Debugging Your Programs

Bart Massey 2013-10-22

What is debugging?

• During/after coding, before/during/after testing

• Bring the program to a state where it appears to be bug-free (but this is a lie)

• Estimate 20-40% of programming effort

• "Secret": No good books, one small chapter in our book

How to debug

• Given a failure of the software:

  • Find the causes ("faults") leading to that failure
  • Find the root causes of those faults
  • Figure out and apply a repair
  • Check the repair
    • Does it fix the failures?
    • Does it cause new failures?

Key activity: diagnosis

• Like in medicine or car repair: "It doesn't work; what's happening and what can be done?"

• Diagnosis is hypothesis formation and testing

  • What possible reasons might there be for observed symptoms?
  • Can those reasons be ruled out by what is known so far?
  • If not, can we do tests to rule each reason out or increase our belief that it is the correct one?
  • Repeat until exactly one possible reason remains, and it looks really likely to be true.

Common bugs

• Two basic kinds:

  • Bad control flow
  • Just plain calculating the wrong thing

• Examples

  • Off-by-one "fencepost" errors
  • Copy-and-paste calculation errors
  • Typos/"Thinkos"
  • Failure to design to the spec
  • Failure to understand/implement the design

Root Cause Analysis

• It's not enough to find the line of code that "causes the bug"

• You want to find out how that line got there

• In software, faults are caused by mistakes ("errors") that were made by a human (usually you)

• With the "root causes" found, you can:

  • Correct all the faults caused by that error
  • Take steps to make that error less likely in the future

Preparing code for debugging

• "Real programmers don't comment. It was hard to write--it should be hard to read and harder to understand."

• Code should have a spec, simple tests, and pseudocode

• Formatting should be as clean as possible

  • Consistent indentation
  • Consistent liberal use of whitespace
  • Good names
  • Idiomatic

• Code should be instrumented appropriately

Debugging pre-inspection

• Read the code in question carefully. Look for things that are wrong or unclear

• Explain the code to someone. Have them look at it too

• Most bugs are easily found and fixed by inspection alone

Debugging tools: your brain

• Are you sure the spec and tests are correct?

• White-box: what kinds of similar inputs might produce the same program misbehavior?

• Black-box: what properties distinguish misbehaving inputs?

• Is the timing as expected?

• Are your current hypotheses consistent with everything you have observed or can observe?

Debugging tools: print() function

• For a specific hypothesis, stick a print() in that will either disconfirm or confirm the hypothesis

  • Works in a huge variety of situations
  • But don't spam instrumentation everywhere, or you will get confused by it

• Can use print() for exploring program behavior ("tracing"), but beware: one can waste a lot of time doing this without learning anything.

• Always best to know what the question is before you start looking for the answer

Debugging tools: "debugger"

• Idle will happily provide you the ability to

  • Step through your program one statement at a time
  • Run until a given program line is reached
  • Examine/change variable values anytime stopped

• Except the debugger is really fragile and hard to use

• In particular, doesn't interact well with input()

• In general, debugger is tool of last resort

Post-diagnosis

• Once you've found the immediate source of a bug, do RCA

• Look for other places where faults may have been inserted due to the same root causes

• Think hard about how those faults got there. What are you going to do to avoid this in the future?

• Craft fixes that fix the faults properly

  • This may involve changing the design or revising the specification

• Apply the fixes, then test everything carefully

  • Did the problem get fixed?
  • Are there new problems?

Backups, versions and source code management

• It is really easy to get the buggy version and the fixed version and the version you are working on right now mixed up

• Tool called source code management system helps here

• It is probably a mistake to have too many backup files around; in any case, use a consistent clear naming scheme for backup files

Parting thoughts

• Don't get stuck!

  • Interrupt yourself every few minutes and see if you're making real progress
  • If you are stuck, many strategies are available:
    • Try a different approach
    • Take a break
    • Ask for help

• Don't get discouraged

  • The most experience programmers still make a lot of bugs and have a hard time fixing them

  • The bugs you will make are all fixable