kw_only python 2 backport

changelog.d/700.change.rst

@@ -0,0 +1 @@
+``kw_only=True`` now works on Python 2.

docs/examples.rst

@@ -146,7 +146,7 @@ On Python 3 it overrides the implicit detection.
 Keyword-only Attributes
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-When using ``attrs`` on Python 3, you can also add `keyword-only <https://docs.python.org/3/glossary.html#keyword-only-parameter>`_ attributes:
+You can also add `keyword-only <https://docs.python.org/3/glossary.html#keyword-only-parameter>`_ attributes:
 
 .. doctest::
 

src/attr/_make.py

@@ -223,6 +223,7 @@ def attrib(
     .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
     .. versionadded:: 19.2.0 *eq* and *order*
     .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.3.0 *kw_only* backported to Python 2
     """
     eq, order = _determine_eq_order(cmp, eq, order, True)
 
@@ -1924,6 +1925,63 @@ def _assign_with_converter(attr_name, value_var, has_on_setattr):
     )
 
 
+if PY2:
+
+    def _unpack_kw_only_py2(attr_name, default=None):
+        """
+        Unpack *attr_name* from _kw_only dict.
+        """
+        if default is not None:
+            arg_default = ", %s" % default
+        else:
+            arg_default = ""
+        return "%s = _kw_only.pop('%s'%s)" % (
+            attr_name,
+            attr_name,
+            arg_default,
+        )
+
+    def _unpack_kw_only_lines_py2(kw_only_args):
+        """
+        Unpack all *kw_only_args* from _kw_only dict and handle errors.
+
+        Given a list of strings "{attr_name}" and "{attr_name}={default}"
+        generates list of lines of code that pop attrs from _kw_only dict and
+        raise TypeError similar to builtin if required attr is missing or
+        extra key is passed.
+
+        >>> print("\n".join(_unpack_kw_only_lines_py2(["a", "b=42"])))
+        try:
+            a = _kw_only.pop('a')
+            b = _kw_only.pop('b', 42)
+        except KeyError as _key_error:
+            raise TypeError(
+                ...
+        if _kw_only:
+            raise TypeError(
+                ...
+        """
+        lines = ["try:"]
+        lines.extend(
+            "    " + _unpack_kw_only_py2(*arg.split("="))
+            for arg in kw_only_args
+        )
+        lines += """\
+except KeyError as _key_error:
+    raise TypeError(
+        '__init__() missing required keyword-only argument: %s' % _key_error
+    )
+if _kw_only:
+    raise TypeError(
+        '__init__() got an unexpected keyword argument %r'
+        % next(iter(_kw_only))
+    )
+""".split(
+            "\n"
+        )
+        return lines
+
+
 def _attrs_to_init_script(
     attrs,
     frozen,
@@ -2178,14 +2236,14 @@ def fmt_setter_with_converter(
     args = ", ".join(args)
     if kw_only_args:
         if PY2:
-            raise PythonTooOldError(
-                "Keyword-only arguments only work on Python 3 and later."
-            )
+            lines = _unpack_kw_only_lines_py2(kw_only_args) + lines
 
-        args += "{leading_comma}*, {kw_only_args}".format(
-            leading_comma=", " if args else "",
-            kw_only_args=", ".join(kw_only_args),
-        )
+            args += "%s**_kw_only" % (", " if args else "",)  # leading comma
+        else:
+            args += "%s*, %s" % (
+                ", " if args else "",  # leading comma
+                ", ".join(kw_only_args),  # kw_only args
+            )
     return (
         """\
 def __init__(self, {args}):

tests/test_make.py

@@ -716,7 +716,6 @@ class SubClass(BaseClass):
         assert hash(ba) == hash(sa)
 
 
-@pytest.mark.skipif(PY2, reason="keyword-only arguments are PY3-only.")
 class TestKeywordOnlyAttributes(object):
     """
     Tests for keyword-only attributes.
@@ -728,7 +727,7 @@ def test_adds_keyword_only_arguments(self):
         """
 
         @attr.s
-        class C:
+        class C(object):
             a = attr.ib()
             b = attr.ib(default=2, kw_only=True)
             c = attr.ib(kw_only=True)
@@ -747,7 +746,7 @@ def test_ignores_kw_only_when_init_is_false(self):
         """
 
         @attr.s
-        class C:
+        class C(object):
             x = attr.ib(init=False, default=0, kw_only=True)
             y = attr.ib()
 
@@ -763,15 +762,34 @@ def test_keyword_only_attributes_presence(self):
         """
 
         @attr.s
-        class C:
+        class C(object):
             x = attr.ib(kw_only=True)
 
         with pytest.raises(TypeError) as e:
             C()
 
-        assert (
-            "missing 1 required keyword-only argument: 'x'"
-        ) in e.value.args[0]
+        if PY2:
+            assert (
+                "missing required keyword-only argument: 'x'"
+            ) in e.value.args[0]
+        else:
+            assert (
+                "missing 1 required keyword-only argument: 'x'"
+            ) in e.value.args[0]
+
+    def test_keyword_only_attributes_unexpected(self):
+        """
+        Raises `TypeError` when unexpected keyword argument passed.
+        """
+
+        @attr.s
+        class C(object):
+            x = attr.ib(kw_only=True)
+
+        with pytest.raises(TypeError) as e:
+            C(x=5, y=10)
+
+        assert "got an unexpected keyword argument 'y'" in e.value.args[0]
 
     def test_keyword_only_attributes_can_come_in_any_order(self):
         """
@@ -782,7 +800,7 @@ def test_keyword_only_attributes_can_come_in_any_order(self):
         """
 
         @attr.s
-        class C:
+        class C(object):
             a = attr.ib(kw_only=True)
             b = attr.ib(kw_only=True, default="b")
             c = attr.ib(kw_only=True)
@@ -811,7 +829,7 @@ def test_keyword_only_attributes_allow_subclassing(self):
         """
 
         @attr.s
-        class Base:
+        class Base(object):
             x = attr.ib(default=0)
 
         @attr.s
@@ -830,7 +848,7 @@ def test_keyword_only_class_level(self):
         """
 
         @attr.s(kw_only=True)
-        class C:
+        class C(object):
             x = attr.ib()
             y = attr.ib(kw_only=True)
 
@@ -849,7 +867,7 @@ def test_keyword_only_class_level_subclassing(self):
         """
 
         @attr.s
-        class Base:
+        class Base(object):
             x = attr.ib(default=0)
 
         @attr.s(kw_only=True)
@@ -872,7 +890,7 @@ def test_init_false_attribute_after_keyword_attribute(self):
         """
 
         @attr.s
-        class KwArgBeforeInitFalse:
+        class KwArgBeforeInitFalse(object):
             kwarg = attr.ib(kw_only=True)
             non_init_function_default = attr.ib(init=False)
             non_init_keyword_default = attr.ib(
@@ -900,7 +918,7 @@ def test_init_false_attribute_after_keyword_attribute_with_inheritance(
         """
 
         @attr.s
-        class KwArgBeforeInitFalseParent:
+        class KwArgBeforeInitFalseParent(object):
             kwarg = attr.ib(kw_only=True)
 
         @attr.s
@@ -927,23 +945,6 @@ class TestKeywordOnlyAttributesOnPy2(object):
     Tests for keyword-only attribute behavior on py2.
     """
 
-    def test_syntax_error(self):
-        """
-        Keyword-only attributes raise Syntax error on ``__init__`` generation.
-        """
-
-        with pytest.raises(PythonTooOldError):
-
-            @attr.s(kw_only=True)
-            class ClassLevel(object):
-                a = attr.ib()
-
-        with pytest.raises(PythonTooOldError):
-
-            @attr.s()
-            class AttrLevel(object):
-                a = attr.ib(kw_only=True)
-
     def test_no_init(self):
         """
         Keyworld-only is a no-op, not any error, if ``init=false``.