ENH: initial implementation of core __array_function__ machinery

I'd like to implement NEP-18 in a multi-step process:
1. (This PR) Pure Python implementation of `__array_function__` machinery,
   based on the prototype implementation from NEP-18.
2. Rewrite this machinery in C as needed to improve performance.
3. Implement overrides on NumPy functions.

Steps 2 and 3 should be able to happen in parallel (and with other people
contributing!).

This PR still needs more tests, especially for `ndarray.__array_function__`

1 files changed, 12 insertions, 0 deletions

diff --git a/numpy/core/_methods.py b/numpy/core/_methods.py
index 33f6d01a8..f49f64feb 100644
--- a/numpy/core/_methods.py
+++ b/numpy/core/_methods.py
@@ -154,3 +154,15 @@ def _ptp(a, axis=None, out=None, keepdims=False):
         umr_minimum(a, axis, None, None, keepdims),
         out
     )
+
+def _array_function(self, func, types, args, kwargs):
+    # TODO: rewrite this in C
+    # Cannot handle items that have __array_function__ other than our own.
+    for t in types:
+        if (hasattr(t, '__array_function__') and
+                t.__array_function__ is not mu.ndarray.__array_function__):
+            return NotImplemented
+
+    # Arguments contain no overrides, so we can safely call the
+    # overloaded function again.
+    return func(*args, **kwargs)