Help text duplicated when using more than 1 CLI instance

[kmvanbrunt]

import sys
from cmd2 import cmd2

instance_1 = cmd2.Cmd()
instance_2 = cmd2.Cmd()
sys.exit(instance_2.cmdloop())

With this program, run alias -h. The subcommand help gets repeated.

  SUBCOMMAND
    create    create or overwrite an alias
    delete    delete aliases
    list      delete aliases
    create    create or overwrite an alias
    delete    delete aliases
    list      delete aliases

Since the parsers (e.g. Cmd.alias_parser) are class objects, they get updated with each instantiation. This problem has existed since we added the argparse decorators, but it's being exercised a bit more since we added CommandSet and as_subcommand_to decorators. This specific issue of repeated text was introduced when cmd2.py started using the as_subcommand_to decorator.

For the 2.0 release, we need to find a way to make deep copies of the parsers and tie each copy to an instance. We won't have 1 parser attached to the functions anymore. Instead the decorators will need to retrieve the instance-specific parser.

[tleonhardt]

I have a very vague recollection of researching how to make a deep copy of a parser and finding that it was possible in Python 3.7+ (or something similar) but not beforehand.

[tleonhardt]

Yeah, inability to deep-copy an ArgumentParser was fixed in Python 3.7. The context is in older issue #522

[tleonhardt]

@kmvanbrunt #1005 appears to have fixed this, at least temporarily. Are we awaiting a more permanent fix? Or should this issue be closed?

[kmvanbrunt]

@tleonhardt The permanent fix needs to be in 2.0. We need a way to support instance-specific parsers and that will most likely be a breaking change.

[kmvanbrunt]

@anselor Do you have any thoughts on this issue?

[anselor]

I'm sure I do I've just been busy with other more immediate deadlines and haven't had time to come back to this and other cmd2 stuff.

[kmvanbrunt]

@anselor and I agree the 2.0 release does not have to wait for this fix. The risk of someone using two cmd2 objects and editing common parsers between them is pretty low.

[tleonhardt]

@kmvanbrunt Is this fixed now? Or are we waiting on some better fix?

[federicoemartinez]

Could this be avoided if the decorators accepted a argparser factory (a callable that returns an instance of argparser) so the instance is created when needed?

[anselor]

I tried this today to be sure but I was pretty sure this suggestion by itself wouldn't work because the decorator, just like the the ArgumentParser instances, only runs once during the class declaration.

I've been digging around the code a bit and I think may have a solution uses the factory suggestion while also maintaining backwards compatibility. I think what we need to do is, as a command is registered in the commandset (be it during construction or later) we need to at that point either clone the provided ArgumentParser or call the factory method and then store that in the Cmd or CommandSet instance. The innermost cmd_wrapper() method inside of with_argparser() would instead look up the ArgumentParser instance from the provider_self rather than the one passed in the decorator.

@federicoemartinez