TST: base test for ExtensionArray.astype to its own type + copy keyword

pandas/core/arrays/boolean.py

@@ -375,7 +375,10 @@ def astype(self, dtype, copy: bool = True) -> ArrayLike:
 
         if isinstance(dtype, BooleanDtype):
             values, mask = coerce_to_array(self, copy=copy)
-            return BooleanArray(values, mask, copy=False)
+            if not copy:
+                return self
+            else:
+                return BooleanArray(values, mask, copy=False)
         elif isinstance(dtype, StringDtype):
             return dtype.construct_array_type()._from_sequence(self, copy=False)
 

pandas/core/arrays/numpy_.py

@@ -5,7 +5,7 @@
 from numpy.lib.mixins import NDArrayOperatorsMixin
 
 from pandas._libs import lib
-from pandas._typing import Scalar
+from pandas._typing import Scalar, ArrayLike
 from pandas.compat.numpy import function as nv
 from pandas.util._decorators import doc
 from pandas.util._validators import validate_fillna_kwargs
@@ -276,6 +276,15 @@ def __setitem__(self, key, value) -> None:
 
         self._ndarray[key] = value
 
+    def astype(self, dtype, copy: bool = True) -> ArrayLike:
+        if dtype == self.dtype:
+            if not copy:
+                return self
+            elif copy:
+                return self.copy()
+        else:
+            return super().astype(dtype, copy)
+
     def isna(self) -> np.ndarray:
         return isna(self._ndarray)
 

pandas/core/arrays/period.py

@@ -577,7 +577,11 @@ def astype(self, dtype, copy: bool = True):
         # We handle Period[T] -> Period[U]
         # Our parent handles everything else.
         dtype = pandas_dtype(dtype)
-
+        if dtype == self._dtype:
+            if not copy:
+                return self
+            elif copy:
+                return self.copy()
         if is_period_dtype(dtype):
             return self.asfreq(dtype.freq)
         return super().astype(dtype, copy=copy)

pandas/core/arrays/sparse/array.py

@@ -1063,6 +1063,11 @@ def astype(self, dtype=None, copy=True):
         IntIndex
         Indices: array([2, 3], dtype=int32)
         """
+        if dtype == self._dtype:
+            if not copy:
+                return self
+            elif copy:
+                return self.copy()
         dtype = self.dtype.update_dtype(dtype)
         subtype = dtype._subtype_with_str
         # TODO copy=False is broken for astype_nansafe with int -> float, so cannot

pandas/core/arrays/string_.py

@@ -263,6 +263,11 @@ def fillna(self, value=None, method=None, limit=None):
         return super().fillna(value, method, limit)
 
     def astype(self, dtype, copy=True):
+        if dtype == self.dtype:
+            if not copy:
+                return self
+            elif copy:
+                return self.copy()
         dtype = pandas_dtype(dtype)
         if isinstance(dtype, StringDtype):
             if copy:

pandas/tests/extension/base/casting.py

@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 import pandas as pd
 from pandas.core.internals import ObjectBlock
@@ -56,3 +57,9 @@ def test_astype_empty_dataframe(self, dtype):
         df = pd.DataFrame()
         result = df.astype(dtype)
         self.assert_frame_equal(result, df)
+
+    @pytest.mark.parametrize("copy", [True, False])
+    def test_astype_own_type(self, data, copy):
+        result = data.astype(data.dtype, copy=copy)
+        assert (result is data) is (not copy)
+        self.assert_extension_array_equal(result, data)

pandas/tests/extension/decimal/array.py

@@ -131,6 +131,11 @@ def copy(self):
         return type(self)(self._data.copy())
 
     def astype(self, dtype, copy=True):
+        if dtype == self._dtype:
+            if not copy:
+                return self
+            elif copy:
+                return self.copy()
         dtype = pandas_dtype(dtype)
         if isinstance(dtype, type(self.dtype)):
             return type(self)(self._data, context=dtype.context)