About MLJ

MLJ (Machine Learning in Julia) is a toolbox written in Julia providing a common interface and meta-algorithms for selecting, tuning, evaluating, composing and comparing over 180 machine learning models written in Julia and other languages. In particular MLJ wraps a large number of scikit-learn models.

MLJ is released under the MIT license.

Lightning tour

For help learning to use MLJ, see Learning MLJ.

A self-contained notebook and julia script of this demonstration is also available here.

The first code snippet below creates a new Julia environment MLJ_tour and installs just those packages needed for the tour. See Installation for more on creating a Julia environment for use with MLJ.

Julia installation instructions are here.

using Pkg
Pkg.activate("MLJ_tour", shared=true)

In MLJ a model is just a container for hyper-parameters, and that's all. Here we will apply several kinds of model composition before binding the resulting "meta-model" to data in a machine for evaluation using cross-validation.

Loading and instantiating a gradient tree-boosting model:

using MLJ
Booster = @load EvoTreeRegressor # loads code defining a model type
booster = Booster(max_depth=2)   # specify hyper-parameter at construction
booster.nrounds=50               # or mutate afterwards

This model is an example of an iterative model. As it stands, the number of iterations nrounds is fixed.

Composition 1: Wrapping the model to make it "self-iterating"

Let's create a new model that automatically learns the number of iterations, using the NumberSinceBest(3) criterion, as applied to an out-of-sample l1 loss:

using MLJIteration
iterated_booster = IteratedModel(model=booster,
                                 controls=[Step(2), NumberSinceBest(3), NumberLimit(300)],

Composition 2: Preprocess the input features

Combining the model with categorical feature encoding:

pipe = ContinuousEncoder() |> iterated_booster

Composition 3: Wrapping the model to make it "self-tuning"

First, we define a hyper-parameter range for optimization of a (nested) hyper-parameter:

max_depth_range = range(pipe,
                        lower = 1,
                        upper = 10)

Now we can wrap the pipeline model in an optimization strategy to make it "self-tuning":

self_tuning_pipe = TunedModel(model=pipe,
                              ranges = max_depth_range,
                              resampling=CV(nfolds=3, rng=456),

Binding to data and evaluating performance

Loading a selection of features and labels from the Ames House Price dataset:

X, y = @load_reduced_ames;

Evaluating the "self-tuning" pipeline model's performance using 5-fold cross-validation (implies multiple layers of nested resampling):

julia> evaluate(self_tuning_pipe, X, y,
                measures=[l1, l2],
                resampling=CV(nfolds=5, rng=123),
PerformanceEvaluation object with these fields:
  measure, measurement, operation, per_fold,
  per_observation, fitted_params_per_fold,
  report_per_fold, train_test_pairs
│ measure       │ measurement │ operation │ per_fold                                      │
│ LPLoss(p = 1) │ 17200.0     │ predict   │ [16500.0, 17100.0, 16300.0, 17500.0, 18900.0] │
│ LPLoss(p = 2) │ 6.83e8      │ predict   │ [6.14e8, 6.64e8, 5.98e8, 6.37e8, 9.03e8]      │

Key goals

  • Offer a consistent way to use, compose and tune machine learning models in Julia,

  • Promote the improvement of the Julia ML/Stats ecosystem by making it easier to use models from a wide range of packages,

  • Unlock performance gains by exploiting Julia's support for parallelism, automatic differentiation, GPU, optimization etc.

Key features

  • Data agnostic, train most models on any data X supported by the Tables.jl interface (needs Tables.istable(X) == true).

  • Extensive, state-of-the-art, support for model composition (pipelines, stacks and, more generally, learning networks). See more below.

  • Convenient syntax to tune and evaluate (composite) models.

  • Consistent interface to handle probabilistic predictions.

  • Extensible tuning interface, to support a growing number of optimization strategies, and designed to play well with model composition.

  • Options to accelerate model evaluation and tuning with multithreading and/or distributed processing.

Model composability

The generic model composition API's provided by other toolboxes we have surveyed share one or more of the following shortcomings, which do not exist in MLJ:

  • Composite models do not inherit all the behavior of ordinary models.

  • Composition is limited to linear (non-branching) pipelines.

  • Supervised components in a linear pipeline can only occur at the end of the pipeline.

  • Only static (unlearned) target transformations/inverse transformations are supported.

  • Hyper-parameters in homogeneous model ensembles cannot be coupled.

  • Model stacking, with out-of-sample predictions for base learners, cannot be implemented (using the generic API alone).

  • Hyper-parameters and/or learned parameters of component models are not easily inspected or manipulated (by tuning algorithms, for example)

  • Composite models cannot implement multiple operations, for example, both a predict and transform method (as in clustering models) or both a transform and inverse_transform method.

Some of these features are demonstrated in this notebook

For more information see the MLJ design paper or our detailed paper on the composition interface.

Getting help and reporting problems

Users are encouraged to provide feedback on their experience using MLJ and to report issues.

For a query to have maximum exposure to maintainers and users, start a discussion thread at Julia Discourse Machine Learning and tag your issue "mlj". Queries can also be posted as issues, or on the #mlj slack workspace in the Julia Slack channel.

Bugs, suggestions, and feature requests can be posted here.

Users are also welcome to join the #mlj Julia slack channel to ask questions and make suggestions.


Initially, it is recommended that MLJ and associated packages be installed in a new environment to avoid package conflicts. You can do this with

julia> using Pkg; Pkg.activate("my_MLJ_env", shared=true)

Installing MLJ is also done with the package manager:

julia> Pkg.add("MLJ")

Optional: To test your installation, run

julia> Pkg.test("MLJ")

It is important to note that MLJ is essentially a big wrapper providing unified access to model-providing packages. For this reason, one generally needs to add further packages to your environment to make model-specific code available. This happens automatically when you use MLJ's interactive load command @iload, as in

julia> Tree = @iload DecisionTreeClassifier # load type
julia> tree = Tree() # instance

where you will also be asked to choose a providing package, for more than one provide a DecisionTreeClassifier model. For more on identifying the name of an applicable model, see Model Search. For non-interactive loading of code (e.g., from a module or function) see Loading Model Code.

It is recommended that you start with models from more mature packages such as DecisionTree.jl, ScikitLearn.jl or XGBoost.jl.

MLJ is supported by several satellite packages (MLJTuning, MLJModelInterface, etc) which the general user is not required to install directly. Developers can learn more about these here.

See also the alternative installation instructions for Modifying Behavior.


MLJ was initially created as a Tools, Practices and Systems project at the Alan Turing Institute in 2019. Current funding is provided by a New Zealand Strategic Science Investment Fund awarded to the University of Auckland.

Citing MLJ

An overview of MLJ design:


  doi = {10.21105/joss.02704},
  url = {https://doi.org/10.21105/joss.02704},
  year = {2020},
  publisher = {The Open Journal},
  volume = {5},
  number = {55},
  pages = {2704},
  author = {Anthony D. Blaom and Franz Kiraly and Thibaut Lienart and Yiannis Simillides and Diego Arenas and Sebastian J. Vollmer},
  title = {{MLJ}: A Julia package for composable machine learning},
  journal = {Journal of Open Source Software}

An in-depth view of MLJ's model composition design:


      title={Flexible model composition in machine learning and its implementation in {MLJ}},
      author={Anthony D. Blaom and Sebastian J. Vollmer},