Errors

Every failure raises a subclass of ustore.Error, itself a RuntimeError, and each carries the status the C ABI reported as its status attribute, a ustore.Status member:

transaction = database.transaction(ustore.Watch.KEYS)
try:
    transaction.set(key, value)
    transaction.commit(ustore.Durability.FLUSHED)
except ustore.ConflictError as error:
    assert error.status in (ustore.Status.WRITE_CONFLICT, ustore.Status.READ_CONFLICT)
finally:
    transaction.close()

SUCCESS and DEFERRED raise nothing, and every other member has its own class, which also derives the built-in naming the same fault where one exists:

• OUT_OF_MEMORY → OutOfMemoryError, a MemoryError
• READ_ONLY → ReadOnlyError
• IO → InputOutputError, an OSError
• DEVICE → DeviceError
• MALFORMED → MalformedError, a ValueError
• INTERNAL → InternalError
• UNINITIALIZED → UninitializedError
• CONTENDED → ContendedError
• CANCELLED → CancelledError
• UNSUPPORTED → UnsupportedError, a NotImplementedError
• INVALID_ARGUMENT → InvalidArgumentError, a ValueError
• NOT_FOUND → NotFoundError, a KeyError
• ALREADY_EXISTS → AlreadyExistsError, a ValueError
• WRITE_CONFLICT → WriteConflictError, a ConflictError
• READ_CONFLICT → ReadConflictError, a ConflictError
• PHANTOM_CONFLICT → PhantomConflictError, a ConflictError
• CAPACITY_EXHAUSTED → CapacityExhaustedError, an OverflowError

So except KeyError catches a missing collection, and except ConflictError is the one clause a retry loop needs for the three conflicts. A database view_rocksdb opened raises ReadOnlyError from every call that would change it: writes, removes, transactions, collection creates and drops, clears and compactions. A failure the binding catches before the boundary raises the class the engine would have raised for the same fault: a closed database, transaction, snapshot or executor raises UninitializedError, a view of a collection holding another modality raises InvalidArgumentError, and a count of values that disagrees with the count of keys raises InvalidArgumentError too. An argument of the wrong Python type raises TypeError, and a snapshot refuses a write with one.