Addition of pytest case coverage of backend and AnalysisBase.run() using different n_workers values

package/CHANGELOG

@@ -82,6 +82,8 @@ Enhancements
    (Issue #4673) 
  * enables parallelization for analysis.dssp.dssp.DSSP (Issue #4674)
  * Enables parallelization for analysis.hydrogenbonds.hbond_analysis.HydrogenBondAnalysis (Issue #4664)
+ * Added pytest for case where backend instance and AnalysisBase.run() have both different
+   n_worker values assigned (Issue #4649)
  * Improve error message for `AtomGroup.unwrap()` when bonds are not present.(Issue #4436, PR #4642)
  * Add `analysis.DSSP` module for protein secondary structure assignment, based on [pydssp](https://github.com/ShintaroMinami/PyDSSP)
  * Added a tqdm progress bar for `MDAnalysis.analysis.pca.PCA.transform()`

testsuite/MDAnalysisTests/analysis/test_base.py

@@ -121,6 +121,18 @@ def test_incompatible_n_workers(u):
     with pytest.raises(ValueError):
         FrameAnalysis(u).run(backend=backend, n_workers=3)
 
+
+def test_n_workers_conflict_raises_value_error(u):
+    analysis = FrameAnalysis(u.trajectory)
+    backend_instance = ManyWorkersBackend(n_workers=4)
+
+    with pytest.raises(ValueError, match="n_workers specified twice"):
+        FrameAnalysis(u.trajectory).run(
+            backend=backend_instance,
+            n_workers=1,
+            unsupported_backend=True
+        )
+
 @pytest.mark.parametrize('run_class,backend,n_workers', [
     (Parallelizable, 'not-existing-backend', 2),
     (Parallelizable, 'not-existing-backend', None),