Overriding commands in plugins

[Michael Gliwinski]

On Monday 08 Mar 2010 15:34:52 Matt Nordhoff wrote:
> Michael Gliwinski wrote:
> > Hi all,
> >
> > What is the best/proper/right/working way to override some functionality
> > of a command in a plugin?
> >
> > Let's say for example that I want to change 'switch' to print a nice
> > message before it goes on doing its work :)
> >
> > From looking at some of the examples I did:
> >
> > class cmd_switch(bzrlib.builtins.cmd_switch):
> >     __doc__ = bzrlib.builtins.cmd_switch.__doc__
> >     def run(**kwargs):
> >         self.outf.write('Hello World!'\n)
> >         super(cmd_switch, self).run(**kwargs)
> >
> > but it seems to me that doesn't take into account the fact that switch
> > may be overriden by other plugins.  So it seems the result of that may be
> > a bit random, depending on which plugin is loaded first?
> >
> > Then I also saw this done in the loom plugin:
> >
> > class cmd_switch(bzrlib.builtins.cmd_switch):
> >     def run_argv_aliases(self, argv, alias_argv=None):
> >         try:
> >             super(cmd_switch, self).run_argv_aliases(list(argv),
> > alias_argv) except (errors.MustUseDecorated, errors.BzrOptionsError): if
> > self._original_command is None:
> >                 raise
> >             self._original_command().run_argv_aliases(argv, alias_argv)
> >
> > so, IIUC this uses the original command (returned by
> > bzrlib.commands.plugin_cmds.register) stored in _original_command
> > attribute, tries to run the overriden command and in case of problems
> > falls back to original command, yes?
> >
> > Is this the more "proper" way?  Does it have to be done in
> > run_argv_aliases? I.e. if I'm only adding certain functionality it would
> > be bit more convenient to do it in run.
> >
> > I'll try to update http://doc.bazaar.canonical.com/plugins/en/plugin-
> > development.html#extending-an-existing-command if I can figure out
> > something solid to add there.
> >
> > Thanks,
> > Michael
> 
> I know of another way, using bzrlib.commands.get_command_object(), and I
> /think/ it's the best way. As an example, this is what Loggerhead does:
> 
>         import bzrlib.builtins
>         from bzrlib.commands import get_cmd_object, register_command
>         from bzrlib.option import Option
> 
>         _original_command = get_cmd_object('serve')
> 
>         class cmd_serve(bzrlib.builtins.cmd_serve):
>             __doc__ = _original_command.__doc__
> 
>             takes_options = _original_command.takes_options + [
>                 Option('http', help=HELP)]
> 
>             def run(self, *args, **kw):
>                 if 'http' in kw:
>                     # ... snip ...
>                 else:
>                     super(cmd_serve, self).run(*args, **kw)
> 
> <http://bazaar.launchpad.net/~loggerhead-team/loggerhead/trunk-rich/annotat
> e/head%3A/__init__.py#L91> (<http://xrl.us/bgxved>)
> 

OK, so from what I can see using get_cmd_object instead of return value of 
register gives you support for command hooks and access to original command 
even if your command is defined in a different module than where the command is 
registered, sounds about right?

Still, in your run() you use `super(cmd_serve, self).run(...)', doesn't that 
bypass any other classes that might have overriden cmd_serve (since you have 
to inherit from builtin so super will use it't run(), yes?)

Thanks for the tip,
Michael


-- 
Michael Gliwinski
Henderson Group Information Services
9-11 Hightown Avenue, Newtownabby, BT36 4RT
Phone: 028 9034 3319

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