Py Error Handling

Python Error-Handling Review

Use these rules when changed code creates, catches, transforms, logs, retries, or suppresses failures.

Freshness: stable (no external references) — review rules based on core Python conventions, not volatile APIs.

Review Rules

Rule: error-validate-boundary

Impact: HIGH Applies when: External input enters the system through API handlers, CLI args, config, files, queues, or network payloads. Skip when: The caller already validated the exact invariant and the contract is local and obvious. Python: any Tools: none Review signal: Code trusts raw strings, dicts, or numeric ranges until deep inside business logic.

Incorrect:

def fetch_page(url: str, page_size: int) -> Page:
    return client.get(url, params={"page_size": page_size})

Correct:

def fetch_page(url: str, page_size: int) -> Page:
    if not url:
        raise ValueError("'url' is required")
    if not 1 <= page_size <= 100:
        raise ValueError(f"'page_size' must be 1-100, got {page_size}")
    return client.get(url, params={"page_size": page_size})

Reason: Boundary validation fails early with useful context instead of allowing vague downstream failures.

Rule: error-specific-exceptions

Impact: HIGH Applies when: Code raises or catches exceptions. Skip when: A truly unknown exception is caught only to add context and then re-raised. Python: any Tools: ruff | project-configured Review signal: raise Exception(...), bare except:, or except Exception: pass appears in changed code.

Incorrect:

try:
    process()
except Exception:
    pass

Correct:

try:
    process()
except ConnectionError as exc:
    logger.warning("Connection failed; retrying", exc_info=exc)
    raise

Reason: Generic or swallowed exceptions hide bugs and make production failures hard to diagnose.

Rule: error-chain-context

Impact: MEDIUM-HIGH Applies when: Code catches an exception and raises a domain-specific exception. Skip when: The original exception intentionally must be hidden from users and is still logged with trace context. Python: any Tools: ruff | project-configured Review signal: except SomeError as e: followed by raise OtherError(...) without from e.

Incorrect:

try:
    return Config.from_file(path)
except OSError as exc:
    raise ConfigError(f"Could not load {path}")

Correct:

try:
    return Config.from_file(path)
except OSError as exc:
    raise ConfigError(f"Could not load {path}") from exc

Reason: Chaining preserves the original traceback while still exposing a domain-level error.

Rule: error-batch-partial-failures

Impact: MEDIUM-HIGH Applies when: A loop processes independent items from a batch, queue, import, or migration. Skip when: The operation must be atomic and rollback is explicit. Python: any Tools: none Review signal: One item failure aborts a batch where other items could safely continue.

Incorrect:

def process_batch(items: list[Item]) -> list[Result]:
    return [process(item) for item in items]

Correct:

def process_batch(items: list[Item]) -> BatchResult:
    succeeded: dict[int, Result] = {}
    failed: dict[int, Exception] = {}
    for index, item in enumerate(items):
        try:
            succeeded[index] = process(item)
        except ProcessingError as exc:
            failed[index] = exc
    return BatchResult(succeeded=succeeded, failed=failed)

Reason: Independent batch work should report successes and failures separately unless atomicity is required.

Rule: error-cleanup-context-manager

Impact: HIGH Applies when: Code opens files, sockets, locks, database sessions, temporary directories, or other closeable resources. Skip when: Ownership is intentionally transferred and documented. Python: any Tools: ruff | project-configured Review signal: open(), locks, sessions, or clients are acquired without with, async with, try/finally, or an explicit close path.

Incorrect:

def read_config(path: str) -> str:
    handle = open(path)
    return handle.read()

Correct:

def read_config(path: str) -> str:
    with open(path) as handle:
        return handle.read()

Reason: Cleanup must run when reads, writes, or downstream processing raise.

Sensitive Evidence Safety

If changed code or tool output reveals a suspected credential, token, private key, secret-bearing URL, or other sensitive value, do not quote or reproduce the value. Report only its existence and location. Treat filename and pattern checks as heuristic evidence, not proof that a repository is secret-free.

If the exposure appears credible, make it the first finding, stop lower-priority review, and recommend revocation or rotation. Never place sensitive values in reports, generated examples, or commit subjects or bodies.