Understand the idea
Debug against a contract, not against one successful example. Pick cases that isolate assumptions: empty, singleton, duplicates, negatives, boundaries, and unusual sizes. Assertions turn expected behavior into a repeatable check.
Work through an example
Initializing best=0 passes [4,1,7] but fails [-8,-3,-5]. The expected -3 exposes the unsupported assumption that a maximum cannot be negative.
Time and space complexity
A few tests are cheap but do not prove correctness. Exhaustive tests grow with the size of the input domain.
Why it works
A test checks a specific claim; the invariant explains all inputs within the contract.
Common mistakes
- Do not silently swallow every exception.
- Random testing should keep a reproducible seed and a trusted comparison implementation.
Chapter lab · Python implementation
Our running task is to find the largest value in a sequence. A program turns a precise input/output contract into small state changes. Read the example from the top, predict each update, and run its assertions. Python examples use ordinary built-in types; statements inside an indented block belong to that block.
This shared lab connects the chapter's topics. Follow the current lesson's worked example first, then trace the complete implementation. Additional algorithms described in topic exercises are variations you can implement independently.
def largest(values):
if not values:
return None
best = values[0]
for value in values[1:]:
if value > best:
best = value
return best
def largest_without_slice(values):
if not values:
return None
best = values[0]
for i in range(1, len(values)):
best = max(best, values[i])
return bestRun and test the lab
Copy the implementation and these assertions into a Python 3 file and run it. An assertion failure means the actual result differs from the expectation. Add the current topic's exercise and boundary tests to deepen your understanding.
assert largest([]) is None
assert largest([-8, -3, -5]) == -3
assert largest_without_slice([4, 1, 7, 2]) == 7
assert largest_without_slice([0]) == 0Which test catches initialization to zero?
Further reading
Original documentation for the implementation details used in this chapter.
Python data structures