API: Added ExtensionArray constructors

Adds two new (private, but part of the interface) constructors to EA.

Closes pandas-dev#19906

Author: TomAugspurger

pandas/core/arrays/base.py

@@ -18,6 +18,8 @@ class ExtensionArray(object):
     The interface includes the following abstract methods that must be
     implemented by subclasses:
 
+    * _from_extension_array
+    * _from_scalars
     * __getitem__
     * __len__
     * dtype
@@ -56,6 +58,53 @@ class ExtensionArray(object):
     # '_typ' is for pandas.core.dtypes.generic.ABCExtensionArray.
     # Don't override this.
     _typ = 'extension'
+
+    # ------------------------------------------------------------------------
+    # Constructors
+    # ------------------------------------------------------------------------
+    @classmethod
+    def _from_extension_array(cls, array, copy=True):
+        """Construct a new ExtensionArray from an existing instance.
+
+        Parameters
+        ----------
+        array : ExtensionArray
+            An extension array of the same type as cls.
+        copy : bool, default True
+            Whether a copy should be made using ``array.copy``. Note that
+            even if ``copy=False`` there's no guarantee that the underlying
+            data of the two arrays is the same.
+
+        Returns
+        -------
+        ExtensionArray
+
+        See Also
+        --------
+        _from_scalars
+        """
+        raise AbstractMethodError(cls)
+
+    @classmethod
+    def _from_scalars(cls, scalars):
+        """Construct a new ExtensionArray from a sequence of scalars.
+
+        Parameters
+        ----------
+        scalars : Sequence
+            Each element will be an instance of the scalar type for this
+            array, ``cls.dtype.type``.
+
+        Returns
+        -------
+        ExtensionArray
+
+        See Also
+        --------
+        _from_extension_array
+        """
+        raise AbstractMethodError(cls)
+
     # ------------------------------------------------------------------------
     # Must be a Sequence
     # ------------------------------------------------------------------------

pandas/tests/extension/decimal/array.py

@@ -32,6 +32,14 @@ def __init__(self, values):
 
         self.values = values
 
+    @classmethod
+    def _from_extension_array(cls, array, copy=True):
+        return cls(array)
+
+    @classmethod
+    def _from_scalars(cls, scalars):
+        return cls(scalars)
+
     def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self.values[item]

pandas/tests/extension/json/array.py

@@ -33,13 +33,21 @@ def __init__(self, values):
                 raise TypeError
         self.data = values
 
+    @classmethod
+    def _from_extension_array(cls, array, copy=True):
+        return cls(array)
+
+    @classmethod
+    def _from_scalars(cls, scalars):
+        return cls(scalars)
+
     def __getitem__(self, item):
         if isinstance(item, numbers.Integral):
             return self.data[item]
         elif isinstance(item, np.ndarray) and item.dtype == 'bool':
-            return type(self)([x for x, m in zip(self, item) if m])
+            return self._from_scalars([x for x, m in zip(self, item) if m])
         else:
-            return type(self)(self.data[item])
+            return self._from_extension_array(self.data[item])
 
     def __setitem__(self, key, value):
         if isinstance(key, numbers.Integral):
@@ -77,10 +85,10 @@ def isna(self):
     def take(self, indexer, allow_fill=True, fill_value=None):
         output = [self.data[loc] if loc != -1 else self._na_value
                   for loc in indexer]
-        return type(self)(output)
+        return self._from_scalars(output)
 
     def copy(self, deep=False):
-        return type(self)(self.data[:])
+        return self._from_extension_array(self.data[:])
 
     @property
     def _na_value(self):