Undocumented or inconsistent :auto/Date behavior?

[droodman]

I was surprised by this difference in results:

julia> using Dates, DataFrames

julia> v = [Date(2010,1,1)]
1-element Vector{Date}:
 2010-01-01

julia> DataFrame(v, :auto)
1×1 DataFrame
 Row │ x1
     │ UTInstan…
─────┼─────────────────────────────
   1 │ UTInstant{Day}(Day(733773))

julia> df[1,1] isa Date
false

julia> DataFrame("x" => v)
1×1 DataFrame
 Row │ x
     │ Date
─────┼────────────
   1 │ 2010-01-01

julia> df[1,1] isa Date
true

So :auto does more than automatically generate column names? It slightly affects how Dates and DateTimes are stored? Or is this an unintentional bug?

Using Julia 1.11.3 and DataFrames 1.7.0.

[bkamins]

The issue is unreladed with :auto. First, what should have been done:

julia> DataFrame([v], :auto)
1×1 DataFrame
 Row │ x1
     │ Date
─────┼────────────
   1 │ 2010-01-01

What you do overrides this behaviour:

julia> DataFrame(v)
1×1 DataFrame
 Row │ instant
     │ UTInstan…
─────┼─────────────────────────────
   1 │ UTInstant{Day}(Day(733773))

(by adding extra auto-generation of column names).

This is a surprising behaviour (but documented). The constructor you are invoking is:

 DataFrame(table; copycols::Union{Bool, Nothing}=nothing)
  DataFrame(table, names::AbstractVector; makeunique::Bool=false, copycols::Union{Bool, Nothing}=nothing)

as v (without being wrapped) is a Tables.jl table. Here is another similar example of this constructor in use (hopefully less surprising):

julia> DataFrame((a=[1, 2], b=[3, 4])) # Tables.jl table constructor
  2×2 DataFrame
   Row │ a      b
       │ Int64  Int64
  ─────┼──────────────
     1 │     1      3
     2 │     2      4

  julia> DataFrame([(a=1, b=0), (a=2, b=0)]) # Tables.jl table constructor
  2×2 DataFrame
   Row │ a      b
       │ Int64  Int64
  ─────┼──────────────
     1 │     1      0
     2 │     2      0