Issue 1:
When trying to run the test-suite on a fresh installation it breaks.

Long story short, doctests require PIL and test_lda_predict fails as well.

This has been done in a Linux machine. @martinagvilas can you replicate in mac? thx.

cc: @lesteve

Here is what we did:

$ conda remove --name test-sklearn --all
$ conda create --name test-sklearn -y
$ conda activate test-sklearn
$ conda install -c conda-forge scikit-learn cython pytest
$ cd ~
$ python -c "import sklearn; print(sklearn.__version__)" # 0.21.3
$ conda remove --name test-sklearn scikit-learn --force-remove # keep dependencies
$ conda list | grep sklearn
$ cd ~/code/scikit-learn
$ pip install -e .
$ pytest sklearn

Here is the full pytest output

========================= test session starts ==========================
platform linux -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: /home/sik/code/scikit-learn, inifile: setup.cfg
collected 13419 items / 2 skipped / 13417 selected

sklearn/_config.py . [ 0%]
sklearn/discriminant_analysis.py .. [ 0%]
sklearn/exceptions.py .. [ 0%]
sklearn/isotonic.py . [ 0%]
sklearn/kernel_approximation.py .... [ 0%]
sklearn/kernel_ridge.py . [ 0%]
sklearn/naive_bayes.py .... [ 0%]
sklearn/pipeline.py .... [ 0%]
sklearn/random_projection.py ... [ 0%]
sklearn/cluster/affinity_propagation_.py . [ 0%]
sklearn/cluster/bicluster.py .. [ 0%]
sklearn/cluster/birch.py . [ 0%]
sklearn/cluster/dbscan_.py . [ 0%]
sklearn/cluster/hierarchical.py .. [ 0%]
sklearn/cluster/k_means_.py .. [ 0%]
sklearn/cluster/mean_shift_.py . [ 0%]
sklearn/cluster/spectral.py . [ 0%]
sklearn/cluster/tests/test_affinity_propagation.py ......... [ 0%]
sklearn/cluster/tests/test_bicluster.py ........s......... [ 0%]
sklearn/cluster/tests/test_birch.py ....... [ 0%]
sklearn/cluster/tests/test_dbscan.py ........................... [ 0%]
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sklearn/cluster/tests/test_feature_agglomeration.py . [ 0%]
sklearn/cluster/tests/test_hierarchical.py ..................... [ 0%]
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sklearn/cluster/tests/test_k_means.py .......................... [ 1%]
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sklearn/cluster/tests/test_mean_shift.py ............ [ 2%]
sklearn/cluster/tests/test_optics.py ........................... [ 2%]
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sklearn/cluster/tests/test_spectral.py .............. [ 2%]
sklearn/compose/_column_transformer.py .. [ 2%]
sklearn/compose/_target.py . [ 2%]
sklearn/compose/tests/test_column_transformer.py .ss.s........s. [ 2%]
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sklearn/compose/tests/test_target.py ............. [ 3%]
sklearn/covariance/elliptic_envelope.py . [ 3%]
sklearn/covariance/empirical_covariance_.py . [ 3%]
sklearn/covariance/graph_lasso_.py .. [ 3%]
sklearn/covariance/robust_covariance.py . [ 3%]
sklearn/covariance/shrunk_covariance_.py .. [ 3%]
sklearn/covariance/tests/test_covariance.py ...... [ 3%]
sklearn/covariance/tests/test_elliptic_envelope.py .. [ 3%]
sklearn/covariance/tests/test_graphical_lasso.py ..... [ 3%]
sklearn/covariance/tests/test_robust_covariance.py ....... [ 3%]
sklearn/cross_decomposition/cca_.py . [ 3%]
sklearn/cross_decomposition/pls_.py ... [ 3%]
sklearn/cross_decomposition/tests/test_pls.py ........ [ 3%]
sklearn/datasets/base.py ....... [ 3%]
sklearn/datasets/samples_generator.py . [ 3%]
sklearn/datasets/tests/test_20news.py sss [ 3%]
sklearn/datasets/tests/test_base.py .................... [ 3%]
sklearn/datasets/tests/test_california_housing.py . [ 3%]
sklearn/datasets/tests/test_covtype.py s [ 3%]
sklearn/datasets/tests/test_kddcup99.py ss [ 3%]
sklearn/datasets/tests/test_lfw.py sssss [ 3%]
sklearn/datasets/tests/test_olivetti_faces.py s [ 3%]
sklearn/datasets/tests/test_openml.py ...........sssss.ssssss... [ 3%]
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sklearn/datasets/tests/test_rcv1.py s [ 4%]
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sklearn/datasets/tests/test_svmlight_format.py ...........s..... [ 4%]
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sklearn/decomposition/base.py . [ 4%]
sklearn/decomposition/factor_analysis.py . [ 4%]
sklearn/decomposition/fastica_.py . [ 4%]
sklearn/decomposition/incremental_pca.py .. [ 4%]
sklearn/decomposition/kernel_pca.py . [ 4%]
sklearn/decomposition/nmf.py .. [ 4%]
sklearn/decomposition/online_lda.py . [ 4%]
sklearn/decomposition/pca.py . [ 4%]
sklearn/decomposition/sparse_pca.py .. [ 4%]
sklearn/decomposition/truncated_svd.py . [ 4%]
sklearn/decomposition/tests/test_dict_learning.py .............. [ 5%]
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sklearn/decomposition/tests/test_factor_analysis.py . [ 5%]
sklearn/decomposition/tests/test_fastica.py ......... [ 5%]
sklearn/decomposition/tests/test_incremental_pca.py ............ [ 5%]
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sklearn/decomposition/tests/test_kernel_pca.py .............. [ 5%]
sklearn/decomposition/tests/test_nmf.py ........................ [ 5%]
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sklearn/decomposition/tests/test_pca.py ........................ [ 6%]
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sklearn/decomposition/tests/test_sparse_pca.py .......s...... [ 7%]
sklearn/decomposition/tests/test_truncated_svd.py .............. [ 7%]
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sklearn/ensemble/forest.py .. [ 7%]
sklearn/ensemble/partial_dependence.py .. [ 7%]
sklearn/ensemble/voting.py .. [ 7%]
sklearn/ensemble/weight_boosting.py .. [ 7%]
sklearn/ensemble/_hist_gradient_boosting/gradient_boosting.py .. [ 7%]
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sklearn/ensemble/_hist_gradient_boosting/tests/test_binning.py . [ 7%]
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sklearn/ensemble/_hist_gradient_boosting/tests/test_loss.py .... [ 8%]
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sklearn/ensemble/_hist_gradient_boosting/tests/test_predictor.py . [ 8%]
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sklearn/ensemble/_hist_gradient_boosting/tests/test_splitting.py . [ 8%]
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sklearn/ensemble/_hist_gradient_boosting/tests/test_warm_start.py . [ 8%]
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sklearn/ensemble/tests/test_bagging.py ......................... [ 9%]
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sklearn/ensemble/tests/test_base.py .... [ 9%]
sklearn/ensemble/tests/test_forest.py .......................... [ 9%]
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sklearn/ensemble/tests/test_gradient_boosting.py ............... [ 10%]
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sklearn/ensemble/tests/test_iforest.py .................... [ 11%]
sklearn/ensemble/tests/test_partial_dependence.py .....sssss [ 11%]
sklearn/ensemble/tests/test_voting.py ......................... [ 11%]
sklearn/ensemble/tests/test_weight_boosting.py ................. [ 12%]
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sklearn/experimental/enable_hist_gradient_boosting.py . [ 12%]
sklearn/experimental/enable_iterative_imputer.py . [ 12%]
sklearn/experimental/tests/test_enable_hist_gradient_boosting.py . [ 12%]
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sklearn/experimental/tests/test_enable_iterative_imputer.py . [ 12%]
sklearn/feature_extraction/dict_vectorizer.py .. [ 12%]
sklearn/feature_extraction/hashing.py . [ 12%]
sklearn/feature_extraction/image.py FF [ 12%]
sklearn/feature_extraction/text.py ... [ 12%]
sklearn/feature_extraction/tests/test_dict_vectorizer.py ....... [ 12%]
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sklearn/feature_extraction/tests/test_feature_hasher.py ........ [ 12%]
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sklearn/feature_extraction/tests/test_image.py ................. [ 12%]
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sklearn/feature_extraction/tests/test_text.py .................. [ 12%]
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sklearn/feature_selection/rfe.py .. [ 13%]
sklearn/feature_selection/univariate_selection.py ...... [ 13%]
sklearn/feature_selection/variance_threshold.py . [ 13%]
sklearn/feature_selection/tests/test_base.py ..... [ 13%]
sklearn/feature_selection/tests/test_chi2.py ..... [ 13%]
sklearn/feature_selection/tests/test_feature_select.py ......... [ 13%]
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sklearn/feature_selection/tests/test_from_model.py ............. [ 13%]
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sklearn/feature_selection/tests/test_mutual_info.py ........ [ 14%]
sklearn/feature_selection/tests/test_rfe.py ............ [ 14%]
sklearn/feature_selection/tests/test_variance_threshold.py ... [ 14%]
sklearn/gaussian_process/gpc.py . [ 14%]
sklearn/gaussian_process/gpr.py . [ 14%]
sklearn/gaussian_process/tests/test_gpc.py ..................... [ 14%]
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sklearn/gaussian_process/tests/test_gpr.py ..................... [ 14%]
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sklearn/impute/_knn.py . [ 17%]
sklearn/impute/tests/test_impute.py ............................ [ 17%]
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sklearn/impute/tests/test_knn.py ............................... [ 18%]
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sklearn/inspection/tests/test_partial_dependence.py ............ [ 18%]
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sklearn/inspection/tests/test_permutation_importance.py ..ss.s. [ 19%]
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sklearn/linear_model/bayes.py .. [ 19%]
sklearn/linear_model/coordinate_descent.py ......... [ 19%]
sklearn/linear_model/huber.py . [ 19%]
sklearn/linear_model/least_angle.py ..... [ 19%]
sklearn/linear_model/logistic.py .. [ 19%]
sklearn/linear_model/omp.py .. [ 19%]
sklearn/linear_model/passive_aggressive.py .. [ 19%]
sklearn/linear_model/perceptron.py . [ 19%]
sklearn/linear_model/ransac.py . [ 19%]
sklearn/linear_model/ridge.py .... [ 19%]
sklearn/linear_model/sag.py . [ 19%]
sklearn/linear_model/stochastic_gradient.py .. [ 19%]
sklearn/linear_model/theil_sen.py . [ 19%]
sklearn/linear_model/tests/test_base.py ....................... [ 20%]
sklearn/linear_model/tests/test_bayes.py ............. [ 20%]
sklearn/linear_model/tests/test_coordinate_descent.py .......... [ 20%]
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sklearn/linear_model/tests/test_huber.py .......... [ 20%]
sklearn/linear_model/tests/test_least_angle.py ................. [ 20%]
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sklearn/linear_model/tests/test_omp.py .................. [ 22%]
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sklearn/linear_model/tests/test_ransac.py ...................... [ 22%]
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sklearn/linear_model/tests/test_sparse_coordinate_descent.py ... [ 26%]
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sklearn/linear_model/tests/test_theil_sen.py ................. [ 26%]
sklearn/manifold/isomap.py . [ 26%]
sklearn/manifold/locally_linear.py . [ 26%]
sklearn/manifold/mds.py . [ 26%]
sklearn/manifold/spectral_embedding_.py . [ 26%]
sklearn/manifold/t_sne.py . [ 26%]
sklearn/manifold/tests/test_isomap.py ...... [ 26%]
sklearn/manifold/tests/test_locally_linear.py ....... [ 26%]
sklearn/manifold/tests/test_mds.py ... [ 26%]
sklearn/manifold/tests/test_spectral_embedding.py ....ss....... [ 27%]
sklearn/manifold/tests/test_t_sne.py ........................... [ 27%]
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sklearn/metrics/classification.py ................. [ 27%]
sklearn/metrics/pairwise.py ...... [ 27%]
sklearn/metrics/ranking.py ........ [ 27%]
sklearn/metrics/regression.py .......... [ 27%]
sklearn/metrics/scorer.py . [ 27%]
sklearn/metrics/_plot/tests/test_plot_roc_curve.py sssssssssssss [ 27%]
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sklearn/metrics/cluster/supervised.py ....... [ 27%]
sklearn/metrics/cluster/tests/test_bicluster.py ... [ 27%]
sklearn/metrics/cluster/tests/test_common.py ................... [ 28%]
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sklearn/metrics/cluster/tests/test_supervised.py ............... [ 28%]
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sklearn/metrics/cluster/tests/test_unsupervised.py ........... [ 28%]
sklearn/metrics/tests/test_classification.py ................... [ 28%]
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sklearn/metrics/tests/test_common.py ........................... [ 29%]
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sklearn/metrics/tests/test_pairwise.py ......................... [ 33%]
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sklearn/metrics/tests/test_ranking.py .......................... [ 35%]
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sklearn/metrics/tests/test_score_objects.py .................... [ 36%]
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sklearn/mixture/tests/test_bayesian_mixture.py ................. [ 36%]
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sklearn/mixture/tests/test_gaussian_mixture.py ................. [ 36%]
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sklearn/mixture/tests/test_mixture.py .. [ 36%]
sklearn/model_selection/_search.py .... [ 36%]
sklearn/model_selection/_split.py .............. [ 36%]
sklearn/model_selection/_validation.py s... [ 36%]
sklearn/model_selection/tests/test_search.py ................... [ 37%]
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sklearn/model_selection/tests/test_split.py .................... [ 37%]
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sklearn/model_selection/tests/test_validation.py ............... [ 38%]
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sklearn/neighbors/base.py .... [ 38%]
sklearn/neighbors/classification.py .. [ 38%]
sklearn/neighbors/graph.py .. [ 38%]
sklearn/neighbors/nca.py . [ 38%]
sklearn/neighbors/nearest_centroid.py . [ 38%]
sklearn/neighbors/regression.py .. [ 38%]
sklearn/neighbors/unsupervised.py . [ 38%]
sklearn/neighbors/tests/test_ball_tree.py ...................... [ 38%]
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sklearn/neighbors/tests/test_kde.py ............................ [ 41%]
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sklearn/neighbors/tests/test_lof.py ............ [ 41%]
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sklearn/neighbors/tests/test_nearest_centroid.py ......... [ 43%]
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sklearn/preprocessing/_encoders.py .. [ 44%]
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sklearn/preprocessing/tests/test_common.py ......... [ 44%]
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sklearn/utils/tests/test_deprecation.py ... [ 96%]
sklearn/utils/tests/test_estimator_checks.py .......... [ 96%]
sklearn/utils/tests/test_extmath.py ........................... [ 96%]
sklearn/utils/tests/test_fast_dict.py .. [ 96%]
sklearn/utils/tests/test_fixes.py ...... [ 96%]
sklearn/utils/tests/test_linear_assignment.py . [ 96%]
sklearn/utils/tests/test_metaestimators.py .. [ 96%]
sklearn/utils/tests/test_multiclass.py ......s... [ 96%]
sklearn/utils/tests/test_murmurhash.py ...... [ 96%]
sklearn/utils/tests/test_optimize.py . [ 96%]
sklearn/utils/tests/test_pprint.py ......... [ 96%]
sklearn/utils/tests/test_random.py ..... [ 96%]
sklearn/utils/tests/test_seq_dataset.py ......... [ 97%]
sklearn/utils/tests/test_shortest_path.py .... [ 97%]
sklearn/utils/tests/test_show_versions.py ... [ 97%]
sklearn/utils/tests/test_sparsefuncs.py ........................ [ 97%]
......................................... [ 97%]
sklearn/utils/tests/test_testing.py ...........s.......... [ 97%]
sklearn/utils/tests/test_utils.py .............................. [ 97%]
.s...sssss...s..s..ssss..s..s..ssss..s..s..ssss..s........ssssss [ 98%]
ssssssssss........ssssssssssssssss..s..ssss..s..ssss..s..ssss... [ 98%]
s..s..s..s...s...s.ss........................................... [ 99%]
... [ 99%]
sklearn/utils/tests/test_validation.py ......................... [ 99%]
....................s.ss........................... [100%]

=============================== FAILURES ===============================
______ [doctest] sklearn.feature_extraction.image.PatchExtractor _______
459 deterministic.
460 See :term:`Glossary <random_state>`.
461
462
463 Examples
464 --------
465 >>> from sklearn.datasets import load_sample_images
466 >>> from sklearn.feature_extraction import image
467 >>> # Use the array data from the second image in this dataset:
468 >>> X = load_sample_images().images[1]
UNEXPECTED EXCEPTION: ImportError('The Python Imaging Library (PIL) is required to load data from jpeg files. Please refer to https://pillow.readthedocs.io/en/stable/installation.html for installing PIL.')
Traceback (most recent call last):

File "/home/sik/miniconda3/envs/test-sklearn/lib/python3.7/doctest.py", line 1329, in __run
compileflags, 1), test.globs)

File "<doctest sklearn.feature_extraction.image.PatchExtractor[2]>", line 1, in <module>

File "/home/sik/code/scikit-learn/sklearn/datasets/base.py", line 801, in load_sample_images
images = [imread(filename) for filename in filenames]

File "/home/sik/code/scikit-learn/sklearn/datasets/base.py", line 801, in <listcomp>
images = [imread(filename) for filename in filenames]

File "/home/sik/code/scikit-learn/sklearn/externals/_pilutil.py", line 204, in imread
raise ImportError(PILLOW_ERROR_MESSAGE)

ImportError: The Python Imaging Library (PIL) is required to load data from jpeg files. Please refer to https://pillow.readthedocs.io/en/stable/installation.html for installing PIL.

/home/sik/code/scikit-learn/sklearn/feature_extraction/image.py:468: UnexpectedException
____ [doctest] sklearn.feature_extraction.image.extract_patches_2d _____
334 The collection of patches extracted from the image, where `n_patches`
335 is either `max_patches` or the total number of patches that can be
336 extracted.
337
338 Examples
339 --------
340 >>> from sklearn.datasets import load_sample_image
341 >>> from sklearn.feature_extraction import image
342 >>> # Use the array data from the first image in this dataset:
343 >>> one_image = load_sample_image("china.jpg")
UNEXPECTED EXCEPTION: ImportError('The Python Imaging Library (PIL) is required to load data from jpeg files. Please refer to https://pillow.readthedocs.io/en/stable/installation.html for installing PIL.')
Traceback (most recent call last):

File "/home/sik/miniconda3/envs/test-sklearn/lib/python3.7/doctest.py", line 1329, in __run
compileflags, 1), test.globs)

File "<doctest sklearn.feature_extraction.image.extract_patches_2d[2]>", line 1, in <module>

File "/home/sik/code/scikit-learn/sklearn/datasets/base.py", line 838, in load_sample_image
images = load_sample_images()

File "/home/sik/code/scikit-learn/sklearn/datasets/base.py", line 801, in load_sample_images
images = [imread(filename) for filename in filenames]

File "/home/sik/code/scikit-learn/sklearn/datasets/base.py", line 801, in <listcomp>
images = [imread(filename) for filename in filenames]

File "/home/sik/code/scikit-learn/sklearn/externals/_pilutil.py", line 204, in imread
raise ImportError(PILLOW_ERROR_MESSAGE)

ImportError: The Python Imaging Library (PIL) is required to load data from jpeg files. Please refer to https://pillow.readthedocs.io/en/stable/installation.html for installing PIL.

/home/sik/code/scikit-learn/sklearn/feature_extraction/image.py:343: UnexpectedException
___________________________ test_lda_predict ___________________________

def test_lda_predict():
# Test LDA classification.
# This checks that LDA implements fit and predict and returns correct
# values for simple toy data.
for test_case in solver_shrinkage:
solver, shrinkage = test_case
clf = LinearDiscriminantAnalysis(solver=solver, shrinkage=shrinkage)
y_pred = clf.fit(X, y).predict(X)
assert_array_equal(y_pred, y, 'solver %s' % solver)

# Assert that it works with 1D data
y_pred1 = clf.fit(X1, y).predict(X1)
assert_array_equal(y_pred1, y, 'solver %s' % solver)

# Test probability estimates
y_proba_pred1 = clf.predict_proba(X1)
assert_array_equal((y_proba_pred1[:, 1] > 0.5) + 1, y,
'solver %s' % solver)
y_log_proba_pred1 = clf.predict_log_proba(X1)
assert_array_almost_equal(np.exp(y_log_proba_pred1), y_proba_pred1,
> 8, 'solver %s' % solver)
E AssertionError:
E Arrays are not almost equal to 8 decimals
E solver svd
E Mismatch: 16.7%
E Max absolute difference: 5.9604645e-08
E Max relative difference: 4.7683716e-07
E x: array([[9.9999994e-01, 1.1253517e-07],
E [9.9966466e-01, 3.3535002e-04],
E [9.9966466e-01, 3.3535002e-04],...
E y: array([[9.99999881e-01, 1.12535155e-07],
E [9.99664664e-01, 3.35350138e-04],
E [9.99664664e-01, 3.35350138e-04],...

sklearn/tests/test_discriminant_analysis.py:80: AssertionError
======================= short test summary info ========================
SKIPPED [3] /home/sik/code/scikit-learn/sklearn/ensemble/_hist_gradient_boosting/tests/test_compare_lightgbm.py:17: could not import 'lightgbm': No module named 'lightgbm'
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: This test is failing on the buildbot, but cannot reproduce. Temporarily disabling it until it can be reproduced and fixed.
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:132: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:263: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:467: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:538: could not import 'pandas': No module named 'pandas'
SKIPPED [9] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:812: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:1054: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:1091: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/compose/tests/test_column_transformer.py:1125: could not import 'pandas': No module named 'pandas'
SKIPPED [3] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Download 20 newsgroups to run this test
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Covertype dataset can not be loaded.
SKIPPED [2] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: kddcup99 dataset can not be loaded.
SKIPPED [5] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: PIL not installed.
SKIPPED [1] sklearn/datasets/tests/test_olivetti_faces.py:20: Download Olivetti faces dataset to run this test
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:289: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:329: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:348: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:389: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:426: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:478: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:490: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:518: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:557: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:595: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/datasets/tests/test_openml.py:633: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Download RCV1 dataset to run this test.
SKIPPED [1] sklearn/datasets/tests/test_svmlight_format.py:188: testing the overflow of 32 bit sparse indexing requires a large amount of memory
SKIPPED [1] sklearn/decomposition/tests/test_sparse_pca.py:134: skipping mini_batch_fit_transform.
SKIPPED [34] /home/sik/code/scikit-learn/sklearn/conftest.py:18: could not import 'matplotlib': No module named 'matplotlib'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/impute/tests/test_impute.py:315: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/impute/tests/test_impute.py:428: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/inspection/tests/test_permutation_importance.py:52: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/inspection/tests/test_permutation_importance.py:114: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/linear_model/tests/test_logistic.py:1306: liblinear does not support multinomial logistic
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/manifold/tests/test_spectral_embedding.py:166: could not import 'pyamg': No module named 'pyamg'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/manifold/tests/test_spectral_embedding.py:196: could not import 'pyamg': No module named 'pyamg'
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/doctest.py:351: all tests skipped by +SKIP option
SKIPPED [4] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_data.py:866: 'with_mean=True' cannot be used with sparse matrix.
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_data.py:1077: RobustScaler cannot center sparse matrix
SKIPPED [3] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_encoders.py:127: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_encoders.py:283: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_encoders.py:378: could not import 'pandas': No module named 'pandas'
SKIPPED [4] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_encoders.py:450: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_encoders.py:584: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/preprocessing/tests/test_function_transformer.py:157: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator ColumnTransformer which requires parameters ['transformers']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator FeatureUnion which requires parameters ['transformer_list']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator GridSearchCV which requires parameters ['estimator', 'param_grid']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator Pipeline which requires parameters ['steps']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator RandomizedSearchCV which requires parameters ['estimator', 'param_distributions']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator SparseCoder which requires parameters ['dictionary']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator VotingClassifier which requires parameters ['estimators']
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Can't instantiate estimator VotingRegressor which requires parameters ['estimators']
SKIPPED [50] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: pandas is not installed: not testing for input of type pandas.Series to class weight.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: score_samples of BernoulliRBM is not invariant when applied to a subset.
SKIPPED [3] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Skipping check_estimators_data_not_an_array for cross decomposition module as estimators are not deterministic.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: predict of DummyClassifier is not invariant when applied to a subset.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: transform of MiniBatchSparsePCA is not invariant when applied to a subset.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Not testing NuSVC class weight as it is ignored.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: decision_function of NuSVC is not invariant when applied to a subset.
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: transform of SparsePCA is not invariant when applied to a subset.
SKIPPED [2] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: numpydoc is required to test the docstrings
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/utils/tests/test_multiclass.py:299: could not import 'pandas': No module named 'pandas'
SKIPPED [37] /home/sik/code/scikit-learn/sklearn/utils/tests/test_utils.py:239: could not import 'pandas': No module named 'pandas'
SKIPPED [39] /home/sik/code/scikit-learn/sklearn/utils/tests/test_utils.py:242: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/utils/tests/test_utils.py:423: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/utils/tests/test_utils.py:444: could not import 'pandas': No module named 'pandas'
SKIPPED [2] /home/sik/code/scikit-learn/sklearn/utils/tests/test_utils.py:466: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/miniconda3/envs/test-sklearn/lib/python3.7/site-packages/_pytest/unittest.py:238: Pandas not found
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/utils/tests/test_validation.py:719: could not import 'pandas': No module named 'pandas'
SKIPPED [1] /home/sik/code/scikit-learn/sklearn/utils/tests/test_validation.py:732: could not import 'pandas': No module named 'pandas'
= 3 failed, 13147 passed, 265 skipped, 6 xfailed, 1572 warnings in 409.77s (0:06:49) =
This problem is unconstrained.
RUNNING THE L-BFGS-B CODE

* * *

Machine precision = 2.220D-16
N = 3 M = 10

At X0 0 variables are exactly at the bounds

At iterate 0 f= 1.38629D+02 |proj g|= 6.27865D+01

* * *

Tit = total number of iterations
Tnf = total number of function evaluations
Tnint = total number of segments explored during Cauchy searches
Skip = number of BFGS updates skipped
Nact = number of active bounds at final generalized Cauchy point
Projg = norm of the final projected gradient
F = final function value

* * *

N Tit Tnf Tnint Skip Nact Projg F
3 1 2 1 0 0 2.422D+01 9.713D+01
F = 97.133816163368223

STOP: TOTAL NO. of ITERATIONS REACHED LIMIT

Cauchy time 0.000E+00 seconds.
Subspace minimization time 0.000E+00 seconds.
Line search time 0.000E+00 seconds.

Total User time 0.000E+00 seconds.

Here is the output of conda list

# packages in environment at /home/sik/miniconda3/envs/test-sklearn:
#
# Name Version Build Channel
_libgcc_mutex 0.1 main
atomicwrites 1.3.0 py_0 conda-forge
attrs 19.1.0 py_0 conda-forge
bzip2 1.0.8 h516909a_0 conda-forge
ca-certificates 2019.6.16 hecc5488_0 conda-forge
certifi 2019.6.16 py37_1 conda-forge
cython 0.29.13 py37he1b5a44_0 conda-forge
importlib_metadata 0.20 py37_0 conda-forge
joblib 0.13.2 pypi_0 pypi
libblas 3.8.0 12_openblas conda-forge
libcblas 3.8.0 12_openblas conda-forge
libffi 3.2.1 he1b5a44_1006 conda-forge
libgcc-ng 9.1.0 hdf63c60_0
libgfortran-ng 7.3.0 hdf63c60_0
liblapack 3.8.0 12_openblas conda-forge
libopenblas 0.3.7 h6e990d7_1 conda-forge
libstdcxx-ng 9.1.0 hdf63c60_0
more-itertools 7.2.0 py_0 conda-forge
ncurses 6.1 hf484d3e_1002 conda-forge
numpy 1.17.1 py37h95a1406_0 conda-forge
openssl 1.1.1c h516909a_0 conda-forge
packaging 19.0 py_0 conda-forge
pip 19.2.3 py37_0 conda-forge
pluggy 0.12.0 py_0 conda-forge
py 1.8.0 py_0 conda-forge
pyparsing 2.4.2 py_0 conda-forge
pytest 5.1.2 py37_0 conda-forge
python 3.7.3 h33d41f4_1 conda-forge
readline 8.0 hf8c457e_0 conda-forge
scikit-learn 0.22.dev0 dev_0 <develop>
scipy 1.3.1 py37h921218d_2 conda-forge
setuptools 41.2.0 py37_0 conda-forge
six 1.12.0 py37_1000 conda-forge
sqlite 3.29.0 hcee41ef_1 conda-forge
tk 8.6.9 hed695b0_1002 conda-forge
wcwidth 0.1.7 py_1 conda-forge
wheel 0.33.6 py37_0 conda-forge
xz 5.2.4 h14c3975_1001 conda-forge
zipp 0.6.0 py_0 conda-forge
zlib 1.2.11 h516909a_1005 conda-forge

Issue 2:
I haven't followed the openmp building in detail but it looks like on OS X and windows building are now failing by default?

That's pretty annoying for sprinting with a bigger group and also can raise the frustration level of new contributors who are likely to be on these OSs