Clarified error messages in LazyTractogram

Author: MarcCote

nibabel/streamlines/tests/test_tractogram.py

@@ -586,6 +586,8 @@ def test_lazy_tractogram_creation(self):
         # Creating LazyTractogram with generators is not allowed as
         # generators get exhausted and are not reusable unlike generator function.
         assert_raises(TypeError, LazyTractogram, streamlines)
+        assert_raises(TypeError, LazyTractogram,
+                      data_per_point={"none": None})
         assert_raises(TypeError, LazyTractogram,
                       data_per_streamline=data_per_streamline)
         assert_raises(TypeError, LazyTractogram, DATA['streamlines'],

nibabel/streamlines/tractogram.py

@@ -165,8 +165,11 @@ def __getitem__(self, key):
         return self.store[key]()
 
     def __setitem__(self, key, value):
-        if value is not None and not callable(value):  # TODO: why None?
-            raise TypeError("`value` must be a generator function or None.")
+        if not callable(value):
+            msg = ("Values in a `LazyDict` must be generator functions."
+                   " Those are functions which whenever are called return an"
+                   " instantiated generator.")
+            raise TypeError(msg)
         self.store[key] = value
 
     def __delitem__(self, key):
@@ -604,7 +607,10 @@ def _apply_affine():
 
     def _set_streamlines(self, value):
         if value is not None and not callable(value):
-            raise TypeError("`streamlines` must be a generator function.")
+            msg = ("`streamlines` must be a generator function. That is a"
+                   " function which whenever is called returns an"
+                   " instantiated generator.")
+            raise TypeError(msg)
         self._streamlines = value
 
     @property