Fixes #5170: Improve msd_type parsing

package/AUTHORS

@@ -265,6 +265,7 @@ Chronological list of authors
   - Raúl Lois-Cuns  
   - Pranay Pelapkar
   - Shreejan Dolai
+  - Tanisha Dubey
 
 External code
 -------------

package/CHANGELOG

@@ -15,7 +15,7 @@ The rules for this file:
 
 -------------------------------------------------------------------------------
 ??/??/?? IAlibay, orbeckst, marinegor, tylerjereddy, ljwoods2, marinegor,
-         spyke7, talagayev
+         spyke7, talagayev, tanii1125
 
  * 2.11.0
 

package/MDAnalysis/analysis/msd.py

@@ -382,14 +382,11 @@ def _parse_msd_type(self):
             "xyz": [0, 1, 2],
         }
 
-        self.msd_type = self.msd_type.lower()
-
         try:
-            self._dim = keys[self.msd_type]
-        except KeyError:
-            raise ValueError(
-                "invalid msd_type: {} specified, please specify one of xyz, "
-                "xy, xz, yz, x, y, z".format(self.msd_type)
+            self._dim = keys[self.msd_type.lower()]
+        except (AttributeError, KeyError):
+            raise TypeError(
+                "msd_type must be a string and one of: xyz, xy, xz, yz, x, y, z"
             )
 
         self.dim_fac = len(self._dim)

testsuite/MDAnalysisTests/analysis/test_msd.py

@@ -117,11 +117,21 @@ def test_updating_ag_rejected(self, u):
         with pytest.raises(TypeError, match=errmsg):
             m = MSD(updating_ag, msd_type="xyz", fft=False)
 
-    @pytest.mark.parametrize("msdtype", ["foo", "bar", "yx", "zyx"])
-    def test_msdtype_error(self, u, SELECTION, msdtype):
-        errmsg = f"invalid msd_type: {msdtype}"
-        with pytest.raises(ValueError, match=errmsg):
-            m = MSD(u, SELECTION, msd_type=msdtype)
+    @pytest.mark.parametrize(
+        "msd_type, exc",
+        [
+            ("Xz", None),  # valid, mixed case.
+            (123, TypeError),  # non-string.
+        ],
+    )
+    def test_msdtype_error(self, u, SELECTION, msd_type, exc):
+        if exc is None:
+            m = MSD(u, SELECTION, msd_type=msd_type, fft=False)
+            assert m.dim_fac == 2
+            assert m._dim == [0, 2]
+        else:
+            with pytest.raises(exc):
+                MSD(u, SELECTION, msd_type=msd_type, fft=False)
 
     @pytest.mark.parametrize(
         "dim, dim_factor",