ion-resource: add meaningful error message when subcommand is missing

[vsoch]

Problem: a subcommand "func" is always required for ion resource, yet the current client spits out an error if the user types the command without help, subcommand, or other args.
Solution: if the func (subcommand) is missing, print the parser help and provide a meaningful message to the user.

Details

I am starting to test out flux ion-resource (to learn about interaction with fluxion and the queue manager) and very naively I built and ran the command and saw this:

$ flux ion-resource

Traceback (most recent call last):
  File "/usr/libexec/flux/cmd/py-runner.py", line 94, in <module>
    runpy.run_path(sys.argv[0], run_name="__main__")
  File "<frozen runpy>", line 291, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/libexec/flux/cmd/flux-ion-resource.py", line 673, in <module>
    main()
  File "/usr/libexec/flux/cmd/flux-ion-resource.py", line 653, in main
    args.func(args)
    ^^^^^^^^^
AttributeError: 'Namespace' object has no attribute 'func'

It's pretty common for the user to try out a command by typing it first, with or without help, and expecting it to dump out usage. So that's what I've updated it to do here. The "func" argument that is missing is a subcommand, so we can check if the attribute is defined on the parser, and if not, print the parser help, and a clear message to the user that a subcommand is required. Here is the new interaction:

$ flux ion-resource

usage: flux-ion-resource.py [-h] [-v]
                            {match,update,info,stats,stats-cancel,cancel,partial-cancel,find,status,set-status,set-property,get-property,ns-info,params}
                            ...

Front-end command for sched-fluxion-resource module for testing. Provide 4 sub-commands. For
sub-command usage, flux-ion-resource.py <sub-command> --help

options:
  -h, --help            show this help message and exit
  -v, --verbose         be verbose

Available Commands:
  Valid commands

  {match,update,info,stats,stats-cancel,cancel,partial-cancel,find,status,set-status,set-property,get-property,ns-info,params}
                        Additional help
    match               Find the best matching resources for a jobspec.
    update              Update the resource database.
    info                Print info on a single job.
    stats               Print overall performance statistics.
    stats-cancel        Clear overall performance statistics.
    cancel              Cancel an allocated or reserved job.
    partial-cancel      Partially cancel an allocated job.
    find                Find resources matching with a criteria.
    status              Display resource status.
    set-status          Set up/down status of a resource vertex.
    set-property        Set property-key=value for specified resource.
    get-property        Get value for specified resource and property-key.
    ns-info             Get remapped ID given raw ID seen by the reader.
    params              Display the module's parameter values.

A subcommand is required.

That should be a fairly straight forward update if the fix is desired. Note that I have not tested nothing else yet - I am going to bed so likely won't until tomorrow. Looking forward to trying it out!

[jameshcorbett]

Great catch! LGTM. Argparse has a way to support this with the required=True argument to add_subparsers but it isn't added until 3.7 :( https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_subparsers otherwise we could have done

diff --git a/src/cmd/flux-ion-resource.py b/src/cmd/flux-ion-resource.py
index 8fa71ef1..1a1a7e22 100755
--- a/src/cmd/flux-ion-resource.py
+++ b/src/cmd/flux-ion-resource.py
@@ -411,7 +411,10 @@ def parse_match(parser_m: argparse.ArgumentParser):
     """
 
     subparsers_m = parser_m.add_subparsers(
-        title="Available Commands", description="Valid commands", help="Additional help"
+        title="Available Commands",
+        description="Valid commands",
+        help="Additional help",
+        required=True,
     )

[vsoch]

Thanks @jameshcorbett ! I think I almost like this way better, regardless of version, because it allows us to give the message to the user. If we just spit out the usage it could only be inferred something is wrong, but not specifically what.

[vsoch]

@jameshcorbett what does an ion resource refer to? I keep thinking of iron man. Iron man is pretty cool so I can't say this is a bad thing, but I'm wondering what an "ion resource" actually is. Is it a reference to ion and if so, what is the relation to our fluxion graphs?

[jameshcorbett]

Thanks @jameshcorbett ! I think I almost like this way better, regardless of version, because it allows us to give the message to the user. If we just spit out the usage it could only be inferred something is wrong, but not specifically what.

That's fair, although I think the way argparse handles it when required=True is similar to the way other flux tools handle required subcommands (and git too I think). But I'm happy either way, and the current way it goes (with an ugly exception) is definitely bad.

@jameshcorbett what does an ion resource refer to? I keep thinking of iron man. Iron man is pretty cool so I can't say this is a bad thing, but I'm wondering what an "ion resource" actually is. Is it a reference to ion and if so, what is the relation to our fluxion graphs?

AFAIK all fluxion commands are prefixed with ion- so that the beginning of the command reads "fluxion". It doesn't refer to anything. I think we could have made a fluxion command so that we could execute things like $ fluxion resource but I guess we decided to make flux the entry point to everything. Also it helps to avoid name collisions, flux ion-resource is very different in its behavior and intended usage from flux resource.

[vsoch]

AFAIK all fluxion commands are prefixed with ion- so that the beginning of the command reads "fluxion"

Ahh of course! And I totally missed that. I think (when I read verbatim) I miss it and wonder what a flux ion is. But that's probably just me - it took me over a year to figure out that flux-sched was the same thing as fluxion.

Thank you for the review!

[jameshcorbett]

You should feel free to set MWP unless you want another review!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.4%. Comparing base (bc9e3e3) to head (06abf95).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1243   +/-   ##
======================================
  Coverage    74.4%   74.4%           
======================================
  Files         104     104           
  Lines       14936   14936           
======================================
  Hits        11118   11118           
  Misses       3818    3818