Error handling in SQLAlchemy

Question

Hello everyone,

How do you handle errors in SQLAlchemy? I am relatively new to SQLAlchemy and do not know yet.

Before I used SQLAlchemy, I would do things like

status = db.query("INSERT INTO users ...")
if (!status):
    raise Error, db.error

But now I am coding in SQLAlchemy and I do things like

user = User('Boda Cydo')
session.add(user)
session.commit()

No error checking whatsoever!

I do not like this coding style without error checking at all.

Please advice on how to check and handle errors in SQLAlchemy!

Sincerely, Boda Cydo.

Answer 1

SQLAlchemy will raise an exception on error....

Answer 2

Your example says:

status = db.query("INSERT INTO users ...")
if (!status):
    raise Error, db.error

That seems to mean that you want to raise an exception if there's some error on the query (with raise Error, db.error). However sqlalchemy already does that for you - so

user = User('Boda Cydo')
session.add(user)
session.commit()

Is just the same. The check-and-raise part is already inside SQLAlchemy.

Here is a list of the errors sqlalchemy itself can raise, taken from help(sqlalchemy.exc) and help(sqlalchemy.orm.exc):

• sqlalchemy.exc:
  • ArgumentError - Raised when an invalid or conflicting function argument is supplied. This error generally corresponds to construction time state errors.
  • CircularDependencyError - Raised by topological sorts when a circular dependency is detected
  • CompileError - Raised when an error occurs during SQL compilation
  • ConcurrentModificationError
  • DBAPIError - Raised when the execution of a database operation fails. If the error-raising operation occured in the execution of a SQL statement, that statement and its parameters will be available on the exception object in the statement and params attributes. The wrapped exception object is available in the orig attribute. Its type and properties are DB-API implementation specific.
  • DataError Wraps a DB-API DataError.
  • DatabaseError - Wraps a DB-API DatabaseError.
  • DisconnectionError - A disconnect is detected on a raw DB-API connection. be raised by a PoolListener so that the host pool forces a disconnect.
  • FlushError
  • IdentifierError - Raised when a schema name is beyond the max character limit
  • IntegrityError - Wraps a DB-API IntegrityError.
  • InterfaceError - Wraps a DB-API InterfaceError.
  • InternalError - Wraps a DB-API InternalError.
  • InvalidRequestError - SQLAlchemy was asked to do something it can't do. This error generally corresponds to runtime state errors.
  • NoReferenceError - Raised by ForeignKey to indicate a reference cannot be resolved.
  • NoReferencedColumnError - Raised by ForeignKey when the referred Column cannot be located.
  • NoReferencedTableError - Raised by ForeignKey when the referred Table cannot be located.
  • NoSuchColumnError - A nonexistent column is requested from a RowProxy.
  • NoSuchTableError - Table does not exist or is not visible to a connection.
  • NotSupportedError - Wraps a DB-API NotSupportedError.
  • OperationalError - Wraps a DB-API OperationalError.
  • ProgrammingError - Wraps a DB-API ProgrammingError.
  • SADeprecationWarning - Issued once per usage of a deprecated API.
  • SAPendingDeprecationWarning - Issued once per usage of a deprecated API.
  • SAWarning - Issued at runtime.
  • SQLAlchemyError - Generic error class.
  • SQLError - Raised when the execution of a database operation fails.
  • TimeoutError - Raised when a connection pool times out on getting a connection.
  • UnboundExecutionError - SQL was attempted without a database connection to execute it on.
  • UnmappedColumnError
• sqlalchemy.orm.exc:
  • ConcurrentModificationError - Rows have been modified outside of the unit of work.
  • FlushError - A invalid condition was detected during flush().
  • MultipleResultsFound - A single database result was required but more than one were found.
  • NoResultFound - A database result was required but none was found.
  • ObjectDeletedError - An refresh() operation failed to re-retrieve an object's row.
  • UnmappedClassError - An mapping operation was requested for an unknown class.
  • UnmappedColumnError - Mapping operation was requested on an unknown column.
  • UnmappedError - TODO
  • UnmappedInstanceError - An mapping operation was requested for an unknown instance.

Answer 3

Take a look at example at sqlalchemy.org http://www.sqlalchemy.org/trac/wiki/UsageRecipes/UniqueConstraintExceptionHandling