[openstack-dev] [nova][oslo] oslo.config and import chains

Matthew Booth mbooth at redhat.com
Thu Aug 7 16:46:06 UTC 2014

On 07/08/14 17:39, Kevin L. Mitchell wrote:
> On Thu, 2014-08-07 at 17:27 +0100, Matthew Booth wrote:
>> On 07/08/14 16:27, Kevin L. Mitchell wrote:
>>> On Thu, 2014-08-07 at 12:15 +0100, Matthew Booth wrote:
>>>> A (the?) solution is to register_opts() in foo before importing any
>>>> modules which might also use oslo.config.
>>> Actually, I disagree.  The real problem here is the definition of
>>> bar_func().  The default value of the parameter "arg" will likely always
>>> be the default value of foo_opt, rather than the configured value,
>>> because "CONF.foo_opt" will be evaluated at module load time.  The way
>>> bar_func() should be defined would be:
>>>         def bar_func(arg=None):
>>>             if not arg:
>>>                 arg = CONF.foo_opt
>>> That ensures that arg will be the configured value, and should also
>>> solve the import conflict.
>> That's different behaviour, because you can no longer pass arg=None. The
>> fix isn't to change the behaviour of the code.
> Well, the point is that the code as written is incorrect.  And if 'None'
> is an input you want to allow, then use an incantation like:
>         _unset = object()
>         def bar_func(arg=_unset):
>             if arg is _unset:
>                 arg = CONF.foo_opt
> In any case, the operative point is that CONF.<attribute> must always be
> evaluated inside run-time code, never at module load time.

...unless you call register_opts() safely, which is what I'm proposing.

Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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