Syllabus Lesson 112 of 141 · Invent your tools
Invent your tools

A test idea finder

Why it matters Most missed tests are the same few: empty, zero, negative, None, left out. A tool that reads a signature and lists them is a checklist you cannot forget to run.

Your team wants a helper for code review: paste a function's def line, get the edge cases its parameters suggest. The rules come from the team's testing guide.

edge_cases("def paginate(items: list[str], page: int, size: int = 20) -> list[str]:")
# items: empty, items: one item, items: many items,
# page: 0, page: negative, page: very large,
# size: 0, size: negative, size: very large, size: left out

Real signatures are not that tidy. They span several lines, have nested types like dict[str, list[int]], defaults like sep: str = ", ", a bare * or /, *args, **kwargs, async def, and return types with commas in them.

Your turn

Write edge_cases(signature): signature is a complete def or async def header ending in :, possibly over several lines. Return the list of ideas, each "name: idea", order not important. Skip self, cls, * and /. By annotation:

  • int: 0, negative, very large. float: 0.0, negative, NaN. str: empty, only spaces, non-ASCII. bool: True, False.
  • list, tuple, set, dict (with or without [...]): empty, one item, many items.
  • X | None, None | X or Optional[X]: None, plus the ideas for X. Any other union, and any other type: typical value.
  • No annotation: None, wrong type.
Also add "name: left out" for a parameter with a default, "args: no extra arguments" for *args and "kw: no keyword arguments" for **kw (with their own names). Trap to beat: eleven hidden signatures with every feature listed above.

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