Precondition checking / Defensive programming

Expresses the requirement that a collection is non‑empty before proceeding with operations that would fail on empty input.

Writing a function that expects a non‑empty list; designing a property‑based test that should only generate non‑empty examples; adding a guard clause before indexing or reducing a sequence.

Python codebases, especially test suites using hypothesis, or libraries with design‑by‑contract utilities.

assume(len(;) > 0)

assume(len(;) > 0)

xs: a sequence or iterable whose length can be checked with len()

• assert
• guard clauses
• precondition decorators
• hypothesis.strategies.filter

• assert len(xs) > 0
• if not xs: raise ValueError('empty')
• using a contract library like icontract.require(lambda xs: len(xs) > 0)

Using assume outside of hypothesis where it has no effect; forgetting that assume only filters test cases and does not raise an exception in production; assuming the check validates input for callers outside tests.

assert len(xs) > 0 – raises AssertionError immediately; hypothesis.assume – filters generated test cases without raising; len(xs) and if‑else guard – explicit handling.

Coming from languages like C or Java where assert(len(xs)>0) is used for runtime checks, you might expect assume to raise an error; in hypothesis it merely discards the current example.

• precondition checks
• defensive programming
• input validation
• hypothesis filtering

assume only affects hypothesis test generation; it is ignored in normal execution, so it does not protect production code. For runtime guarantees use assert or explicit checks.

Keeps test suites focused on relevant inputs, reducing wasted examples and improving the signal‑to‑noise ratio of property‑based tests.

Assume the list ain’t empty – think of a bouncer at the door.

assume only affects hypothesis test generation; it is ignored in normal execution and does not raise an exception in production code. For runtime guarantees use assert or explicit checks.

Replace hypothesis.assume with a proper precondition decorator (e.g., icontract.require) for runtime enforcement.