Error handling in SQLAlchemy
Question:
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.
Answers:
SQLAlchemy will raise an exception on error….
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 detectedCompileError – Raised when an error occurs during SQL compilationConcurrentModificationErrorDBAPIError – 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.FlushErrorIdentifierError – Raised when a schema name is beyond the max character limitIntegrityError – 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 – A refresh() operation failed to re-retrieve an object’s row.UnmappedClassError – A mapping operation was requested for an unknown class.UnmappedColumnError – Mapping operation was requested on an unknown column.UnmappedError – TODOUnmappedInstanceError – A mapping operation was requested for an unknown instance.
I tried this and it showed me the specific error message.
from sqlalchemy.exc import SQLAlchemyError
try:
# try something
except SQLAlchemyError as e:
error = str(e.__dict__['orig'])
return error
Hope this helps
To get type of exception, you can simply call :
Firstly, import exc from sqlalchemy
from sqlalchemy import exc
To catch types of errors:
try:
# any query
except exc.SQLAlchemyError as e:
print(type(e))
This will give you exception type:
Output:
<class 'sqlalchemy.exc.IntegrityError'>
<class 'sqlalchemy.exc.IntegrityError'>
My two cents on handling errors in SQLAlchemy: a simple python’s try-except will not work as MySQL is persistent. For example, if you try to insert a record to the database but it is a duplicate, the program will take the exception route but MySQL will stop based on the insert command that did not go through. Avoid try-except in combination with SQLAlchemy commands or be prepared for these situations.
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.
SQLAlchemy will raise an exception on error….
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 detectedCompileError– Raised when an error occurs during SQL compilationConcurrentModificationErrorDBAPIError– 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 thestatementandparamsattributes.
The wrapped exception object is available in theorigattribute.
Its type and properties are DB-API implementation specific.DataErrorWraps a DB-APIDataError.DatabaseError– Wraps a DB-APIDatabaseError.DisconnectionError– A disconnect is detected on a raw DB-API connection.
be raised by aPoolListenerso that the host pool forces a disconnect.FlushErrorIdentifierError– Raised when a schema name is beyond the max character limitIntegrityError– Wraps a DB-APIIntegrityError.InterfaceError– Wraps a DB-APIInterfaceError.InternalError– Wraps a DB-APIInternalError.InvalidRequestError– SQLAlchemy was asked to do something it can’t do. This error generally corresponds to runtime state errors.NoReferenceError– Raised byForeignKeyto indicate a reference cannot be resolved.NoReferencedColumnError– Raised byForeignKeywhen the referredColumncannot be located.NoReferencedTableError– Raised byForeignKeywhen the referredTablecannot be located.NoSuchColumnError– A nonexistent column is requested from aRowProxy.NoSuchTableError– Table does not exist or is not visible to a connection.NotSupportedError– Wraps a DB-APINotSupportedError.OperationalError– Wraps a DB-APIOperationalError.ProgrammingError– Wraps a DB-APIProgrammingError.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 duringflush().MultipleResultsFound– A single database result was required but more than one were found.NoResultFound– A database result was required but none was found.ObjectDeletedError– Arefresh()operation failed to re-retrieve an object’s row.UnmappedClassError– A mapping operation was requested for an unknown class.UnmappedColumnError– Mapping operation was requested on an unknown column.UnmappedError– TODOUnmappedInstanceError– A mapping operation was requested for an unknown instance.
I tried this and it showed me the specific error message.
from sqlalchemy.exc import SQLAlchemyError
try:
# try something
except SQLAlchemyError as e:
error = str(e.__dict__['orig'])
return error
Hope this helps
To get type of exception, you can simply call :
Firstly, import exc from sqlalchemy
from sqlalchemy import exc
To catch types of errors:
try:
# any query
except exc.SQLAlchemyError as e:
print(type(e))
This will give you exception type:
Output:
<class 'sqlalchemy.exc.IntegrityError'>
<class 'sqlalchemy.exc.IntegrityError'>
My two cents on handling errors in SQLAlchemy: a simple python’s try-except will not work as MySQL is persistent. For example, if you try to insert a record to the database but it is a duplicate, the program will take the exception route but MySQL will stop based on the insert command that did not go through. Avoid try-except in combination with SQLAlchemy commands or be prepared for these situations.