Improve error message for mandatory attributes after defaults

When a mandatory attribute appears after an attribute with a default
value or factory, the error message now includes which attribute has
the default, making it easier to identify the problem.

Fixes #437

Author: Topgearfanboy

src/attr/_make.py

@@ -483,13 +483,15 @@ def _transform_attrs(
     # the end and can be mandatory or non-mandatory in any order, as they will
     # be specified as keyword args anyway). Check the order of those attrs:
     had_default = False
+    which_had_default = None
     for a in (a for a in attrs if a.init is not False and a.kw_only is False):
         if had_default is True and a.default is NOTHING:
-            msg = f"No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: {a!r}"
+            msg = f"No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: {a!r} appears after {which_had_default!r}"
             raise ValueError(msg)
 
         if had_default is False and a.default is not NOTHING:
             had_default = True
+            which_had_default = a
 
     # Resolve default field alias for any new attributes that the
     # field_transformer may have added without setting an alias.

tests/test_hooks.py

@@ -179,7 +179,12 @@ class C:
                 "eq=True, eq_key=None, order=True, order_key=None, "
                 "hash=None, init=True, "
                 "metadata=mappingproxy({'field_order': 1}), type='int', converter=None, "
-                "kw_only=False, inherited=False, on_setattr=None, alias='x')"
+                "kw_only=False, inherited=False, on_setattr=None, alias='x') "
+                "appears after Attribute(name='y', default=0, validator=None, "
+                "repr=True, eq=True, eq_key=None, order=True, order_key=None, "
+                "hash=None, init=True, metadata=mappingproxy({'field_order': 0}), "
+                "type='int', converter=None, kw_only=False, inherited=False, "
+                "on_setattr=None, alias='y')"
             ),
         ) == e.value.args
 

tests/test_make.py

@@ -288,7 +288,12 @@ class C:
                 "eq=True, eq_key=None, order=True, order_key=None, "
                 "hash=None, init=True, "
                 "metadata=mappingproxy({}), type=None, converter=None, "
-                "kw_only=False, inherited=False, on_setattr=None, alias='y')"
+                "kw_only=False, inherited=False, on_setattr=None, alias='y') "
+                "appears after Attribute(name='x', default=None, validator=None, "
+                "repr=True, eq=True, eq_key=None, order=True, order_key=None, "
+                "hash=None, init=True, metadata=mappingproxy({}), type=None, "
+                "converter=None, kw_only=False, inherited=False, on_setattr=None, "
+                "alias='x')"
             ),
         ) == e.value.args