RFC: Base.propertynames(x), analogous to fieldnames(typeof(x))

[stevengj]

As discussed in #24960 and #1974, it would be useful to have a propertynames(x) function, analogous to fieldnames(typeof(x)), that returns (if possible!) an array of valid symbols s for passing to getproperty(x,s) or x.s. This is especially useful for REPL tab completion.

Note that, unlike fieldnames, it needs to take an instance rather than a type, since in many cases the property names (e.g. of a dataframe or Python object) will depend on the instance.

Questions:

• Since this is mainly for things like tab-completion, maybe it should be documented as possibly returning only "user-visible" symbols? e.g. for a LinAlg.LU, object, maybe it should return [:L,:U,:p,:P] but not "internal" fields like :ipiv?
  • if so, should the function have a different name, e.g. userpropertynames?
• Should Base.propertynames be exported, like fieldnames?

Answers:

• I decided to expand the api to propertynames(x, private=false): propertynames(x) by default may only return user-visible (documented) properties, but the optional private=true argument can indicate that all valid properties are requested. Tab completion uses private=false.
• Since getproperty is not exported, I decided not to export propertynames

[JeffBezanson]

Seems reasonable to me!