Exceptions

In programming exceptions are conditions of the program that require special processing. Handling exceptions is important, because to the user the program should function as if it were in a normal state whereby any exceptions that are encountered are handled in the background. See also ​Exception_handling.

This page lists the most common exceptions in Trac core and database communication.

Custom Trac Exceptions

Trac defines a set of custom exception types:

• Exception
  • TracBaseError (trac.core​): Exception base class for errors in Trac. (since 1.1.2)
    • TracError (trac.core​): Standard Exception for errors in Trac.
      • AdminCommandError (trac.admin.api​): Exception raised when an admin command cannot be executed.
      • ConfigurationError (trac.config​): Exception raised when a value in the configuration file is not valid.
      • InvalidAttachment (trac.attachment​): Exception raised when attachment validation fails.
      • InvalidRepository (trac.versioncontrol.api​): Exception raised when initializing a repository fails.
      • InvalidTicket (trac.ticket.web_ui​): Exception raised when a ticket fails validation.
      • InvalidWikiPage (trac.wiki.web_ui​): Exception raised when a Wiki page fails validation. Deprecated since Trac 0.11 and removed in Trac 1.1.2.
      • MacroError (trac.wiki.formatter​) Exception raised in macro when an argument is invalid (Since Trac 1.0.11).
      • ProcessorError (trac.wiki.formatter​)
      • QuerySyntaxError (trac.ticket.query​): Exception raised when a ticket query cannot be parsed from a string.
      • QueryValueError (trac.ticket.query​): Exception raised when a ticket query has bad constraint values.
      • ResourceExistsError (trac.resource​): Thrown when attempting to insert an existing resource
      • ResourceNotFound (trac.resource​): Thrown when a non-existent resource is requested
      • TimeoutError (trac.db.pool​): Exception raised by the connection pool when no connection has become available after a given timeout (Prior to 1.0.2dev-r12305, inherited directly from Exception).
      • TracNotImplementedError (trac.web.api​) Exception raised when a NotImplementedError is trapped. This exception is for internal use only and should not be raised by plugins. Plugin should use NotImplementedError. (Since 1.0.11)
      • TracValueError (trac.core​) Exception raised when a function or operator receives an argument that is the correct type, but inappropriate value. (Since 1.2.1)
    • HTTPException (trac.web.api​): Representing a HTTP status code
      • HTTPBadRequest (trac.web.api​): 400 Bad Request
      • HTTPUnauthorized (trac.web.api​): 401 Unauthorized
      • HTTPPaymentRequired (trac.web.api​): 402 Payment Required
      • HTTPForbidden (trac.web.api​): 403 Forbidden
      • HTTPNotFound (trac.web.api​): 404 Not Found
      • HTTPMethodNotAllowed (trac.web.api​): 405 Method Not Allowed
      • HTTPNotAcceptable (trac.web.api​): 406 Not Acceptable
      • HTTPProxyAuthenticationRequired (trac.web.api​): 407 Proxy Authentication Required
      • HTTPRequestTimeout (trac.web.api​): 408 Request Timeout
      • HTTPConflict (trac.web.api​): 409 Conflict
      • HTTPGone (trac.web.api​): 410 Gone
      • HTTPLengthRequired (trac.web.api​): 411 Length Required
      • HTTPPreconditionFailed (trac.web.api​): 412 Precondition Failed
      • HTTPRequestEntityTooLarge (trac.web.api​): 413 Request Entity Too Large
      • HTTPRequestUriTooLong (trac.web.api​): 414 Request-Uri Too Long
      • HTTPUnsupportedMediaType (trac.web.api​): 415 Unsupported Media Type
      • HTTPRequestedRangeNotSatisfiable (trac.web.api​): 416 Requested Range Not Satisfiable
      • HTTPExpectationFailed (trac.web.api​): 417 Expectation Failed
      • HTTPInternalError (trac.web.api​): 500 Internal Server Error
      • HTTPNotImplemented (trac.web.api​): 501 Not Implemented
      • HTTPBadGateway (trac.web.api​): 502 Bad Gateway
      • HTTPServiceUnavailable (trac.web.api​): 503 Service Unavailable
      • HTTPGatewayTimeout (trac.web.api​): 504 Gateway Timeout
      • HTTPVersionNotSupported (trac.web.api​): 505 Http Version Not Supported
    • RequestDone (trac.web.api​): Marker exception that indicates whether request processing has completed and a response was sent.
    • ParseError (trac.versioncontrol.svn_authz​): Exception thrown for parse errors in authz files
    • GitError (tracopt.versioncontrol.git.PyGIT​)
      • GitErrorSha (tracopt.versioncontrol.git.PyGIT​)
    • Error (trac.db.tests.api​)
  • StandardError, TracBaseError
    • PermissionError (trac.perm​): Insufficient permissions to complete the operation
  • StandardError
    • RuntimeError, TracBaseError
      • BackupError (trac.env​): Exception raised during an upgrade when the DB backup fails.

Hierarchy is inconsistent, even after addition of TracBaseError class (#11568). There doesn't seem to be any consistent pattern in inheriting from TracError. Some exceptions also inherit from built-in exception classes other than Exception.

There also seem to be multiple naming conventions. Consider following ​PEP:0008#exception-names.

But changing the existing hierarchy might be difficult without breaking backward compatibility.

Database Exceptions

We can catch ​Python DB API exceptions in a database neutral way. (Since Trac 1.0, see #6348.)

try: 
    with env.db_transaction as db: 
        ... 
except env.db_exc.IntegrityError, e: 
    ... 

The above example catches sqlite.IntegrityError, MySQLdb.IntegrityError or psycopg.IntegrityError, depending on the selected DatabaseBackend.

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See tickets