apacheGH-41389: [Python] Expose byte_width and bit_width of Extension…

…Type in terms of the storage type (apache#41413)

### Rationale for this change

This update aligns the Python API with Arrow C++ by exposing the actual byte and bit widths of extension types from their storage type.

### What changes are included in this PR?

- Expose byte_width and bit_width properties for ExtensionType in Python, reflecting the underlying storage type.
- Add  unit tests to verify these properties

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes

* GitHub Issue: apache#41389

Lead-authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

python/pyarrow/includes/libarrow.pxd

@@ -2801,6 +2801,8 @@ cdef extern from "arrow/extension_type.h" namespace "arrow":
     cdef cppclass CExtensionType" arrow::ExtensionType"(CDataType):
         c_string extension_name()
         shared_ptr[CDataType] storage_type()
+        int byte_width()
+        int bit_width()
 
         @staticmethod
         shared_ptr[CArray] WrapArray(shared_ptr[CDataType] ext_type,

python/pyarrow/tests/test_extension_type.py

@@ -251,14 +251,14 @@ def test_ext_type_repr():
     assert repr(ty) == "IntegerType(DataType(int64))"
 
 
-def test_ext_type__lifetime():
+def test_ext_type_lifetime():
     ty = UuidType()
     wr = weakref.ref(ty)
     del ty
     assert wr() is None
 
 
-def test_ext_type__storage_type():
+def test_ext_type_storage_type():
     ty = UuidType()
     assert ty.storage_type == pa.binary(16)
     assert ty.__class__ is UuidType
@@ -267,6 +267,32 @@ def test_ext_type__storage_type():
     assert ty.__class__ is ParamExtType
 
 
+def test_ext_type_byte_width():
+    # Test for fixed-size binary types
+    ty = UuidType()
+    assert ty.byte_width == 16
+    ty = ParamExtType(5)
+    assert ty.byte_width == 5
+
+    # Test for non fixed-size binary types
+    ty = LabelType()
+    with pytest.raises(ValueError, match="Non-fixed width type"):
+        _ = ty.byte_width
+
+
+def test_ext_type_bit_width():
+    # Test for fixed-size binary types
+    ty = UuidType()
+    assert ty.bit_width == 128
+    ty = ParamExtType(5)
+    assert ty.bit_width == 40
+
+    # Test for non fixed-size binary types
+    ty = LabelType()
+    with pytest.raises(ValueError, match="Non-fixed width type"):
+        _ = ty.bit_width
+
+
 def test_ext_type_as_py():
     ty = UuidType()
     expected = uuid4()

python/pyarrow/types.pxi

@@ -1519,6 +1519,24 @@ cdef class BaseExtensionType(DataType):
         """
         return pyarrow_wrap_data_type(self.ext_type.storage_type())
 
+    @property
+    def byte_width(self):
+        """
+        The byte width of the extension type.
+        """
+        if self.ext_type.byte_width() == -1:
+            raise ValueError("Non-fixed width type")
+        return self.ext_type.byte_width()
+
+    @property
+    def bit_width(self):
+        """
+        The bit width of the extension type.
+        """
+        if self.ext_type.bit_width() == -1:
+            raise ValueError("Non-fixed width type")
+        return self.ext_type.bit_width()
+
     def wrap_array(self, storage):
         """
         Wrap the given storage array as an extension array.