oslo.config print_help() and print_usage() crash or are not "useful"

[Andrew Forrest]

Adapting the "common usage pattern in OpenStack" cited in [1]:

    from oslo.config import cfg

    opts = [
        cfg.StrOpt('bind_host', default='0.0.0.0'),
        cfg.IntOpt('bind_port', default=9292),
    ]

    CONF = cfg.CONF
    CONF.register_opts(opts)

Attempts to call "CONF.print_usage()" or "CONF.print_help()" after this point result in a traceback owing to the fact that the private member _oparser of the CONF object (class ConfigOpts()) is not yet set:

    Traceback (most recent call last):
      File "demo.py", line 19, in <module>
        CONF.print_help()
      File "[...]/python2.7/site-packages/oslo/config/cfg.py", line 1603, in print_help
        self._oparser.print_help(file)
    AttributeError: 'NoneType' object has no attribute 'print_help'

This member is set via three public methods:
    + ConfigOpts.reset() or ConfigOpts.clear() [reset calls clear]
    + calling the ConfigOpts object itself

In the first bullet immediately above there is not much beyond the default -h to report in the help or usage message afterwards. In the second case, a call to the object itself triggers the processing of the command line options which will then respond to the necessity of a help or usage message itself precluding the need (or even the opportunity) for the main program to call these functions.

---
[1] http://docs.openstack.org/developer/oslo.config/api/oslo.config.cfg.html#module-oslo.config.cfg

[Mark McLoughlin (markmc)]

Andrew - could you go into some more detail about why exactly you want to print help/usage before parsing the command line?

[Andrew Forrest (forrest-r)]

It's not that I want to do those things before parsing the command line.

I was converting a program with its own help and usage message routines to use oslo.config and I noticed these facilities were available and I tried to use them but, as you can see, that isn't possible: either the program crashes because _oparser is not yet set or it exits under the control of ConfigOpts.__call__() after producing its own help or usage message (as appropriate) precluding a future call to print_help() or print_usage().

If these methods were removed future oslo.config users would not have to rediscover this issue.

Does that help?

[Mark McLoughlin (markmc)]

Ok, it sounds like your concern is mostly that this isn't obvious. We've added explicit error handling to make it more clear: https://review.openstack.org/#/c/35326/

[Andrew Forrest (forrest-r)]

I appreciate you trying to understand my concern but that's not it.

Apparently I don't understand the use-case for these functions. Would you please indulge me once more? Disregard the AttributeError case for the moment, when (after a call to the ConfigOpts object) would it ever be useful to call either of these methods?

[Mark McLoughlin (markmc)]

You'd use it after calling CONF() to parse the arguments - if, for example, the user supplied incompatible values

[Mark McLoughlin (markmc)]

Basically, we don't initialize the argument parser at until CONF() is called because not all CLI options may have been registered before then

[Andrew Forrest (forrest-r)]

Re #5: okay, I can see that now.

Sorry for suggesting that these functions might not be useful.