What function (display?) does IJulia call at the end of each cell?

General Usage
1

I have an AbstractArray type with its own display method that gets called correctly in the REPL but in Jupyter/IJulia whatever function is called to display the last argument in each cell is apparently calling a default display type for AbstractArray rather than my object’s specialized method. What display type function is called at the end of each cell?
Here is a MWE where the exact same code displays differently in the REPL and in IJulia:

struct SqrV <: AbstractArray{Int, 1}
	count::Int
end

Base.size(S::SqrV) = (S.count,)
Base.IndexStyle(::Type{<:SqrV}) = IndexLinear()
Base.getindex(S::SqrV, i::Int) = i*i
Base.display(s::SqrV) = println("SV1 count=$(s.count)")
Base.Multimedia.display(d::AbstractDisplay, s::SqrV) = println("SV2 $(s.count)")
Base.Multimedia.display(d::AbstractDisplay, m::MIME, s::SqrV) = println("SV3 $(s.count)")
s = SqrV(4)
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2

IJulia calls display, but with its own AbstractDisplay type, so your method isn’t being called.

The problem is that you shouldn’t be overriding display in the first place. You should override show, as is explained in the manual. For example:

Base.show(io::IO, s::SqrV) = print(io, "SV1 count=$(s.count)")
Base.show(io::IO, m::MIME"text/plain", s::SqrV) = print(io, "SV2 $(s.count)")

Then if you do print(s) it will use the first method, whereas REPL display and IJulia will use the second method, as is explained in the manual.

Note that you want to call print(io, ...) in show, not println(....). First, show output does not typically include a trailing newline (that is added when show is called, e.g. by println). Second, you want to output to the supplied io stream — that way your custom show method can work with streams besides stdout (e.g. output to a file or buffer).

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3

Thanks so much! I hadn’t realized that from the display documentation so I made a 1-line PR to that I hope might save others from the same question:
https://github.com/JuliaLang/julia/pull/34438

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