Assigning settings on the class is an error

You probably meant to assign them on an (or the default) *instance*.

hypothesis-python/RELEASE.rst

@@ -0,0 +1,7 @@
+RELEASE_TYPE: minor
+
+To adjust the current settings profile, you can assign to attributes of the
+current settings *instance* ``hypothesis.settings.default``.  This release
+makes it an error to omit the ``.default``, because assigning to attributes
+of ``hypothesis.settings`` had no effect whatsoever - except to confuse
+users about their settings!

hypothesis-python/src/hypothesis/_settings.py

@@ -110,13 +110,23 @@ def default(self):
             assert default_variable.value is not None
         return default_variable.value
 
-    @default.setter
-    def default(self, value):
-        raise AttributeError('Cannot assign settings.default')
-
     def _assign_default_internal(self, value):
         default_variable.value = value
 
+    def __setattr__(self, name, value):
+        if name == 'default':
+            raise AttributeError(
+                'Cannot assign to the property settings.default - '
+                'consider using settings.load_profile instead.'
+            )
+        elif not (isinstance(value, settingsProperty) or name.startswith('_')):
+            raise AttributeError(
+                'Cannot setattr %s=%r on class hypothesis.settings - did '
+                'you mean the current instance, hypothesis.settings.default?'
+                % (name, value)
+            )
+        return type.__setattr__(self, name, value)
+
 
 class settings(
     settingsMeta('settings', (object,), {})  # type: ignore

hypothesis-python/tests/cover/test_settings.py

@@ -394,3 +394,10 @@ def test_setting_use_coverage_is_deprecated(value):
 def test_settings_apply_for_explicit_examples(x):
     # Regression test for #1521
     assert settings.default.verbosity == Verbosity.verbose
+
+
+def test_setattr_on_settings_singleton_is_error():
+    # https://github.com/pandas-dev/pandas/pull/22679#issuecomment-420750921
+    # Should be setting attributes on settings.default, not settings!
+    with pytest.raises(AttributeError):
+        settings.max_examples = 10