Default factory with self argument

[kalekundert]

I find myself somewhat often wanting default values that depend on other attributes of the object, e.g. the default output path is a function of the input path, etc. The way to do this currently is to use the get argument:

appcli.param(
    '--output',
    default=None,
    get=lambda self, x: x or self.path.with_suffix('.out'),
)

However, this is less-than-ideal for two reasons:

• It's not intuitive: the default is just there to makes things work, it's not a true default.
• Get functions aren't cached, and in some cases caching these defaults would be desireable.

Possible solutions:

• Add another kind of default argument, e.g. default_self, that is like default_factory but is passed the object itself as an argument:

  appcli.param(
    '--output',
    default_self=lambda self: self.path.with_suffix('.out'),
  )

  I'm not crazy about the name, though. Some alternatives:

  • default_self
  • default_method: more likely to be a lambda than a true method, but it does kinda convey the idea.

• Inspect default_factory, and pass it the object if it takes a single positional argument. I don't really like this; it just seems to hacky and fragile. I can imagine scenarios with default arguments that would give surprising results.

[kalekundert]

Closing this in favor of #12, which addresses the issue in a more comprehensive way.