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

Inheritance diagram of qubes.exc

Module contents

Qubes OS exception hierarchy

exception qubes.exc.BackupCancelledError(msg=None)[source]

Bases: qubes.exc.QubesException

Thrown at user when backup was manually cancelled

exception qubes.exc.QubesException[source]

Bases: Exception

Exception that can be shown to the user

exception qubes.exc.QubesFeatureNotFoundError(domain, feature)[source]

Bases: qubes.exc.QubesException, KeyError

Feature not set for a given domain

exception qubes.exc.QubesMemoryError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError, MemoryError

Cannot start domain, because not enough memory is available

exception qubes.exc.QubesNoSuchPropertyError(holder, prop_name, msg=None)[source]

Bases: qubes.exc.QubesException, AttributeError

Requested property does not exist

exception qubes.exc.QubesNoTemplateError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

Cannot start domain, because there is no template

exception qubes.exc.QubesNotImplementedError(msg=None)[source]

Bases: qubes.exc.QubesException, NotImplementedError

Thrown at user when some feature is not implemented

exception qubes.exc.QubesPropertyValueError(holder, prop, value, msg=None)[source]

Bases: qubes.exc.QubesValueError

Cannot set value of qubes.property, because user-supplied value is wrong.

exception qubes.exc.QubesTagNotFoundError(domain, tag)[source]

Bases: qubes.exc.QubesException, KeyError

Tag not set for a given domain

exception qubes.exc.QubesVMError(vm, msg)[source]

Bases: qubes.exc.QubesException

Some problem with domain state.

exception qubes.exc.QubesVMInUseError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

VM is in use, cannot remove.

exception qubes.exc.QubesVMNotFoundError(vmname)[source]

Bases: qubes.exc.QubesException, KeyError

Domain cannot be found in the system

exception qubes.exc.QubesVMNotHaltedError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

Domain is not halted.

This exception is thrown when machine should be halted, but is not (either running or paused).

exception qubes.exc.QubesVMNotPausedError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMNotStartedError

Domain is not paused.

This exception is thrown when machine should be paused, but is not.

exception qubes.exc.QubesVMNotRunningError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMNotStartedError

Domain is not running.

This exception is thrown when machine should be running but is either halted or paused.

exception qubes.exc.QubesVMNotStartedError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

Domain is not started.

This exception is thrown when machine is halted, but should be started (that is, either running or paused).

exception qubes.exc.QubesVMNotSuspendedError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

Domain is not suspended.

This exception is thrown when machine should be suspended but is either halted or running.

exception qubes.exc.QubesVMShutdownTimeoutError(vm, msg=None)[source]

Bases: qubes.exc.QubesVMError

Domain shutdown timed out.

exception qubes.exc.QubesValueError[source]

Bases: qubes.exc.QubesException, ValueError

Cannot set some value, because it is invalid, out of bounds, etc.