Better error messages when model object is missing or not a model object

doc/whats_new.rst

@@ -11,6 +11,8 @@ Bug fixes
 
 - Fix misinterpreted tuples passed as ``allowed_dims`` argument of
   ``Variable`` init (:issue:`17`).
+- Better error message when a Model instance is expected but no object
+  is found or a different object is provided (:issue:`13`).
 
 v0.1.0 (8 October 2017)
 -----------------------

xsimlab/tests/test_xr_accessor.py

@@ -3,6 +3,7 @@
 import numpy as np
 
 from xsimlab import xr_accessor, create_setup
+from xsimlab.xr_accessor import _maybe_get_model_from_context
 
 
 def test_filter_accessor():
@@ -297,10 +298,6 @@ def test_run_multi(self):
 
 
 def test_create_setup(model, input_dataset):
-    with pytest.raises(TypeError) as excinfo:
-        create_setup()
-    assert "No context on context stack" in str(excinfo.value)
-
     expected = xr.Dataset()
     actual = create_setup(model=model)
     xr.testing.assert_identical(actual, expected)
@@ -327,12 +324,14 @@ def test_create_setup(model, input_dataset):
     xr.testing.assert_identical(ds, input_dataset)
 
 
-def test_model_context(model):
+def test_get_model_from_context(model):
     with pytest.raises(TypeError) as excinfo:
-        create_setup()
-    assert "No context on context stack" in str(excinfo.value)
+        _maybe_get_model_from_context(None)
+    assert "no model found in context" in str(excinfo.value)
 
-    expected = xr.Dataset()
-    with model:
-        actual = create_setup(model=model)
-    xr.testing.assert_identical(actual, expected)
+    with model as m:
+        assert _maybe_get_model_from_context(None) is m
+
+    with pytest.raises(TypeError) as excinfo:
+        _maybe_get_model_from_context('not a model')
+    assert "is not an instance of xsimlab.Model" in str(excinfo.value)

xsimlab/xr_accessor.py

@@ -31,7 +31,14 @@ def _maybe_get_model_from_context(model):
     none supplied.
     """
     if model is None:
-        return Model.get_context()
+        try:
+            return Model.get_context()
+        except TypeError:
+            raise TypeError("no model found in context")
+
+    if not isinstance(model, Model):
+        raise TypeError("%s is not an instance of xsimlab.Model" % model)
+
     return model