Improve error for in_(a_string) with a non-string value

changelog.d/383.change.rst

@@ -0,0 +1,3 @@
+``in_()`` validators now raise a ValueError with a useful message
+even if the options are a string and the value is not a string.
+By contrast, e.g. ``1 in "abc"`` raises an unhelpful TypeError.

src/attr/validators.py

@@ -135,7 +135,12 @@ class _InValidator(object):
     options = attrib()
 
     def __call__(self, inst, attr, value):
-        if value not in self.options:
+        try:
+            in_options = value in self.options
+        except TypeError as e:  # e.g. `1 in "abc"`
+            in_options = False
+
+        if not in_options:
             raise ValueError(
                 "'{name}' must be in {options!r} (got {value!r})"
                 .format(name=attr.name, options=self.options, value=value)

tests/test_validators.py

@@ -243,6 +243,19 @@ def test_fail(self):
             "'test' must be in [1, 2, 3] (got None)",
         ) == e.value.args
 
+    def test_fail_with_string(self):
+        """
+        Raise ValueError if the value is outside our options when the
+        options are specified as a string and the value is not a string.
+        """
+        v = in_("abc")
+        a = simple_attr("test")
+        with pytest.raises(ValueError) as e:
+            v(None, a, None)
+        assert (
+            "'test' must be in 'abc' (got None)",
+        ) == e.value.args
+
     def test_repr(self):
         """
         Returned validator has a useful `__repr__`.