DEP: Deprecate 4 `ndarray.ctypes` methods (#19031)

* DEP: Deprecate 4 `ndarray.ctypes` methods

* `get_data`
* `get_shape`
* `get_strides`
* `get_as_parameter`

* TST: Add deprecation tests for 4 `ndarray.ctypes` methods

* DOC: Add a release note for the `ndarray.ctypes` method deprecation

* MAINT: Deprecate via `__getattr__` instead of `__getattribute__

Co-Authored-By: Sebastian Berg <sebastian@sipsolutions.net>

* MAINT: Deprecate the methods via proper function definitions

Co-Authored-By: Eric Wieser <425260+eric-wieser@users.noreply.github.com>

* DOC: Added a missing "the"

* DOC: typo fix: property -> properties

Co-Authored-By: Eric Wieser <425260+eric-wieser@users.noreply.github.com>

Co-authored-by: Sebastian Berg <sebastian@sipsolutions.net>
Co-authored-by: Eric Wieser <425260+eric-wieser@users.noreply.github.com>

4 files changed, 78 insertions, 11 deletions

diff --git a/doc/release/upcoming_changes/19031.deprecation.rst b/doc/release/upcoming_changes/19031.deprecation.rst
new file mode 100644
index 000000000..de92e18df
--- /dev/null
+++ b/doc/release/upcoming_changes/19031.deprecation.rst
@@ -0,0 +1,12 @@
+Four `ndarray.ctypes` methods have been deprecated
+--------------------------------------------------
+Four methods of the `ndarray.ctypes` object have been deprecated,
+as they are (undocumentated) implementation artifacts of their respective
+properties.
+
+The methods in question are:
+
+* ``_ctypes.get_data`` (use ``_ctypes.data`` instead)
+* ``_ctypes.get_shape`` (use ``_ctypes.shape`` instead)
+* ``_ctypes.get_strides`` (use ``_ctypes.strides`` instead)
+* ``_ctypes.get_as_parameter`` (use ``_ctypes._as_parameter_`` instead)
diff --git a/doc/source/user/misc.rst b/doc/source/user/misc.rst
index 031ce4efa..f0a7f5e4c 100644
--- a/doc/source/user/misc.rst
+++ b/doc/source/user/misc.rst
@@ -149,11 +149,12 @@ Only a survey of the choices. Little detail on how each works.
    - good numpy support: arrays have all these in their ctypes
      attribute: ::
 
-       a.ctypes.data              a.ctypes.get_strides
-       a.ctypes.data_as           a.ctypes.shape
-       a.ctypes.get_as_parameter  a.ctypes.shape_as
-       a.ctypes.get_data          a.ctypes.strides
-       a.ctypes.get_shape         a.ctypes.strides_as
+       a.ctypes.data
+       a.ctypes.data_as
+       a.ctypes.shape
+       a.ctypes.shape_as
+       a.ctypes.strides
+       a.ctypes.strides_as
 
  - Minuses:
 
diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
index 449926f58..4eebbaea3 100644
--- a/numpy/core/_internal.py
+++ b/numpy/core/_internal.py
@@ -8,6 +8,7 @@ import ast
 import re
 import sys
 import platform
+import warnings
 
 from .multiarray import dtype, array, ndarray
 try:
@@ -350,11 +351,45 @@ class _ctypes:
         """
         return self.data_as(ctypes.c_void_p)
 
-    # kept for compatibility
-    get_data = data.fget
-    get_shape = shape.fget
-    get_strides = strides.fget
-    get_as_parameter = _as_parameter_.fget
+    # Numpy 1.21.0, 2021-05-18
+
+    def get_data(self):
+        """Deprecated getter for the `_ctypes.data` property.
+
+        .. deprecated:: 1.21
+        """
+        warnings.warn('"get_data" is deprecated. Use "data" instead',
+                      DeprecationWarning, stacklevel=2)
+        return self.data
+
+    def get_shape(self):
+        """Deprecated getter for the `_ctypes.shape` property.
+
+        .. deprecated:: 1.21
+        """
+        warnings.warn('"get_shape" is deprecated. Use "shape" instead',
+                      DeprecationWarning, stacklevel=2)
+        return self.shape
+
+    def get_strides(self):
+        """Deprecated getter for the `_ctypes.strides` property.
+
+        .. deprecated:: 1.21
+        """
+        warnings.warn('"get_strides" is deprecated. Use "strides" instead',
+                      DeprecationWarning, stacklevel=2)
+        return self.strides
+
+    def get_as_parameter(self):
+        """Deprecated getter for the `_ctypes._as_parameter_` property.
+
+        .. deprecated:: 1.21
+        """
+        warnings.warn(
+            '"get_as_parameter" is deprecated. Use "_as_parameter_" instead',
+            DeprecationWarning, stacklevel=2,
+        )
+        return self._as_parameter_
 
 
 def _newnames(datatype, order):
diff --git a/numpy/core/tests/test_deprecations.py b/numpy/core/tests/test_deprecations.py
index ed1688374..6eab2505d 100644
--- a/numpy/core/tests/test_deprecations.py
+++ b/numpy/core/tests/test_deprecations.py
@@ -906,7 +906,7 @@ else:
 class TestNoseDecoratorsDeprecated(_DeprecationTestCase):
     class DidntSkipException(Exception):
         pass
-    
+
     def test_slow(self):
         def _test_slow():
             @np.testing.dec.slow
@@ -1172,3 +1172,22 @@ class TestComparisonBadObjectDType(_DeprecationTestCase):
         self.assert_deprecated(lambda: np.equal(1, 1, dtype=object))
         self.assert_deprecated(
                 lambda: np.equal(1, 1, sig=(None, None, object)))
+
+
+class TestCtypesGetter(_DeprecationTestCase):
+    # Deprecated 2021-05-18, Numpy 1.21.0
+    warning_cls = DeprecationWarning
+    ctypes = np.array([1]).ctypes
+
+    @pytest.mark.parametrize(
+        "name", ["get_data", "get_shape", "get_strides", "get_as_parameter"]
+    )
+    def test_deprecated(self, name: str) -> None:
+        func = getattr(self.ctypes, name)
+        self.assert_deprecated(lambda: func())
+
+    @pytest.mark.parametrize(
+        "name", ["data", "shape", "strides", "_as_parameter_"]
+    )
+    def test_not_deprecated(self, name: str) -> None:
+        self.assert_not_deprecated(lambda: getattr(self.ctypes, name))