Support for converting keyword only arguments

py_backwards/transformers/__init__.py

@@ -2,6 +2,7 @@
 from .dict_unpacking import DictUnpackingTransformer
 from .formatted_values import FormattedValuesTransformer
 from .functions_annotations import FunctionsAnnotationsTransformer
+from .keyword_only_arguments import KeywordOnlyArgumentsTransformer
 from .starred_unpacking import StarredUnpackingTransformer
 from .variables_annotations import VariablesAnnotationsTransformer
 from .yield_from import YieldFromTransformer
@@ -27,6 +28,7 @@
     YieldFromTransformer,
     ReturnFromGeneratorTransformer,
     # 2.7
+    KeywordOnlyArgumentsTransformer,
     FunctionsAnnotationsTransformer,
     SuperWithoutArgumentsTransformer,
     ClassWithoutBasesTransformer,

py_backwards/transformers/keyword_only_arguments.py

@@ -0,0 +1,37 @@
+from .base import BaseNodeTransformer
+
+
+class KeywordOnlyArgumentsTransformer(BaseNodeTransformer):
+    """Compiles:
+        def fn(x, *, a=None):
+            pass
+    To:
+        def fn(x, a=None):
+            pass
+
+    """
+    target = (2, 7)
+
+    def visit_arguments(self, node):
+        if node.kwonlyargs:
+            self._tree_changed = True
+
+            if node.defaults:
+                required_args = node.args[:-len(node.defaults)]
+                optional_args = node.args[-len(node.defaults):]
+            else:
+                required_args = node.args
+                optional_args = []
+
+            for kwarg, default in zip(node.kwonlyargs, node.kw_defaults):
+                if default is None:
+                    required_args.append(kwarg)
+                else:
+                    optional_args.append(kwarg)
+                    node.defaults.append(default)
+
+            node.args = required_args + optional_args
+            node.kwonlyargs = []
+            node.kw_defaults = []
+
+        return self.generic_visit(node)