context: tweak erros and add tests for context lookup errors

This commit tweaks the error message for the conext generated
errors and adds tests.

src/otk/context.py

@@ -107,16 +107,17 @@ def variable(self, name: str) -> Any:
                 value = value[part]
             elif isinstance(value, list):
                 if not part.isnumeric():
-                    raise TransformVariableIndexTypeError("part is not numeric")
+                    raise TransformVariableIndexTypeError(f"part is not numeric but {type(part)}")
 
                 try:
                     value = value[int(part)]
                 except IndexError as exc:
-                    raise TransformVariableIndexRangeError(f"{part} is out of range") from exc
+                    raise TransformVariableIndexRangeError(f"{part} is out of range for {value}") from exc
             else:
                 prefix = ".".join(parts[:i])
                 raise TransformVariableTypeError(
-                    f"tried to look up '{prefix}.{part}', but prefix '{prefix}' value {value!r} is not a dictionary")
+                    f"tried to look up '{prefix}.{part}', but the value of "
+                    f"prefix '{prefix}' is not a dictionary but {type(value)}")
 
         return value
 

test/test_context.py

@@ -111,18 +111,27 @@ def test_context_nonexistent():
 
 def test_context_unhappy():
     ctx = CommonContext()
+
+    with pytest.raises(TransformVariableLookupError) as exc:
+        ctx.variable("foo")
+    assert "could not resolve 'foo' as 'foo' is not defined" in str(exc.value)
+
     ctx.define("foo", "foo")
 
-    with pytest.raises(TransformVariableTypeError):
+    with pytest.raises(TransformVariableTypeError) as exc:
         ctx.variable("foo.bar")
+    assert "tried to look up 'foo.bar', but the value of prefix 'foo' is not a dictionary but <class 'str'>" in str(
+        exc.value)
 
     ctx.define("bar", ["bar"])
 
-    with pytest.raises(TransformVariableIndexTypeError):
+    with pytest.raises(TransformVariableIndexTypeError) as exc:
         ctx.variable("bar.bar")
+    assert "part is not numeric but <class 'str'>" in str(exc.value)
 
-    with pytest.raises(TransformVariableIndexRangeError):
+    with pytest.raises(TransformVariableIndexRangeError) as exc:
         ctx.variable("bar.3")
+    assert "3 is out of range for ['bar']" in str(exc.value)
 
 
 @pytest.mark.parametrize(

test/test_substitute_vars.py

@@ -60,7 +60,7 @@ def test_sub_var_missing_var_in_context():
         substitute_vars(context, state, "${a.b}")
     # no subtree
     context.define("a", "foo")
-    expected_err = r"tried to look up 'a.b', but prefix 'a' value 'foo' is not a dictionary"
+    expected_err = r"tried to look up 'a.b', but the value of prefix 'a' is not a dictionary but <class 'str'>"
     with pytest.raises(TransformVariableTypeError, match=expected_err):
         substitute_vars(context, state, "${a.b}")