeError:'str'objectisnotcallable
Here's another example:
>>>len=5>>>len('hi')Traceback(mostrecentcalllast):File"<stdin>",line1,in<module>TypeError:'int'objectisnotcallable
As an exercise, create an empty file named as
math.py
. In the same directory, create another program file that imports themath
module and then uses some feature, sayprint(math.pi)
. What happens if you execute this program?
See also:
- Think Python: Debugging chapter
- inventwithpython: common runtime errors andcommon python gotchas
- pythonforbiologists: common beginner errors
- stackoverflow: common pitfalls in Python
pdb
Python comes with a handy built-in librarypdb
that you can use from the CLI to debug small programs. Seedocs.python: pdb for documentation. Here's some of the frequently used commands (only short form is shown, see documentation for long form and more details).
l
prints code around the current statement the debugger is at, useful to visualize the progress of debug effortll
prints entire code for the current function or frames
execute current line, steps inside function callsn
execute current line, treats function as a single execution stepc
continue execution until the next breakpointp expression
print value of an expression in the current context, usually used to see the current value of a variableh
list of available commandsh c
help onc
command
q
quit the debugger
Here's an example invocation of the debugger for thenum_funcs.py
program seen earlier in theImporting your own module section. Only then
command is used below. The line with>
prefix tells you about the program file being debugged, current line number and function name. The line with->
prefix is the code present at the current line.(Pdb)
is the prompt for this interactive session. You can also see the output ofprint()
function for the lastn
command in the illustration below.
$python3.9-m pdb num_funcs.py> /home/learnbyexample/Python/programs/num_funcs.py(1)<module>()-> def sqr(n):(Pdb) n> /home/learnbyexample/Python/programs/num_funcs.py(4)<module>()-> def fact(n):(Pdb) n> /home/learnbyexample/Python/programs/num_funcs.py(10)<module>()-> num= 5(Pdb) n> /home/learnbyexample/Python/programs/num_funcs.py(11)<module>()-> print(f'square of {num} is {sqr(num)}')(Pdb) nsquare of 5 is 25> /home/learnbyexample/Python/programs/num_funcs.py(12)<module>()-> print(f'factorial of {num} is {fact(num)}')
Continuation of the above debugging session is shown below, this time withs
command to step into the function. User
while you are still inside the function to skip until the function encounters areturn
statement. Examples forp
andll
commands are also shown below.
(Pdb) s--Call--> /home/learnbyexample/Python/programs/num_funcs.py(4)fact()-> def fact(n):(Pdb) ll 4 -> def fact(n): 5 total= 1 6foriinrange(2, n+1): 7 total*= i 8returntotal(Pdb) n> /home/learnbyexample/Python/programs/num_funcs.py(5)fact()-> total= 1(Pdb) p n5(Pdb) r--Return--> /home/learnbyexample/Python/programs/num_funcs.py(8)fact()->120->returntotal(Pdb) nfactorial of 5 is 120--Return--> /home/learnbyexample/Python/programs/num_funcs.py(12)<module>()->None-> print(f'factorial of {num} is {fact(num)}')
If you continue beyond the last instruction, you can restart from the beginning if you wish. Useq
to end the session.
(Pdb) n--Return--> <string>(1)<module>()->None(Pdb) nThe program finished and will be restarted> /home/learnbyexample/Python/programs/num_funcs.py(1)<module>()-> def sqr(n):(Pdb) q
You can callbreakpoint() orpdb.set_trace() to set breakpoints in the code and use it in combination with
c
command.
See also:
IDLE debugging
Sites likePythontutor allow you to visually debug a program — you can execute a program step by step and see the current value of variables. Similar feature is typically provided by IDEs. Under the hood, these visualizations would likely be using thepdb
module discussed in the previous section.
This section will show an example withIDLE
. Before you can run the program, first selectDebugger option underDebug menu. You can also useidle3.9 -d
to launch IDLE in debug mode directly. You'll see a new window pop up as shown below:
Then, with debug mode active, run the program. Use the buttons and options to go over the code. Variable values will be automatically available, as shown below.
You can right-click on a line from the text editor to set/clear breakpoints.
Seerealpython: Debug With IDLE for a more detailed tutorial.
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