`DefaultDict` and `haskey`

[goretkin]

I didn't find a conversation around whether haskey should always return true for DefaultDict.

julia> using DataStructures: DefaultDict

julia> d1 = DefaultDict(0, :A=>1, :B=>2)
DefaultDict{Symbol,Int64,Int64} with 2 entries:
  :A => 1
  :B => 2

julia> haskey(d1, :C)
false

julia> d1[:C]
0

This would make it inconsistent with keys, but consistent with getindex.

[mbauman]

I've found this a little unexpected myself, along with get's behavior. I actually started writing an algorithm that has a heterogeneous vector contains both Dicts and DefaultDicts, and wanted to use get generically.

The use-case is a little complicated, but involves iterating over a collection of dictionaries, wherein some dictionaries are known to be complete keysets whereas others are known to be incomplete.

The complete keysets can be regular Dict, accessed via get(d::Dict, key, nothing)

I wanted the known-to-be-incompelete keysets to be DefaultDict(()->missing). But unfortunately get(d::DefaultDict, key, nothing) simply gives me nothing, even though d[key] is missing!

[mbauman]

The convert behaviors here make it a little tricky to define a DefaultDict-aware haskey method that acts like all possible keys are available. I think it'd be:

haskey(::DefaultDict{K}, key) where {K} = key isa K || try convert(K, key); true; catch _ false; end

and similarly:

get(d::DefaultDict{K}, key::K, _) where {K} = d[key]
get(d::DefaultDict, key, default) = try d[key]; catch _ default; end