ScikitLearn declared inside a module causes segmentation error

[ppalmes]

Tested in Julia 1.0.3 and Julia 1.1 and Julia 0.7

To recreate the problem:
create package A
pkg] generate A
bash> cd A
pkg] activate .
pkg] add ScikitLearn
julia> edit("src/A.jl")
-----
module A
using ScikitLearn
@sk_import linear_model: LogisticRegression

function testme()
model = LogisticRegression()
end

end
---
julia> using A
julia> A.testme() -> causes segmentation error

However, if you use:
julia> include("src/A.jl")
julia> A.testme() -> works

[ppalmes]

The purpose for this thing is to create a wrapper to have common API between caret of RCall and scikitlearn of PyCall

[cstjean]

Thank you for the report. I've hit several segmentation faults myself in Julia 1.1, but this is the first occurrence with ScikitLearn. It might be interesting to reduce it as much as possible and report that to Julialang. That said, it's not surprising that @sk_import doesn't work inside a module, although it really should be documented and warned against. The proper way is documented in PyCall. Use @macroexpand @sk_import ..., and it should be clear.

[ppalmes]

Thanks for the reply. I resolved it by adding:
__precompile__(false)
in the main module. It seems that precompiling is the issue and this also happens with PyCall.

[ppalmes]

I think the bug occurred when they decided to make precompiling as the default which was changed in this PR: JuliaLang/julia#26282

[cstjean]

Yeah. It's that when precompiling, it stores the values of all global variables. @sk_import or @pyimport create a global that holds a pointer to the loaded Python module. Obviously, storing that in the precompiled code file is a very bad idea. When reloading your module, it reads the pointer address, which now points to whatever, thus -> segfault. That's why you need the __init__ trick if you want precompilation.

[Red-Portal]

For anyone who's still wondering here's a snippet that worked for me.
Note that the loading is done by PyCall but the rest is done using the ScikitLearn.jl API.

using ScikitLearn
using PyCall
const mixture = PyNULL()
function __init__()
    copy!(mixture, pyimport("sklearn.mixture"))
end

### Usage
    gmm_config = mixture.BayesianGaussianMixture(n_components=10,
                                                 max_iter=1000,
                                                 weight_concentration_prior=1.0)
    model = fit!(gmm_config, samples[:,:])
    w = model.weights_[:,1,1]
    μ = model.means_[:,1,1]
    σ = sqrt.(model.covariances_[:,1,1])
###