TEST: warn user if gurobipy not installed

skipif() has the advantage of explicitly warning the user if tests
are not included due to gurobipy not being installed. This is entirely
similar to behavior in plot_test.py with matplotlib.

This changeset replaces the `nonfree` marker, which was not friendly
to newcomers: they would need to know to `pytest -m nonfree` if they
wanted to test with Gurobi. Otherwise, `pytest` (without the marker)
would silently not include those tests.

The advantages of using the `nonfree` marker are:
1. emphasizing to the user that tests involve behavior by code without
   open source,
2. failing if the corresponding packages (gurobipy, possibly others in
   the future) are not installed.
However, users who know enough to use `pytest -m nonfree` likely are
aware of Gurobi's license and likely would notice a warning when tests
are running, so indeed, skipif() is the best choice.

.github/workflows/main.yml

@@ -91,20 +91,10 @@ jobs:
                 export CVXOPT_BUILD_GLPK=1
                 pip install cvxopt
                 python -c 'import cvxopt.glpk'
-            - name: Run all tests, using `cvxopt.glpk`
-              run: |
-                set -o posix
-                echo 'Exported environment variables:'
-                export -p
-                cd tests/
-                pytest \
-                    -v \
-                    --continue-on-collection-errors \
-                    .
-            - name: Install `gurobipy` (nonfree)
+            - name: Install optional solvers with restrictive licenses
               run: |
                 pip install gurobipy
-            - name: Run tests involving nonfree solvers
+            - name: Run all tests, using `cvxopt.glpk`
               run: |
                 set -o posix
                 echo 'Exported environment variables:'
@@ -113,5 +103,4 @@ jobs:
                 pytest \
                     -v \
                     --continue-on-collection-errors \
-                    -m nonfree \
                     .

tests/polytope_test.py

@@ -8,6 +8,11 @@
 import pytest
 import scipy.optimize
 
+try:
+    import gurobipy
+except ImportError:
+    gurobipy = None
+
 import polytope as pc
 import polytope.polytope as alg
 from polytope import solvers
@@ -616,7 +621,9 @@ def test_reduce():
     assert_allclose(u, np.array([[50.], [1.]]), rtol=1e-07, atol=1e-07)
 
 
-@pytest.mark.nonfree
+@pytest.mark.skipif(
+    gurobipy is None,
+    reason='`gurobipy` is not installed')
 def test_gurobipy_return_same_result_as_scipy():
     c, A, b = example_1d()
     result_gurobi = solvers.lpsolve(c, A, b, solver='gurobi')

tests/pytest.ini

@@ -1,4 +1,2 @@
 [pytest]
-addopts = --strict-markers -m 'not nonfree'
-markers =
-    nonfree: marks tests as requiring nonfree (i.e., having restrictive licenses) dependencies
+addopts = --strict-markers