Bug Ensure cache_readonly is only applied on instance

doc/source/whatsnew/v1.3.0.rst

@@ -795,6 +795,7 @@ ExtensionArray
 
 - Bug in :meth:`DataFrame.where` when ``other`` is a :class:`Series` with :class:`ExtensionArray` dtype (:issue:`38729`)
 - Fixed bug where :meth:`Series.idxmax`, :meth:`Series.idxmin` and ``argmax/min`` fail when the underlying data is :class:`ExtensionArray` (:issue:`32749`, :issue:`33719`, :issue:`36566`)
+- Fixed a bug where some properties of subclasses of :class:`PandasExtensionDtype` where improperly cached (:issue:`40329`)
 -
 
 Other

pandas/core/dtypes/dtypes.py

@@ -81,7 +81,7 @@ class PandasExtensionDtype(ExtensionDtype):
     base: DtypeObj | None = None
     isbuiltin = 0
     isnative = 0
-    _cache: dict[str_type, PandasExtensionDtype] = {}
+    _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
     def __str__(self) -> str_type:
         """
@@ -105,7 +105,7 @@ def __getstate__(self) -> dict[str_type, Any]:
     @classmethod
     def reset_cache(cls) -> None:
         """ clear the cache """
-        cls._cache = {}
+        cls._cache_dtypes = {}
 
 
 class CategoricalDtypeType(type):
@@ -177,7 +177,7 @@ class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     str = "|O08"
     base = np.dtype("O")
     _metadata = ("categories", "ordered")
-    _cache: dict[str_type, PandasExtensionDtype] = {}
+    _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
     def __init__(self, categories=None, ordered: Ordered = False):
         self._finalize(categories, ordered, fastpath=False)
@@ -671,7 +671,7 @@ class DatetimeTZDtype(PandasExtensionDtype):
     na_value = NaT
     _metadata = ("unit", "tz")
     _match = re.compile(r"(datetime64|M8)\[(?P<unit>.+), (?P<tz>.+)\]")
-    _cache: dict[str_type, PandasExtensionDtype] = {}
+    _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
     def __init__(self, unit: str_type | DatetimeTZDtype = "ns", tz=None):
         if isinstance(unit, DatetimeTZDtype):
@@ -837,7 +837,7 @@ class PeriodDtype(dtypes.PeriodDtypeBase, PandasExtensionDtype):
     num = 102
     _metadata = ("freq",)
     _match = re.compile(r"(P|p)eriod\[(?P<freq>.+)\]")
-    _cache: dict[str_type, PandasExtensionDtype] = {}
+    _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
     def __new__(cls, freq=None):
         """
@@ -859,12 +859,12 @@ def __new__(cls, freq=None):
             freq = cls._parse_dtype_strict(freq)
 
         try:
-            return cls._cache[freq.freqstr]
+            return cls._cache_dtypes[freq.freqstr]
         except KeyError:
             dtype_code = freq._period_dtype_code
             u = dtypes.PeriodDtypeBase.__new__(cls, dtype_code)
             u._freq = freq
-            cls._cache[freq.freqstr] = u
+            cls._cache_dtypes[freq.freqstr] = u
             return u
 
     def __reduce__(self):
@@ -1042,7 +1042,7 @@ class IntervalDtype(PandasExtensionDtype):
     _match = re.compile(
         r"(I|i)nterval\[(?P<subtype>[^,]+)(, (?P<closed>(right|left|both|neither)))?\]"
     )
-    _cache: dict[str_type, PandasExtensionDtype] = {}
+    _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
     def __new__(cls, subtype=None, closed: str_type | None = None):
         from pandas.core.dtypes.common import (
@@ -1099,12 +1099,12 @@ def __new__(cls, subtype=None, closed: str_type | None = None):
 
         key = str(subtype) + str(closed)
         try:
-            return cls._cache[key]
+            return cls._cache_dtypes[key]
         except KeyError:
             u = object.__new__(cls)
             u._subtype = subtype
             u._closed = closed
-            cls._cache[key] = u
+            cls._cache_dtypes[key] = u
             return u
 
     @property

pandas/tests/dtypes/test_dtypes.py

@@ -66,15 +66,15 @@ def test_pickle(self, dtype):
 
         # clear the cache
         type(dtype).reset_cache()
-        assert not len(dtype._cache)
+        assert not len(dtype._cache_dtypes)
 
         # force back to the cache
         result = tm.round_trip_pickle(dtype)
         if not isinstance(dtype, PeriodDtype):
             # Because PeriodDtype has a cython class as a base class,
             #  it has different pickle semantics, and its cache is re-populated
             #  on un-pickling.
-            assert not len(dtype._cache)
+            assert not len(dtype._cache_dtypes)
         assert result == dtype
 
 
@@ -791,14 +791,14 @@ def test_basic_dtype(self):
     def test_caching(self):
         IntervalDtype.reset_cache()
         dtype = IntervalDtype("int64", "right")
-        assert len(IntervalDtype._cache) == 1
+        assert len(IntervalDtype._cache_dtypes) == 1
 
         IntervalDtype("interval")
-        assert len(IntervalDtype._cache) == 2
+        assert len(IntervalDtype._cache_dtypes) == 2
 
         IntervalDtype.reset_cache()
         tm.round_trip_pickle(dtype)
-        assert len(IntervalDtype._cache) == 0
+        assert len(IntervalDtype._cache_dtypes) == 0
 
     def test_not_string(self):
         # GH30568: though IntervalDtype has object kind, it cannot be string