Latticework — Master the skills engineers actually use

Explanation

A code smell isn't a bug — the code runs fine — but it's a signal that something about the design will likely cause pain later. A magic number is a classic one: an unexplained numeric literal buried in the middle of logic, with no indication of what it means or why that specific value was chosen.

def has_magic_number(code_values, allowed):
    return any(v not in allowed for v in code_values)

# 86400 shows up with no explanation -- what IS 86400?
print(has_magic_number([0, 1, 86400], {0, 1}))   # True -- flagged

86400 is the number of seconds in a day — obvious once you know that, completely opaque otherwise. SECONDS_PER_DAY = 86400 fixes it immediately: now the constant explains itself everywhere it's used.

Try it

Both functions compute the exact same thing -- but only the second one tells you WHY '30' and '5' matter, and only the second one is safe to change (rename GRACE_PERIOD_DAYS's value once, instead of hunting for every '30' that might or might not mean 'grace period').

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Explanation

Duplicated code is another classic smell — the same logic copy-pasted in multiple places means every future bugfix or change has to be applied EVERYWHERE it was copied, and it's easy to miss a spot:

from collections import Counter

def count_duplicate_lines(lines):
    counts = Counter(lines)
    return sum(1 for line, c in counts.items() if c > 1)

code = ["x = compute_total()", "y = 2", "x = compute_total()"]
print(count_duplicate_lines(code))   # 1 -- that line appears twice

The fix for duplication is usually extraction: pull the repeated logic into its own function, called from every place that needs it — exactly the single-responsibility principle from the previous module, applied to eliminate the duplication rather than just tolerate it.

Exercise

Write `has_magic_number(code_values, allowed)`: given a list of numeric literals found in some code and a set of self-explanatory `allowed` values (like `{0, 1}`), return True if any value ISN'T in `allowed` — a 'magic number' that should probably be a named constant.

Exercise

Write `count_duplicate_lines(lines)`: given a list of code lines, return how many DISTINCT lines appear more than once — a simple proxy for duplicated logic.

Quiz

What is a 'code smell'?

Checkpoint

You can identify magic numbers and duplicated code as code smells, and know the standard fix for each (named constants; extraction into a shared function).

Code Smells