fix: make copy_function work with keyword-only defaults (#336)

* fix: make copy_function work with keyword-only defaults

* move kwdefaults test to py3-only file

---------

Co-authored-by: Mahmoud Hashemi <mahmoud@hatnote.com>

boltons/funcutils.py

@@ -215,6 +215,8 @@ def copy_function(orig, copy_dict=True):
                        name=orig.__name__,
                        argdefs=getattr(orig, "__defaults__", None),
                        closure=getattr(orig, "__closure__", None))
+    if hasattr(orig, "__kwdefaults__"):
+        ret.__kwdefaults__ = orig.__kwdefaults__
     if copy_dict:
         ret.__dict__.update(orig.__dict__)
     return ret

tests/test_funcutils_fb_py3.py

@@ -5,7 +5,7 @@
 
 import pytest
 
-from boltons.funcutils import wraps, FunctionBuilder, update_wrapper
+from boltons.funcutils import wraps, FunctionBuilder, update_wrapper, copy_function
 import boltons.funcutils as funcutils
 
 
@@ -53,6 +53,13 @@ def kwonly_non_roundtrippable_repr(*, x=lambda y: y + 1):
         True, 'kwonly_non_roundtrippable_repr', 2)
 
 
+def test_copy_function_kw_defaults_py3():
+    # test that the copy works with keyword-only defaults
+    f = lambda x, *, y=2: x * y
+    f_copy = copy_function(f)
+    assert f(21) == f_copy(21) == 42
+
+
 @pytest.mark.parametrize('partial_kind', (functools, funcutils))
 def test_update_wrapper_partial(partial_kind):
     wrapper = partial_kind.partial(wrappable_varkw_func, b=1)