Switched SortedList base class to Sequence

Since SortedList does not implement the MutableSequence interface (many
of MutableSequence's mixin methods return NotImplemented), it should not
derive from MutableSequence.

This change will allow type checkers to properly reject SortedList as an
argument to a function that takes MutableSequence.

This is a breaking change (albeit a minor one).

docs/introduction.rst

@@ -166,19 +166,19 @@ at an index which is not supported by :class:`SortedList`.
     >>> sl.reverse()
     Traceback (most recent call last):
       ...
-    NotImplementedError: use ``reversed(sl)`` instead
+    AttributeError: use ``reversed(sl)`` instead
     >>> sl.append('f')
     Traceback (most recent call last):
       ...
-    NotImplementedError: use ``sl.add(value)`` instead
+    AttributeError: use ``sl.add(value)`` instead
     >>> sl.extend(['f', 'g', 'h'])
     Traceback (most recent call last):
       ...
-    NotImplementedError: use ``sl.update(values)`` instead
+    AttributeError: use ``sl.update(values)`` instead
     >>> sl.insert(5, 'f')
     Traceback (most recent call last):
       ...
-    NotImplementedError: use ``sl.add(value)`` instead
+    AttributeError: use ``sl.add(value)`` instead
 
 Comparison with :class:`SortedList` uses lexicographical ordering as with other
 sequence types.

docs/sortedlist.rst

@@ -41,10 +41,6 @@ SortedList
    .. automethod:: __repr__
    .. automethod:: _check
    .. automethod:: _reset
-   .. automethod:: append
-   .. automethod:: extend
-   .. automethod:: insert
-   .. automethod:: reverse
    .. automethod:: __setitem__
 
 

sortedcontainers/sortedlist.py

@@ -30,9 +30,9 @@
 ###############################################################################
 
 try:
-    from collections.abc import Sequence, MutableSequence
+    from collections.abc import Sequence
 except ImportError:
-    from collections import Sequence, MutableSequence
+    from collections import Sequence
 
 from functools import wraps
 from sys import hexversion
@@ -82,7 +82,7 @@ def wrapper(self):
 ###############################################################################
 
 
-class SortedList(MutableSequence):
+class SortedList(Sequence):
     """Sorted list is a sorted mutable sequence.
 
     Sorted list values are maintained in sorted order.
@@ -134,8 +134,14 @@ class SortedList(MutableSequence):
     Sorted lists use lexicographical ordering semantics when compared to other
     sequences.
 
-    Some methods of mutable sequences are not supported and will raise
-    not-implemented error.
+    .. versionchanged:: 3.0
+
+        SortedLists are mutable sequences but they do not implement the
+        :class:`collections.abc.MutableSequence` interface. In version 3.0, the
+        base class was switched to :class:`collections.abc.Sequence` and the
+        ``append``, ``extend``, ``reverse`` and ``insert`` methods, which
+        previously raised :py:exc:`NotImplementedError` when called, were
+        removed.
 
     """
     DEFAULT_LOAD_FACTOR = 1000
@@ -932,24 +938,6 @@ def __reversed__(self):
         return chain.from_iterable(map(reversed, reversed(self._lists)))
 
 
-    def reverse(self):
-        """Raise not-implemented error.
-
-        Sorted list maintains values in ascending sort order. Values may not be
-        reversed in-place.
-
-        Use ``reversed(sl)`` for an iterator over values in descending sort
-        order.
-
-        Implemented to override `MutableSequence.reverse` which provides an
-        erroneous default implementation.
-
-        :raises NotImplementedError: use ``reversed(sl)`` instead
-
-        """
-        raise NotImplementedError('use ``reversed(sl)`` instead')
-
-
     def islice(self, start=None, stop=None, reverse=False):
         """Return an iterator that slices sorted list from `start` to `stop`.
 
@@ -1273,38 +1261,20 @@ def copy(self):
 
     __copy__ = copy
 
+    def __getattr__(self, key):
+        if key == 'append':
+            msg = 'use ``sl.add(value)`` instead'
+        elif key == 'extend':
+            msg = 'use ``sl.update(values)`` instead'
+        elif key == 'reverse':
+            msg = 'use ``reversed(sl)`` instead'
+        elif key == 'insert':
+            msg = 'use ``sl.add(value)`` instead'
+        else:
+            msg = "'%s' object has no attribute '%s'" % (type(self).__name__,
+                                                         key)
 
-    def append(self, value):
-        """Raise not-implemented error.
-
-        Implemented to override `MutableSequence.append` which provides an
-        erroneous default implementation.
-
-        :raises NotImplementedError: use ``sl.add(value)`` instead
-
-        """
-        raise NotImplementedError('use ``sl.add(value)`` instead')
-
-
-    def extend(self, values):
-        """Raise not-implemented error.
-
-        Implemented to override `MutableSequence.extend` which provides an
-        erroneous default implementation.
-
-        :raises NotImplementedError: use ``sl.update(values)`` instead
-
-        """
-        raise NotImplementedError('use ``sl.update(values)`` instead')
-
-
-    def insert(self, index, value):
-        """Raise not-implemented error.
-
-        :raises NotImplementedError: use ``sl.add(value)`` instead
-
-        """
-        raise NotImplementedError('use ``sl.add(value)`` instead')
+        raise AttributeError(msg)
 
 
     def pop(self, index=-1):

tests/test_coverage_sortedkeylist_modulo.py

@@ -326,7 +326,7 @@ def test_reversed():
 
 def test_reverse():
     slt = SortedKeyList(range(10000), key=modulo)
-    with pytest.raises(NotImplementedError):
+    with pytest.raises(AttributeError):
         slt.reverse()
 
 def test_islice():

tests/test_coverage_sortedkeylist_negate.py

@@ -281,7 +281,7 @@ def test_reversed():
 
 def test_reverse():
     slt = SortedKeyList(range(10000), key=negate)
-    with pytest.raises(NotImplementedError):
+    with pytest.raises(AttributeError):
         slt.reverse()
 
 def test_islice():

tests/test_coverage_sortedlist.py

@@ -8,6 +8,11 @@
 from itertools import chain
 import pytest
 
+try:
+    from collections import abc
+except ImportError:
+    import collections as abc
+
 if hexversion < 0x03000000:
     from itertools import izip as zip
     range = xrange
@@ -266,7 +271,7 @@ def test_reversed():
 
 def test_reverse():
     slt = SortedList(range(10000))
-    with pytest.raises(NotImplementedError):
+    with pytest.raises(AttributeError):
         slt.reverse()
 
 def test_islice():
@@ -604,3 +609,29 @@ def test_check():
     slt._len = 5
     with pytest.raises(AssertionError):
         slt._check()
+
+
+@pytest.mark.parametrize("base_class_name",
+                         ["Sequence", "Reversible", "Iterable",
+                          "Sized", "Collection"])
+def test_abstract_base_classes(base_class_name):
+    # Some of these were introduced in later versions of Python
+    base_class = getattr(abc, base_class_name, None)
+    if base_class is None:
+        pytest.skip("%s introduced in a later version of Python" %
+                    base_class_name)
+
+    sl = SortedList()
+    assert isinstance(sl, base_class)
+
+
+def test_not_mutable_sequence_instance():
+    """
+    SortedList does not implement the MutableSequence interface, but has custom
+    behavior for all the methods of a MutableSquence, so we want to make sure
+    that it still fails an `isinstance` check.
+    """
+
+    sl = SortedList()
+    assert not isinstance(sl, abc.MutableSequence)
+