Ability to have `kw_only` attributes defined anywhere, as an optional feature (#559)

* Ability to disable the keyword-only order checks that forbid normal attrs after kw_only attrs

* Updated typing info

* Updated tests, and added a new test for the new ordering feature

* Pep8

* Updated docstring

* Rename to a name that reflects the usage, not the implementation

* Update tests after the rename

* Huge simplification: never check kw_only and non-init attributes order

* No more new parameter, remove from typing info

* Update tests after the simplification, and add new test

src/attr/_make.py

@@ -374,38 +374,24 @@ def _transform_attrs(cls, these, auto_attribs, kw_only):
 
     attrs = AttrsClass(base_attrs + own_attrs)
 
+    # mandatory vs non-mandatory attr order only matters when they are part of
+    # the __init__ signature and when they aren't kw_only (which are moved to
+    # 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:
+    attrs_to_check = [
+        a for a in attrs if a.init is not False and a.kw_only is False
+    ]
+
     had_default = False
-    was_kw_only = False
-    for a in attrs:
-        if (
-            was_kw_only is False
-            and had_default is True
-            and a.default is NOTHING
-            and a.init is True
-            and a.kw_only is False
-        ):
+    for a in attrs_to_check:
+        if had_default is True and a.default is NOTHING:
             raise ValueError(
                 "No mandatory attributes allowed after an attribute with a "
                 "default value or factory.  Attribute in question: %r" % (a,)
             )
-        elif (
-            had_default is False
-            and a.default is not NOTHING
-            and a.init is not False
-            and
-            # Keyword-only attributes without defaults can be specified
-            # after keyword-only attributes with defaults.
-            a.kw_only is False
-        ):
+
+        if had_default is False and a.default is not NOTHING:
             had_default = True
-        if was_kw_only is True and a.kw_only is False and a.init is True:
-            raise ValueError(
-                "Non keyword-only attributes are not allowed after a "
-                "keyword-only attribute (unless they are init=False).  "
-                "Attribute in question: {a!r}".format(a=a)
-            )
-        if was_kw_only is False and a.init is True and a.kw_only is True:
-            was_kw_only = True
 
     return _Attributes((attrs, base_attrs, base_attr_map))
 

tests/test_make.py

@@ -620,27 +620,35 @@ class TestKeywordOnlyAttributes(object):
             "missing 1 required keyword-only argument: 'x'"
         ) in e.value.args[0]
 
-    def test_conflicting_keyword_only_attributes(self):
+    def test_keyword_only_attributes_can_come_in_any_order(self):
         """
-        Raises `ValueError` if keyword-only attributes are followed by
-        regular (non keyword-only) attributes.
+        Mandatory vs non-mandatory attr order only matters when they are part
+        of the __init__ signature and when they aren't kw_only (which are
+        moved to the end and can be mandatory or non-mandatory in any order,
+        as they will be specified as keyword args anyway).
         """
 
+        @attr.s
         class C(object):
-            x = attr.ib(kw_only=True)
-            y = attr.ib()
+            a = attr.ib(kw_only=True)
+            b = attr.ib(kw_only=True, default="b")
+            c = attr.ib(kw_only=True)
+            d = attr.ib()
+            e = attr.ib(default="e")
+            f = attr.ib(kw_only=True)
+            g = attr.ib(kw_only=True, default="g")
+            h = attr.ib(kw_only=True)
 
-        with pytest.raises(ValueError) as e:
-            _transform_attrs(C, None, False, False)
+        c = C("d", a="a", c="c", f="f", h="h")
 
-        assert (
-            "Non keyword-only attributes are not allowed after a "
-            "keyword-only attribute (unless they are init=False).  "
-            "Attribute in question: Attribute"
-            "(name='y', default=NOTHING, validator=None, repr=True, "
-            "cmp=True, hash=None, init=True, metadata=mappingproxy({}), "
-            "type=None, converter=None, kw_only=False)",
-        ) == e.value.args
+        assert c.a == "a"
+        assert c.b == "b"
+        assert c.c == "c"
+        assert c.d == "d"
+        assert c.e == "e"
+        assert c.f == "f"
+        assert c.g == "g"
+        assert c.h == "h"
 
     def test_keyword_only_attributes_allow_subclassing(self):
         """