Error Handling

In case of erroneous transactions, connection problems and a lot of other cases, PyLinkAhead may raise specific errors in order to pinpoint the problem as precisely as possible. Some of these errors a representations of errors in the LinkAhead server, others stem from problems that occurred on the client side.

The errors and exceptions are ordered hierarchically form the most general exceptions to specific transaction or connection problems. The most important error types and the hierarchy will be explained in the following. For more information on specific error types, see also the source code.


Starting from PyCaosDB 0.5, the error handling has changed significantly. New error classes have been introduced and the behavior of TransactionError and EntityError has been re-worked. In the following, only the “new” errors are discussed. Please refer to the documentation of PyCaosDB 0.4.1 and earlier for the old error handling.


LinkAheadException is the most generic exception and all other error classes inherit from this one. Because of its generality, it doesn’t tell you much except that some component of PyLinkAhead raised an exception. If you want to catch all possible LinkAhead errors, this is the class to use.


Every transaction (calling insert, update, retrieve, or delete on a container or an entity) may finish with errors. They indicate, for instance, that an entity does not exist or that you need to specify a data type for your property and much more. If and only if one or more errors occur during a transaction a TransactionError will be raised by the transaction method. The TransactionError class is a container for all errors which occur during a transaction. It usually contains one or more entity errors which you can inspect in order to learn why the transaction failed. For this inspection, there are some helpful attributes and methods provided by the TransactionError:

  • entities: a list of all entities that directly caused at least one error in this transaction.

  • errors: a list of all EntityError objects that directly caused the transaction to fail.

  • all_entities, all_errors: sets of all entities and errors that, directly or indirectly, caused either this TransactionError or any of the EntityError objects it contains.

  • has_error(error_t): Check whether an error of type error_t occurred during the transaction.

Additionally, print(transaction_error) prints a tree-like representation of all errors regarding the transaction in question.


An EntityError specifies the entity and the error proper that caused a transaction to fail. It is never raised on its own but is contained in a TransactionError (which may or may not contain other EntityError objects) which is then raised. EntityError has several subclasses that further specify the error that occurred.

The EntityError class is in fact a subclass of TransactionError. Thus, it has the same methods and attributes as the TransactionError explained above. This is important in case of an EntityError that was caused by other faulty entities (e.g., broken parents or properties). In that case these problematic entities and errors can again be inspected by visiting the entities and errors lists as above.

Special Errors

Subclasses of EntityError for special purposes:

  • EntityDoesNotExistError

  • EntityHasNoDataTypeError

  • UniqueNamesError

  • UnqualifiedParentsError

  • UnqualifiedPropertiesError

  • ConsistencyError

  • AuthorizationError

  • AmbiguousEntityError


A BadQueryError is raised when a query could not be processed by the server. In contrast to a TransactionError it is not necessarily caused by problematic entities or containers. BadQueryError has the two important subclasses EmptyUniqueQueryError and QueryNotUniqueError for queries with unique=True which found no or ambiguous entities, respectively.

HTTP Errors

An HTTPClientError or an HTTPServerError is raised in case of http(s) connection problems caused by the Python client or the LinkAhead server, respectively. There are the following subclasses of HTTPClientError that are used to specify the connection problem:

  • HTTPURITooLongError: The URI of the request was too long to be processed by the server.

  • HTTPForbiddenError: You’re not allowed to access this resource.

  • HTTPResourceNotFoundError: The requested resource doesn’t exist.

Other Errors

There are further subclasses of LinkAheadException that are raised in case of faulty configurations or other problems. They should be rather self-explanatory from their names; see the source code for further information.

  • ConfigurationError

  • LoginFailedError

  • MismatchingEntitiesError

  • ServerConfigurationException


import linkahead as db

def link_and_insert(entity, linked, link=True):
  """Link the ENTITY to LINKED and insert it."""
  if link:
    entity.add_property(db.Property(name="link", value=linked))
  except db.TransactionError as tre:
    # Unique names problem may be worked around by using another name
    if tre.has_error(db.UniqueNamesError):
      for ent_error in tre.errors:
        if (isinstance(ent_error, db.UniqueNamesError)
            and entity in ent_error.entities):
 = + "_new"  # Try again with new name.
          link_and_insert(entity, linked, link=False)
    # Unqualified properties will be handled by the caller
    elif tre.has_error(db.UnqualifiedPropertiesError):
      for ent_error in tre.errors:
        if (isinstance(ent_error, db.UnqualifiedPropertiesError_
            and entity in ent_error.entities):
          raise RuntimeError("One of the properties was unqualified: " + str(ent_error))
    # Other problems are not covered by this tutorial
      raise NotImplementedError("Unhandled TransactionError: " + str(tre))