kszucs commented on a change in pull request #7519:
URL: https://github.com/apache/arrow/pull/7519#discussion_r447731034



##########
File path: python/pyarrow/scalar.pxi
##########
@@ -16,1198 +16,745 @@
 # under the License.
 
 
-_NULL = NA = None
+import collections
 
 
 cdef class Scalar:
     """
-    The base class for all array elements.
+    The base class for scalars.
     """
 
+    def __init__(self):
+        raise TypeError("Do not call {}'s constructor directly, use "
+                        "pa.scalar() instead.".format(self.__class__.__name__))
 
-cdef class NullType(Scalar):
-    """
-    Singleton for null array elements.
-    """
-    # TODO rename this NullValue?
+    cdef void init(self, const shared_ptr[CScalar]& wrapped):
+        self.wrapped = wrapped
 
-    def __cinit__(self):
-        global NA
-        if NA is not None:
-            raise Exception('Cannot create multiple NAType instances')
+    @staticmethod
+    cdef wrap(const shared_ptr[CScalar]& wrapped):
+        cdef:
+            Scalar self
+            Type type_id = wrapped.get().type.get().id()
+
+        if type_id == _Type_NA:
+            return _NULL
+
+        typ = _scalar_classes[type_id]
+        self = typ.__new__(typ)
+        self.init(wrapped)
+
+        return self
+
+    cdef inline shared_ptr[CScalar] unwrap(self) nogil:
+        return self.wrapped
+
+    @property
+    def type(self):
+        return pyarrow_wrap_data_type(self.wrapped.get().type)
 
-        self.type = null()
+    @property
+    def is_valid(self):
+        return self.wrapped.get().is_valid
 
     def __repr__(self):
-        return 'NULL'
+        return '<pyarrow.{}: {!r}>'.format(
+            self.__class__.__name__, self.as_py()
+        )
 
-    def as_py(self):
-        """
-        Return None
-        """
-        return None
+    def __str__(self):
+        return str(self.as_py())
+
+    def equals(self, Scalar other):
+        return self.wrapped.get().Equals(other.unwrap().get()[0])
 
     def __eq__(self, other):
-        return NA
+        try:
+            if not isinstance(other, Scalar):
+                other = scalar(other, type=self.type)
+            return self.equals(other)
+        except (TypeError, ValueError, ArrowInvalid):
+            return NotImplemented
+
+    def __hash__(self):
+        cdef CScalarHash hasher
+        return hasher(self.wrapped)
+
+    def as_py(self):
+        raise NotImplementedError()
 
 
-_NULL = NA = NullType()
+_NULL = NA = None
 
 
-cdef class ArrayValue(Scalar):
+cdef class NullScalar(Scalar):
     """
-    The base class for non-null array elements.
+    Concrete class for null scalars.
     """
 
-    def __init__(self):
-        raise TypeError("Do not call {}'s constructor directly, use array "
-                        "subscription instead."
-                        .format(self.__class__.__name__))
+    def __cinit__(self):
+        global NA
+        if NA is not None:
+            raise Exception('Cannot create multiple NAType instances')
+        self.init(shared_ptr[CScalar](new CNullScalar()))
 
-    cdef void init(self, DataType type, const shared_ptr[CArray]& sp_array,
-                   int64_t index):
-        self.type = type
-        self.index = index
-        self._set_array(sp_array)
+    def __init__(self):
+        pass
 
-    cdef void _set_array(self, const shared_ptr[CArray]& sp_array):
-        self.sp_array = sp_array
+    def __eq__(self, other):
+        return NA

Review comment:
       Well, it was the behavior before the refactor. Shall we check proper 
equality with NullScalar instead?




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