Meaning
The guard clause checks the `closed` attribute of a file‑like object and calls its `close()` method only when the object is still open. This prevents raising exceptions or unwanted side effects from attempting to close an already‑closed resource. Use it in cleanup code when automatic context‑manager handling is not employed.
Primary Function
Resource management
Communicative Purpose
Ensures a file descriptor is closed only if it remains open, avoiding double‑close errors.
Pattern
if not file_obj.closed: file_obj.close()
Core Structure
if not ... .closed: ... .close()
Função primária
Resource management
Propósito comunicativo
Ensures a file descriptor is closed only if it remains open, avoiding double‑close errors.
Situações de gatilho
Scripting: manually closing files without a context manager; Data processing: cleaning up file handles in a finally block; Testing: re‑opening a file that may have been closed earlier
Contextos
Standalone scripts, legacy codebases, low‑level I/O handling, custom resource wrappers.
Padrão
if not file_obj.closed: file_obj.close()
Estrutura central
if not ... .closed: ... .close()
Slots de substituição
file_obj: identifier (the file variable, used twice)
Colocados típicos
- try/except
- finally
- with open
- os.path
- sys
Substituições comuns
- using `with open(...) as f:`
- calling `f.flush()` before close
- using `os.close(fd)` for low‑level descriptors.
Erros comuns
Calling f.close() without checking (assuming close is idempotent) → may raise exception on custom file-like objects where close is not idempotent; Using if f.closed: instead of if not f.closed: (negating condition incorrectly) → attempts to close already-closed handles, causing exceptions; Forgetting to close the file altogether (overlooking cleanup) → resource leaks, file descriptors remain open.
Similar / contraste
Contrast with the `with` statement which auto‑closes; differs from a plain `try: ... finally: f.close()` without the guard.
Interferências
Coming from C: assume close() is always safe → in Python it may raise on some custom objects; Coming from Java: assume close() is always safe → in Python it may raise on some custom objects.
Família do chunk
- resource cleanup
- context manager
- try/finally
- guard clause
Nuance
Only needed when not using a context manager; for objects with idempotent close the check adds negligible overhead and can be omitted; however, if the close method has additional side effects, invoking it multiple times may cause unintended behavior.
Efeito pragmático
Prevents exceptions from double‑close and makes cleanup intent explicit.
Dica de memória
Like checking a door is unlocked before locking it again to avoid jamming the lock.
Nota
While the `close()` method of standard file objects is idempotent (making this check redundant in most cases), it is essential for custom file‑like objects where `close()` may raise an exception when invoked multiple times, ensuring robust resource cleanup.
Upgrade path
Replace with a `with open(...) as f:` context manager or use `contextlib.ExitStack` for handling multiple resources.
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