PROGRAMMING · PYTHON · STUDY GUIDE
Debug the reasoning, not just the Python error.
When a loop behaves unexpectedly, replacing random lines of code is rarely the fastest way to learn. A small reproducible example and a trace table make the bug visible.
BY Mohammed Ehab ElNomany
STEP BY STEP
A practical study method.
1. First, make the problem small
Write down the expected result and the actual result. Reduce the input to three or four items and remove unrelated code. Decide whether the problem is a syntax error, an exception, an infinite loop or a wrong-but-running answer. These are different failure modes.
2. Worked example: a missing item
Consider this loop: numbers = [2, 4, 6] and for i in range(len(numbers) - 1): print(numbers[i]). It prints 2 and 4, but misses 6. A trace table shows why: len(numbers) is 3, so range(2) produces indexes 0 and 1 only. If you want every value, use for number in numbers: print(number). If you need indexes, use enumerate(numbers) or the correct bound.
3. Trace what changes at each iteration
For each pass, record the loop variable, any state modified and the output. In a while-loop, also record the condition before and after the body. Example: if count = 0 and the condition is count < 3, forgetting count += 1 means the condition never becomes false. The fix is to understand the update rule, not simply to add an arbitrary break.
4. Use the tools at the right moment
Add brief print() statements or set a breakpoint to inspect the current value of relevant variables. Confirm what the program actually does before changing it. Then remove temporary logging or use a proper debugger. Read exception messages carefully: the line that raises an exception is useful evidence, though the faulty assumption may be earlier.
5. Retest the boundary cases
After fixing a loop, try an empty list, a single item, multiple items and any expected unusual input. Ask whether the loop should run zero, one or many times and whether the first and last elements are handled. Explain the bug in one sentence, then try to write the corrected loop from scratch.
PERSONAL SUPPORT
Apply the method to your own course.
Tell me your level, current topic and learning goal. For a learner under 18, a parent or guardian should make contact. Lessons are primarily online; any in-person request in Egypt is considered case by case.