1. Removed module self test in favor of unittests -- Timbot's suggestion.

2. Replaced calls to Set([]) with Set() -- Timbot's suggestion
3. Fixed subtle bug in sets of sets:

The following code did not work (will add to test suite):
    d = Set('d')
    s = Set([d])  # Stores inner set as an ImmutableSet
    s.remove(d)   # For comparison, wraps d in _TemporarilyImmutableSet

The comparison proceeds by computing the hash of the
_TemporarilyImmutableSet and finding it in the dictionary.
It then verifies equality by calling ImmutableSet.__eq__()
and crashes from the binary sanity check.

The problem is that the code assumed equality would be checked
with _TemporarilyImmutableSet.__eq__().

The solution is to let _TemporarilyImmutableSet derive from BaseSet
so it will pass the sanity check and then to provide it with the
._data element from the wrapped set so that ImmutableSet.__eq__()
will find ._data where it expects.

Since ._data is now provided and because BaseSet is the base class,
_TemporarilyImmutableSet no longer needs .__eq__() or .__ne__().

Note that inheriting all of BaseSet's methods is harmless because
none of those methods (except ones starting with an underscore)
can mutate the .data element.  Also _TemporarilyImmutableSet is only
used internally as is not otherwise visible.

Lib/sets.py

@@ -133,7 +133,7 @@ class BaseSet(object):
 
     def copy(self):
         """Return a shallow copy of a set."""
-        result = self.__class__([])
+        result = self.__class__()
         result._data.update(self._data)
         return result
 
@@ -147,7 +147,7 @@ class BaseSet(object):
         # it can certainly contain an object that has a reference to
         # itself.
         from copy import deepcopy
-        result = self.__class__([])
+        result = self.__class__()
         memo[id(self)] = result
         data = result._data
         value = True
@@ -188,7 +188,7 @@ class BaseSet(object):
             little, big = self, other
         else:
             little, big = other, self
-        result = self.__class__([])
+        result = self.__class__()
         data = result._data
         value = True
         for elt in little:
@@ -210,7 +210,7 @@ class BaseSet(object):
         """
         if not isinstance(other, BaseSet):
             return NotImplemented
-        result = self.__class__([])
+        result = self.__class__()
         data = result._data
         value = True
         for elt in self:
@@ -235,7 +235,7 @@ class BaseSet(object):
         """
         if not isinstance(other, BaseSet):
             return NotImplemented
-        result = self.__class__([])
+        result = self.__class__()
         data = result._data
         value = True
         for elt in self:
@@ -467,7 +467,7 @@ class Set(BaseSet):
         return _TemporarilyImmutableSet(self)
 
 
-class _TemporarilyImmutableSet(object):
+class _TemporarilyImmutableSet(BaseSet):
     # Wrap a mutable set as if it was temporarily immutable.
     # This only supplies hashing and equality comparisons.
 
@@ -475,111 +475,9 @@ class _TemporarilyImmutableSet(object):
 
     def __init__(self, set):
         self._set = set
+        self._data = set._data  # Needed by ImmutableSet.__eq__()
 
     def __hash__(self):
         if self._hashcode is None:
             self._hashcode = self._set._compute_hash()
         return self._hashcode
-
-    def __eq__(self, other):
-        return self._set == other
-
-    def __ne__(self, other):
-        return self._set != other
-
-
-# Rudimentary self-tests
-
-def _test():
-
-    # Empty set
-    red = Set()
-    assert `red` == "Set([])", "Empty set: %s" % `red`
-
-    # Unit set
-    green = Set((0,))
-    assert `green` == "Set([0])", "Unit set: %s" % `green`
-
-    # 3-element set
-    blue = Set([0, 1, 2])
-    assert blue._repr(True) == "Set([0, 1, 2])", "3-element set: %s" % `blue`
-
-    # 2-element set with other values
-    black = Set([0, 5])
-    assert black._repr(True) == "Set([0, 5])", "2-element set: %s" % `black`
-
-    # All elements from all sets
-    white = Set([0, 1, 2, 5])
-    assert white._repr(True) == "Set([0, 1, 2, 5])", "4-element set: %s" % `white`
-
-    # Add element to empty set
-    red.add(9)
-    assert `red` == "Set([9])", "Add to empty set: %s" % `red`
-
-    # Remove element from unit set
-    red.remove(9)
-    assert `red` == "Set([])", "Remove from unit set: %s" % `red`
-
-    # Remove element from empty set
-    try:
-        red.remove(0)
-        assert 0, "Remove element from empty set: %s" % `red`
-    except LookupError:
-        pass
-
-    # Length
-    assert len(red) == 0,   "Length of empty set"
-    assert len(green) == 1, "Length of unit set"
-    assert len(blue) == 3,  "Length of 3-element set"
-
-    # Compare
-    assert green == Set([0]), "Equality failed"
-    assert green != Set([1]), "Inequality failed"
-
-    # Union
-    assert blue  | red   == blue,  "Union non-empty with empty"
-    assert red   | blue  == blue,  "Union empty with non-empty"
-    assert green | blue  == blue,  "Union non-empty with non-empty"
-    assert blue  | black == white, "Enclosing union"
-
-    # Intersection
-    assert blue  & red   == red,   "Intersect non-empty with empty"
-    assert red   & blue  == red,   "Intersect empty with non-empty"
-    assert green & blue  == green, "Intersect non-empty with non-empty"
-    assert blue  & black == green, "Enclosing intersection"
-
-    # Symmetric difference
-    assert red ^ green == green,        "Empty symdiff non-empty"
-    assert green ^ blue == Set([1, 2]), "Non-empty symdiff"
-    assert white ^ white == red,        "Self symdiff"
-
-    # Difference
-    assert red - green == red,           "Empty - non-empty"
-    assert blue - red == blue,           "Non-empty - empty"
-    assert white - black == Set([1, 2]), "Non-empty - non-empty"
-
-    # In-place union
-    orange = Set([])
-    orange |= Set([1])
-    assert orange == Set([1]), "In-place union"
-
-    # In-place intersection
-    orange = Set([1, 2])
-    orange &= Set([2])
-    assert orange == Set([2]), "In-place intersection"
-
-    # In-place difference
-    orange = Set([1, 2, 3])
-    orange -= Set([2, 4])
-    assert orange == Set([1, 3]), "In-place difference"
-
-    # In-place symmetric difference
-    orange = Set([1, 2, 3])
-    orange ^= Set([3, 4])
-    assert orange == Set([1, 2, 4]), "In-place symmetric difference"
-
-    print "All tests passed"
-
-
-if __name__ == "__main__":
-    _test()