Docs on multi vs. single line display of custom types

[baggepinnen]

After having read https://docs.julialang.org/en/latest/manual/types/#man-custom-pretty-printing-1 I was confused as to why displaying my type p::MyType and a vector of such object [p, p] both appeared to call the same three-arg method of show. It appears that if both three and two-arg methods

Base.show(io::IO, p::MyType)
Base.show(io::IO, ::MIME"text/plain", p::MyType)

are defined, a check is performed to see whether or not

Base.show(io::IO, ::MIME"text/plain", p::MyType)

actually prints on several lines or not, and the

Base.show(io::IO, p::MyType)

is only called if the printed string spans multiple lines. This caused me quite a bit of confusion and I wonder if this is the intended behavior and needs better documentation, or if it's a bug.

Below is an example that demonstrates what's confusing.

julia> struct MyType
           a
       end

julia> function Base.show(io::IO, p::MyType)
           print(io, "2-arg: ", p.a)
       end

julia> function Base.show(io::IO, ::MIME"text/plain", p::MyType) 
           print(io, "3-arg: ", p.a)
       end

julia> p = MyType(1)
3-arg: 1

julia> [p,p] # This still calles 3-arg, which confused me
2-element Array{MyType,1}:
 3-arg: 1
 3-arg: 1

julia> function Base.show(io::IO, ::MIME"text/plain", p::MyType) 
           print(io, "3-arg with newline:\n", p.a)
       end

julia> p = MyType(1)
3-arg with newline:
1

julia> [p,p] # Now the 2-arg method is called
2-element Array{MyType,1}:
 2-arg: 1
 2-arg: 1

[mcabbott]

I was confused by this too, updating some code. This doesn't seem to happen on Julia 1.3 and earlier, i.e. [p,p] prints the 2-arg form on 1.1 - 1.3.

[tkf]

A dup of #36072? #36076 already clarifies this a lot in the docstrings:

https://docs.julialang.org/en/v1.6-dev/base/io-network/#Base.show-Tuple{IO,Any,Any}
https://docs.julialang.org/en/v1.6-dev/base/io-network/#Base.show-Tuple{IO,Any}

[thofma]

A dup of #36072? #36076 already clarifies this a lot in the docstrings:

https://docs.julialang.org/en/v1.6-dev/base/io-network/#Base.show-Tuple{IO,Any,Any} https://docs.julialang.org/en/v1.6-dev/base/io-network/#Base.show-Tuple{IO,Any}

I could not find where it is documented, that the 2-arg version is used if the 3-arg version prints over multiple lines. Which docstring contains this?

[exaexa]

Adding myself to the cc: list of confused people here.

EDIT: #37072 and #36076 didn't help.