DEP: Deprecate the coercion of dtype-likes to their superclass

eric-wieser · eric-wieser · commit 091b8c3b16d3 · 2017-08-05T21:56:03.000-05:00
In many cases, this coercion is surprising, or would be if the user knew about it:

* [('a', int)] -&gt; np.flexible
* str - &gt; str (!) - not even a numpy type
* 'float32' -&gt; np.floating (discards size)
* int -&gt; np.signed_integer (not np.integer, as is usually meant)
diff --git a/doc/release/1.14.0-notes.rst b/doc/release/1.14.0-notes.rst
@@ -25,6 +25,17 @@ Deprecations
 Future Changes
 ==============
 
+``np.issubdtype`` will stop downcasting dtype-like arguments
+------------------------------------------------------------
+It would be expected that ``issubdtype(np.float32, 'float64')`` and
+``issubdtype(np.float32, np.float64)`` mean the same thing - however, there
+was an undocumented special case that translated the former into
+``issubdtype(np.float32, np.floating)``, giving the surprising result of True.
+
+This translation now gives a warning explaining what translation is occuring.
+In future, the translation will be disabled, and the first example will be made
+equivalent to the second.
+
 
 Build System Changes
 ====================
diff --git a/numpy/core/numerictypes.py b/numpy/core/numerictypes.py
@@ -85,6 +85,7 @@
 import types as _types
 import sys
 import numbers
+import warnings
 
 from numpy.compat import bytes, long
 from numpy.core.multiarray import (
@@ -763,6 +764,27 @@ def issubdtype(arg1, arg2):
             mro = arg2.mro()
             arg2 = mro[1] if len(mro) > 1 else mro[0]
 
+            def type_repr(x):
+                """ Helper to produce clear error messages """
+                if not isinstance(x, type):
+                    return repr(x)
+                elif issubclass(x, generic):
+                    return "np.{}".format(x.__name__)
+                else:
+                    return x.__name__
+
+            # 1.14, 2017-08-01
+            warnings.warn(
+                "Conversion of the second argument of issubdtype from `{raw}` "
+                "to `{abstract}` is deprecated. In future, it will be treated "
+                "as `{concrete} == np.dtype({raw}).type`.".format(
+                    raw=type_repr(arg2_orig),
+                    abstract=type_repr(arg2),
+                    concrete=type_repr(dtype(arg2_orig).type)
+                ),
+                FutureWarning, stacklevel=2
+            )
+
     return issubclass(arg1, arg2)
 
 
