`qubes.exc` – Exceptions¶

As most of the modern programming languages, Python has exceptions, which can be thrown (raised) when something goes bad. What exactly means “bad” depends on several circumstances.

One of those circumstances is who exactly is to blame: programmer or user? Some errors are commited by programmer and will most probably result it program failure. But most errors are caused by the user, notably by specifying invalid commands or data input. Those errors should not result in failure, but fault and be handled gracefuly. One more time, what “gracefuly” means depends on specific program and its interface (for example GUI programs should most likely display some admonition, but will not crash).

In Qubes we have special exception class, qubes.exc.QubesException, which is dedicated to handling user-caused problems. Programmer errors should not result in raising QubesException, but it should instead result in one of the standard Python exception. QubesExceptions should have a nice message that can be shown to the user. On the other hand, some children classes of QubesException also inherit from children of StandardException to allow uniform except clauses.

Often the error relates to some domain, because we expect it to be in certain state, but it is not. For example to start a machine, it should be halted. For that we have the children of the qubes.exc.QubesVMError class. They all take the domain in question as their first argument and an (optional) message as the second. If not specified, there is stock message which is generally informative enough.

On writing error messages¶

As a general rule, error messages should be short but precise. They should not blame user for error, but the user should know, what had been done wrong and what to do next.

If possible, write the message that is stating the fact, for example “Domain is not running” instead of “You forgot to start the domain” (you fool!). Avoid commanding user, like “Start the domain first” (user is not a function you can call for effect). Instead consider writing in negative form, implying expected state: “Domain is not running” instead of “Domain is paused” (yeah, what’s wrong with that?).

Also avoid implying the personhood of the computer, including adressing user in second person. For example, write “Sending message failed” instead of “I failed to send the message”.

Inheritance diagram¶

digraph inheritanceddb8932c08 { bgcolor=transparent; rankdir=LR; size="8.0, 12.0"; "qubes.exc.BackupAlreadyRunningError" [URL="#qubes.exc.BackupAlreadyRunningError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Thrown at user when they try to run the same backup twice at"]; "qubes.exc.QubesException" -> "qubes.exc.BackupAlreadyRunningError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.BackupCancelledError" [URL="#qubes.exc.BackupCancelledError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Thrown at user when backup was manually cancelled"]; "qubes.exc.QubesException" -> "qubes.exc.BackupCancelledError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesException" [URL="#qubes.exc.QubesException",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Exception that can be shown to the user"]; "qubes.exc.QubesFeatureNotFoundError" [URL="#qubes.exc.QubesFeatureNotFoundError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Feature not set for a given domain"]; "qubes.exc.QubesException" -> "qubes.exc.QubesFeatureNotFoundError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesLabelNotFoundError" [URL="#qubes.exc.QubesLabelNotFoundError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Label does not exists"]; "qubes.exc.QubesException" -> "qubes.exc.QubesLabelNotFoundError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesMemoryError" [URL="#qubes.exc.QubesMemoryError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Cannot start domain, because not enough memory is available"]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesMemoryError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesNoSuchPropertyError" [URL="#qubes.exc.QubesNoSuchPropertyError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Requested property does not exist"]; "qubes.exc.QubesException" -> "qubes.exc.QubesNoSuchPropertyError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesNoTemplateError" [URL="#qubes.exc.QubesNoTemplateError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Cannot start domain, because there is no template"]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesNoTemplateError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesNotImplementedError" [URL="#qubes.exc.QubesNotImplementedError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Thrown at user when some feature is not implemented"]; "qubes.exc.QubesException" -> "qubes.exc.QubesNotImplementedError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesPoolInUseError" [URL="#qubes.exc.QubesPoolInUseError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="VM is in use, cannot remove."]; "qubes.exc.QubesException" -> "qubes.exc.QubesPoolInUseError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesPropertyValueError" [URL="#qubes.exc.QubesPropertyValueError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Cannot set value of qubes.property, because user-supplied value is wrong."]; "qubes.exc.QubesValueError" -> "qubes.exc.QubesPropertyValueError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesTagNotFoundError" [URL="#qubes.exc.QubesTagNotFoundError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Tag not set for a given domain"]; "qubes.exc.QubesException" -> "qubes.exc.QubesTagNotFoundError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMError" [URL="#qubes.exc.QubesVMError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Some problem with domain state."]; "qubes.exc.QubesException" -> "qubes.exc.QubesVMError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMInUseError" [URL="#qubes.exc.QubesVMInUseError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="VM is in use, cannot remove."]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesVMInUseError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotFoundError" [URL="#qubes.exc.QubesVMNotFoundError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain cannot be found in the system"]; "qubes.exc.QubesException" -> "qubes.exc.QubesVMNotFoundError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotHaltedError" [URL="#qubes.exc.QubesVMNotHaltedError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain is not halted."]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesVMNotHaltedError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotPausedError" [URL="#qubes.exc.QubesVMNotPausedError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain is not paused."]; "qubes.exc.QubesVMNotStartedError" -> "qubes.exc.QubesVMNotPausedError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotRunningError" [URL="#qubes.exc.QubesVMNotRunningError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain is not running."]; "qubes.exc.QubesVMNotStartedError" -> "qubes.exc.QubesVMNotRunningError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotStartedError" [URL="#qubes.exc.QubesVMNotStartedError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain is not started."]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesVMNotStartedError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMNotSuspendedError" [URL="#qubes.exc.QubesVMNotSuspendedError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain is not suspended."]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesVMNotSuspendedError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesVMShutdownTimeoutError" [URL="#qubes.exc.QubesVMShutdownTimeoutError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Domain shutdown timed out."]; "qubes.exc.QubesVMError" -> "qubes.exc.QubesVMShutdownTimeoutError" [arrowsize=0.5,style="setlinewidth(0.5)"]; "qubes.exc.QubesValueError" [URL="#qubes.exc.QubesValueError",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",target="_top",tooltip="Cannot set some value, because it is invalid, out of bounds, etc."]; "qubes.exc.QubesException" -> "qubes.exc.QubesValueError" [arrowsize=0.5,style="setlinewidth(0.5)"]; }