MAINT: disable __array_function__ dispatch unless environment variable set

Per discussion on the mailing list, __array_function__ isn't quite ready to
release as part of NumPy 1.16:
https://mail.python.org/pipermail/numpy-discussion/2018-November/078949.html

We'd like to improve performance a bit, and it will be easier to support
introspection on NumPy functions if we support Python 3 only.

So for now, you need to set the environment variable
``NUMPY_EXPERIMENTAL_ARRAY_FUNCTION=1`` to enable dispatching.

2 files changed, 28 insertions, 13 deletions

diff --git a/numpy/core/overrides.py b/numpy/core/overrides.py
index 088c15e65..0bbb39123 100644
--- a/numpy/core/overrides.py
+++ b/numpy/core/overrides.py
@@ -4,6 +4,7 @@ TODO: rewrite this in C for performance.
 """
 import collections
 import functools
+import os
 
 from numpy.core._multiarray_umath import ndarray
 from numpy.compat._inspect import getargspec
@@ -12,6 +13,9 @@ from numpy.compat._inspect import getargspec
 _NDARRAY_ARRAY_FUNCTION = ndarray.__array_function__
 _NDARRAY_ONLY = [ndarray]
 
+ENABLE_ARRAY_FUNCTION = bool(
+    int(os.environ.get('NUMPY_EXPERIMENTAL_ARRAY_FUNCTION', 0)))
+
 
 def get_overloaded_types_and_args(relevant_args):
     """Returns a list of arguments on which to call __array_function__.
@@ -149,17 +153,18 @@ def verify_matching_signatures(implementation, dispatcher):
 def array_function_dispatch(dispatcher, module=None, verify=True):
     """Decorator for adding dispatch with the __array_function__ protocol."""
     def decorator(implementation):
-        # TODO: only do this check when the appropriate flag is enabled or for
-        # a dev install. We want this check for testing but don't want to
-        # slow down all numpy imports.
-        if verify:
-            verify_matching_signatures(implementation, dispatcher)
-
-        @functools.wraps(implementation)
-        def public_api(*args, **kwargs):
-            relevant_args = dispatcher(*args, **kwargs)
-            return array_function_implementation_or_override(
-                implementation, public_api, relevant_args, args, kwargs)
+        if ENABLE_ARRAY_FUNCTION:
+            # __array_function__ requires an explicit opt-in for now
+            public_api = implementation
+        else:
+            if verify:
+                verify_matching_signatures(implementation, dispatcher)
+
+            @functools.wraps(implementation)
+            def public_api(*args, **kwargs):
+                relevant_args = dispatcher(*args, **kwargs)
+                return array_function_implementation_or_override(
+                    implementation, public_api, relevant_args, args, kwargs)
 
         if module is not None:
             public_api.__module__ = module
diff --git a/numpy/core/tests/test_overrides.py b/numpy/core/tests/test_overrides.py
index ee6d5da4a..1c8ca760a 100644
--- a/numpy/core/tests/test_overrides.py
+++ b/numpy/core/tests/test_overrides.py
@@ -7,8 +7,14 @@ from numpy.testing import (
     assert_, assert_equal, assert_raises, assert_raises_regex)
 from numpy.core.overrides import (
     get_overloaded_types_and_args, array_function_dispatch,
-    verify_matching_signatures)
+    verify_matching_signatures, ENABLE_ARRAY_FUNCTION)
 from numpy.core.numeric import pickle
+import pytest
+
+
+requires_array_function = pytest.mark.skipif(
+    not ENABLE_ARRAY_FUNCTION,
+    reason="__array_function__ dispatch not enabled.")
 
 
 def _get_overloaded_args(relevant_args):
@@ -165,6 +171,7 @@ def dispatched_one_arg(array):
     return 'original'
 
 
+@requires_array_function
 class TestArrayFunctionDispatch(object):
 
     def test_pickle(self):
@@ -204,6 +211,7 @@ class TestArrayFunctionDispatch(object):
             dispatched_one_arg(array)
 
 
+@requires_array_function
 class TestVerifyMatchingSignatures(object):
 
     def test_verify_matching_signatures(self):
@@ -256,6 +264,7 @@ def _new_duck_type_and_implements():
     return (MyArray, implements)
 
 
+@requires_array_function
 class TestArrayFunctionImplementation(object):
 
     def test_one_arg(self):
@@ -322,7 +331,7 @@ class TestNDArrayMethods(object):
         assert_equal(repr(array), 'MyArray(1)')
         assert_equal(str(array), '1')
 
-        
+
 class TestNumPyFunctions(object):
 
     def test_module(self):
@@ -331,6 +340,7 @@ class TestNumPyFunctions(object):
         assert_equal(np.fft.fft.__module__, 'numpy.fft')
         assert_equal(np.linalg.solve.__module__, 'numpy.linalg')
 
+    @requires_array_function
     def test_override_sum(self):
         MyArray, implements = _new_duck_type_and_implements()