MAINT: revert __skip_array_function__ from NEP-18

xref GH-13624, GH-12028

TODO: update tests/CI for NUMPY_EXPERIMENTAL_ARRAY_FUNCTION=0

5 files changed, 33 insertions, 29 deletions

diff --git a/numpy/core/overrides.py b/numpy/core/overrides.py
index ad4d1c721..347e71bd4 100644
--- a/numpy/core/overrides.py
+++ b/numpy/core/overrides.py
@@ -1,6 +1,7 @@
 """Implementation of __array_function__ overrides from NEP-18."""
 import collections
 import functools
+import os
 import textwrap
 
 from numpy.core._multiarray_umath import (
@@ -8,6 +9,10 @@ from numpy.core._multiarray_umath import (
 from numpy.compat._inspect import getargspec
 
 
+ENABLE_ARRAY_FUNCTION = bool(
+    int(os.environ.get('NUMPY_EXPERIMENTAL_ARRAY_FUNCTION', 1)))
+
+
 add_docstring(
     implement_array_function,
     """
@@ -137,6 +142,22 @@ def array_function_dispatch(dispatcher, module=None, verify=True,
     Function suitable for decorating the implementation of a NumPy function.
     """
 
+    if not ENABLE_ARRAY_FUNCTION:
+        def decorator(implementation):
+            if docs_from_dispatcher:
+                add_docstring(implementation, dispatcher.__doc__)
+
+            public_api = implementation
+
+            if module is not None:
+                public_api.__module__ = module
+
+            public_api._implementation = implementation
+
+            return public_api
+
+        return decorator
+
     def decorator(implementation):
         if verify:
             verify_matching_signatures(implementation, dispatcher)
@@ -172,7 +193,7 @@ def array_function_dispatch(dispatcher, module=None, verify=True,
         if module is not None:
             public_api.__module__ = module
 
-        public_api.__skip_array_function__ = implementation
+        public_api._implementation = implementation
 
         return public_api
 
diff --git a/numpy/core/src/multiarray/arrayfunction_override.c b/numpy/core/src/multiarray/arrayfunction_override.c
index 02078306c..62e597764 100644
--- a/numpy/core/src/multiarray/arrayfunction_override.c
+++ b/numpy/core/src/multiarray/arrayfunction_override.c
@@ -173,7 +173,7 @@ array_function_method_impl(PyObject *func, PyObject *types, PyObject *args,
         }
     }
 
-    implementation = PyObject_GetAttr(func, npy_ma_str_skip_array_function);
+    implementation = PyObject_GetAttr(func, npy_ma_str_implementation);
     if (implementation == NULL) {
         return NULL;
     }
diff --git a/numpy/core/src/multiarray/multiarraymodule.c b/numpy/core/src/multiarray/multiarraymodule.c
index e15ab5172..269b5ee70 100644
--- a/numpy/core/src/multiarray/multiarraymodule.c
+++ b/numpy/core/src/multiarray/multiarraymodule.c
@@ -4498,7 +4498,7 @@ NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_array_prepare = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_array_wrap = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_array_finalize = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_ufunc = NULL;
-NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_skip_array_function = NULL;
+NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_implementation = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_order = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_copy = NULL;
 NPY_VISIBILITY_HIDDEN PyObject * npy_ma_str_dtype = NULL;
@@ -4514,8 +4514,7 @@ intern_strings(void)
     npy_ma_str_array_wrap = PyUString_InternFromString("__array_wrap__");
     npy_ma_str_array_finalize = PyUString_InternFromString("__array_finalize__");
     npy_ma_str_ufunc = PyUString_InternFromString("__array_ufunc__");
-    npy_ma_str_skip_array_function = PyUString_InternFromString(
-        "__skip_array_function__");
+    npy_ma_str_implementation = PyUString_InternFromString("_implementation");
     npy_ma_str_order = PyUString_InternFromString("order");
     npy_ma_str_copy = PyUString_InternFromString("copy");
     npy_ma_str_dtype = PyUString_InternFromString("dtype");
@@ -4525,7 +4524,7 @@ intern_strings(void)
 
     return npy_ma_str_array && npy_ma_str_array_prepare &&
            npy_ma_str_array_wrap && npy_ma_str_array_finalize &&
-           npy_ma_str_ufunc && npy_ma_str_skip_array_function &&
+           npy_ma_str_ufunc && npy_ma_str_implementation &&
            npy_ma_str_order && npy_ma_str_copy && npy_ma_str_dtype &&
            npy_ma_str_ndmin && npy_ma_str_axis1 && npy_ma_str_axis2;
 }
diff --git a/numpy/core/src/multiarray/multiarraymodule.h b/numpy/core/src/multiarray/multiarraymodule.h
index 5cf082fbb..dd437e091 100644
--- a/numpy/core/src/multiarray/multiarraymodule.h
+++ b/numpy/core/src/multiarray/multiarraymodule.h
@@ -6,7 +6,7 @@ NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_array_prepare;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_array_wrap;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_array_finalize;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_ufunc;
-NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_skip_array_function;
+NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_implementation;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_order;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_copy;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_dtype;
diff --git a/numpy/core/tests/test_overrides.py b/numpy/core/tests/test_overrides.py
index 7f02399b2..8b87fba13 100644
--- a/numpy/core/tests/test_overrides.py
+++ b/numpy/core/tests/test_overrides.py
@@ -190,17 +190,13 @@ class TestNDArrayArrayFunction(object):
         result = np.concatenate((array, override_sub))
         assert_equal(result, expected.view(OverrideSub))
 
-    def test_skip_array_function(self):
-        assert_(dispatched_one_arg.__skip_array_function__
-                is dispatched_one_arg.__wrapped__)
-
     def test_no_wrapper(self):
         # This shouldn't happen unless a user intentionally calls
         # __array_function__ with invalid arguments, but check that we raise
         # an appropriate error all the same.
         array = np.array(1)
-        func = dispatched_one_arg.__skip_array_function__
-        with assert_raises_regex(AttributeError, '__skip_array_function__'):
+        func = dispatched_one_arg._implementation
+        with assert_raises_regex(AttributeError, '_implementation'):
             array.__array_function__(func=func, types=(np.ndarray,),
                                      args=(array,), kwargs={})
 
@@ -385,9 +381,6 @@ class TestNumPyFunctions(object):
 
         assert_equal(np.sum(MyArray()), 'yes')
 
-    def test_sum_implementation_on_list(self):
-        assert_equal(np.sum.__skip_array_function__([1, 2, 3]), 6)
-
     def test_sum_on_mock_array(self):
 
         # We need a proxy for mocks because __array_function__ is only looked
@@ -408,25 +401,16 @@ class TestNumPyFunctions(object):
             np.sum, (ArrayProxy,), (proxy,), {})
         proxy.value.__array__.assert_not_called()
 
-        proxy = ArrayProxy(mock.Mock(spec=ArrayProxy))
-        proxy.value.__array__.return_value = np.array(2)
-        result = np.sum.__skip_array_function__(proxy)
-        assert_equal(result, 2)
-        # TODO: switch to proxy.value.__array__.assert_called() and
-        # proxy.value.__array_function__.assert_not_called() once we drop
-        # Python 3.5 support.
-        ((called_method_name, _, _),) = proxy.value.mock_calls
-        assert_equal(called_method_name, '__array__')
-
     def test_sum_forwarding_implementation(self):
 
-        class MyArray(object):
+        class MyArray(np.ndarray):
 
             def sum(self, axis, out):
                 return 'summed'
 
             def __array_function__(self, func, types, args, kwargs):
-                return func.__skip_array_function__(*args, **kwargs)
+                return super().__array_function__(func, types, args, kwargs)
 
         # note: the internal implementation of np.sum() calls the .sum() method
-        assert_equal(np.sum(MyArray()), 'summed')
+        array = np.array(1).view(MyArray)
+        assert_equal(np.sum(array), 'summed')