DEP: Do not allow "abstract" dtype conversion/creation

These dtypes do not really make sense as instances. We can (somewhat)
reasonably define np.dtype(np.int64) as the default (machine endianess)
int64. (Arguably, it is unclear that `np.array(arr_of_>f8, dtype="f")`
should return arr_of_<f8, but that would be very noisy!)

However, `np.integer` as equivalent to long, is not well defined.
Similarly, `dtype=Decimal` may be neat to spell `dtype=object` when you
intend to put Decimal objects into the array. But it is misleading,
since there is no special meaning to it at this time.

The biggest issue with it, is that `arr.astype(np.floating)` looks
like it will let float32 or float128 pass, but it will force a
float64 output! Arguably downcasting is a bug in this case.

A related issue is `np.dtype("S")` and especially "S0". The dtype "S"
does make sense for most or all places where `dtype=...` can be
passed. However, it is conceptionally different from other dtypes, since
it will not end up being attached to the array (unlike "S2" which
would be). The dtype "S" really means the type number/DType class
of String, and not a specific dtype instance.

6 files changed, 41 insertions, 8 deletions

diff --git a/numpy/core/tests/test_defchararray.py b/numpy/core/tests/test_defchararray.py
index 39600553d..bbb94f7d3 100644
--- a/numpy/core/tests/test_defchararray.py
+++ b/numpy/core/tests/test_defchararray.py
@@ -119,14 +119,14 @@ class TestVecString:
     def test_invalid_result_type(self):
 
         def fail():
-            _vec_string(['a'], np.integer, 'strip')
+            _vec_string(['a'], np.int_, 'strip')
 
         assert_raises(TypeError, fail)
 
     def test_broadcast_error(self):
 
         def fail():
-            _vec_string([['abc', 'def']], np.integer, 'find', (['a', 'd', 'j'],))
+            _vec_string([['abc', 'def']], np.int_, 'find', (['a', 'd', 'j'],))
 
         assert_raises(ValueError, fail)
 
diff --git a/numpy/core/tests/test_deprecations.py b/numpy/core/tests/test_deprecations.py
index 01b35ec90..a89fc70d5 100644
--- a/numpy/core/tests/test_deprecations.py
+++ b/numpy/core/tests/test_deprecations.py
@@ -547,3 +547,32 @@ def test_deprecate_ragged_arrays():
     with assert_warns(np.VisibleDeprecationWarning):
         np.array(arg)
 
+
+class TestAbstractDTypeCoercion(_DeprecationTestCase):
+    # 2020-02-06 1.19.0
+    message = "Converting .* to a dtype .*is deprecated"
+    deprecated_types = [
+        # The builtin scalar super types:
+        np.generic, np.flexible, np.number,
+        np.inexact, np.floating, np.complexfloating,
+        np.integer, np.unsignedinteger, np.signedinteger,
+        # character is a deprecated S1 special case:
+        np.character,
+        # Test python types that do not map to a NumPy type cleanly
+        # (currenlty map to object)
+        type, list, tuple, dict,
+    ]
+
+    def test_dtype_coercion(self):
+        for scalar_type in self.deprecated_types:
+            self.assert_deprecated(np.dtype, args=(scalar_type,))
+
+    def test_array_construction(self):
+        for scalar_type in self.deprecated_types:
+            self.assert_deprecated(np.array, args=([], scalar_type,))
+
+    def test_not_deprecated(self):
+        # All specific types are not deprecated:
+        for group in np.sctypes.values():
+            for scalar_type in group:
+                self.assert_not_deprecated(np.dtype, args=(scalar_type,))
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index ad38911cb..f6181c900 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -6456,7 +6456,7 @@ class TestRepeat:
 NEIGH_MODE = {'zero': 0, 'one': 1, 'constant': 2, 'circular': 3, 'mirror': 4}
 
 
-@pytest.mark.parametrize('dt', [float, Decimal], ids=['float', 'object'])
+@pytest.mark.parametrize('dt', [float, object], ids=['float', 'object'])
 class TestNeighborhoodIter:
     # Simple, 2d tests
     def test_simple2d(self, dt):
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
index 3bc4cd187..0f7ac036f 100644
--- a/numpy/core/tests/test_numeric.py
+++ b/numpy/core/tests/test_numeric.py
@@ -2543,7 +2543,7 @@ class TestCorrelate:
         assert_array_almost_equal(z, self.zs)
 
     def test_object(self):
-        self._setup(Decimal)
+        self._setup(object)
         z = np.correlate(self.x, self.y, 'full')
         assert_array_almost_equal(z, self.z1)
         z = np.correlate(self.y, self.x, 'full')
diff --git a/numpy/core/tests/test_numerictypes.py b/numpy/core/tests/test_numerictypes.py
index c72d13947..22fc2ed58 100644
--- a/numpy/core/tests/test_numerictypes.py
+++ b/numpy/core/tests/test_numerictypes.py
@@ -3,7 +3,8 @@ import itertools
 
 import pytest
 import numpy as np
-from numpy.testing import assert_, assert_equal, assert_raises, IS_PYPY
+from numpy.testing import (
+    assert_, assert_equal, assert_raises, assert_warns, IS_PYPY)
 
 # This is the structure of the table used for plain objects:
 #
@@ -451,8 +452,11 @@ class Test_sctype2char:
 
     def test_other_type(self):
         assert_equal(np.sctype2char(float), 'd')
-        assert_equal(np.sctype2char(list), 'O')
-        assert_equal(np.sctype2char(np.ndarray), 'O')
+        assert_equal(np.sctype2char(object), 'O')
+        with assert_warns(DeprecationWarning):
+            assert_equal(np.sctype2char(list), 'O')
+        with assert_warns(DeprecationWarning):
+            assert_equal(np.sctype2char(np.ndarray), 'O')
 
     def test_third_party_scalar_type(self):
         from numpy.core._rational_tests import rational
diff --git a/numpy/core/tests/test_regression.py b/numpy/core/tests/test_regression.py
index 321723b9b..50a543625 100644
--- a/numpy/core/tests/test_regression.py
+++ b/numpy/core/tests/test_regression.py
@@ -1152,7 +1152,7 @@ class TestRegression:
         assert_(dat.argmax(1).info == 'jubba')
         assert_(dat.argmin(1).info == 'jubba')
         assert_(dat.argsort(1).info == 'jubba')
-        assert_(dat.astype(TestArray).info == 'jubba')
+        assert_(dat.astype(object).info == 'jubba')
         assert_(dat.byteswap().info == 'jubba')
         assert_(dat.clip(2, 7).info == 'jubba')
         assert_(dat.compress([0, 1, 1]).info == 'jubba')