Meaning
Ensures a precondition that a collection contains at least one element. If the collection is empty, an AssertionError is raised, halting execution during development or testing. Use it to catch logic errors early.
Primary Function
Defensive programming
Communicative Purpose
Expresses the expectation that a sequence is non‑empty before further processing.
Pattern
assert len(seq) > 0
Core Structure
assert len(...) > 0
Função primária
Defensive programming
Propósito comunicativo
Expresses the expectation that a sequence is non‑empty before further processing.
Situações de gatilho
Data processing: function receives a list that must contain at least one element; Testing: unit test verifies that a collection returned by a function is non‑empty
Contextos
General Python codebases, data‑processing scripts, algorithm implementations, unit tests.
Padrão
assert len(seq) > 0
Estrutura central
assert len(...) > 0
Slots de substituição
collection_expr: any expression yielding a sequence
Colocados típicos
- assert
- len
- > 0
- collection
Substituições comuns
- if not collection: raise ValueError(...)
- assert collection
Erros comuns
Using assert for user input validation – cause: assumes asserts run in production; consequence: validation is skipped when Python is executed with -O, leading to unexpected errors Writing assert len(collection) > 0 without an explanatory message – cause: lack of context; consequence: AssertionError provides no helpful information for debugging Placing the assert after the collection has been mutated – cause: condition may no longer hold; consequence: false positive or missed error
Similar / contraste
Explicit check with if not collection: raise ValueError – differs by raising a specific exception and never being stripped.
Interferências
Coming from C/C++ where assert is compiled out in release builds; Python's assert behaves similarly when -O is used.
Família do chunk
- precondition checks
- defensive asserts
- input validation
Nuance
Do not use for validating external user input, because asserts can be disabled with the -O flag. The performance impact is negligible, but the check disappears in optimized mode, removing the safety net. It also fails to catch empty sequences that are lazily generated, such as generators that raise StopIteration instead of having a length.
Efeito pragmático
Fails early with a clear AssertionError, making debugging easier and preventing downstream errors.
Dica de memória
Think of an assert len check as a guard at the entrance of a building, stopping anyone from walking into an empty hallway.
Nota
Assertions may be removed with the -O flag; use explicit checks for production validation.
Upgrade path
Replace with explicit validation: if not collection: raise ValueError('collection must not be empty')
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