Ouch. The test suite *really* needs work!!!!! There were several

superficial errors and one deep one that aren't currently caught.  I'm
headed for bed after this checkin.

- Fixed several typos introduced by Raymond Hettinger (through
  cut-n-paste from my template): it's _as_temporarily_immutable, not
  _as_temporary_immutable, and moreover when the element is added, we
  should use _as_immutable.

- Made the seq argument to ImmutableSet.__init__ optional, so we can
  write ImmutableSet() to create an immutable empty set.

- Rename the seq argument to Set and ImmutableSet to iterable.

- Add a Set.__hash__ method that raises a TypeError.  We inherit a
  default __hash__ implementation from object, and we don't want that.
  We can then catch this in update(), so that
  e.g. s.update([Set([1])]) will transform the Set([1]) to
  ImmutableSet([1]).

- Added the dance to catch TypeError and try _as_immutable in the
  constructors too (by calling _update()).  This is needed so that
  Set([Set([1])]) is correctly interpreted as
  Set([ImmutableSet([1])]).  (I was puzzled by a side effect of this
  and the inherited __hash__ when comparing two sets of sets while
  testing different powerset implementations: the Set element passed
  to a Set constructor wasn't transformed to an ImmutableSet, and then
  the dictionary didn't believe the Set found in one dict it was the
  same as ImmutableSet in the other, because the hashes were
  different.)

- Refactored Set.update() and both __init__() methods; moved the body
  of update() into BaseSet as _update(), and call this from __init__()
  and update().

- Changed the NotImplementedError in BaseSet.__init__ to TypeError,
  both for consistency with basestring() and because we have to use
  TypeError when denying Set.__hash__.  Together those provide
  sufficient evidence that an unimplemented method needs to raise
  TypeError.

Lib/sets.py

@@ -52,8 +52,8 @@ what's tested is actually `z in y'.
 # - Guido van Rossum rewrote much of the code, made some API changes,
 #   and cleaned up the docstrings.
 #
-# - Raymond Hettinger implemented a number of speedups and other
-#   improvements.
+# - Raymond Hettinger added a number of speedups and other
+#   bugs^H^H^H^Himprovements.
 
 
 __all__ = ['BaseSet', 'Set', 'ImmutableSet']
@@ -70,9 +70,8 @@ class BaseSet(object):
         """This is an abstract class."""
         # Don't call this from a concrete subclass!
         if self.__class__ is BaseSet:
-            # XXX Maybe raise TypeError instead, like basestring()?
-            raise NotImplementedError, ("BaseSet is an abstract class.  "
-                                        "Use Set or ImmutableSet.")
+            raise TypeError, ("BaseSet is an abstract class.  "
+                              "Use Set or ImmutableSet.")
 
     # Standard protocols: __len__, __repr__, __str__, __iter__
 
@@ -233,7 +232,7 @@ class BaseSet(object):
         try:
             return element in self._data
         except TypeError:
-            transform = getattr(element, "_as_temporary_immutable", None)
+            transform = getattr(element, "_as_temporarily_immutable", None)
             if transform is None:
                 raise # re-raise the TypeError exception we caught
             return transform() in self._data
@@ -279,6 +278,21 @@ class BaseSet(object):
             result ^= hash(elt)
         return result
 
+    def _update(self, iterable):
+        # The main loop for update() and the subclass __init__() methods.
+        # XXX This can be optimized a bit by first trying the loop
+        # without setting up a try/except for each element.
+        data = self._data
+        value = True
+        for element in iterable:
+            try:
+                data[element] = value
+            except TypeError:
+                transform = getattr(element, "_as_immutable", None)
+                if transform is None:
+                    raise # re-raise the TypeError exception we caught
+                data[transform()] = value
+
 
 class ImmutableSet(BaseSet):
     """Immutable set class."""
@@ -287,26 +301,12 @@ class ImmutableSet(BaseSet):
 
     # BaseSet + hashing
 
-    def __init__(self, seq):
-        """Construct an immutable set from a sequence."""
-        # XXX Maybe this should default seq to None?
-        # XXX Creating an empty immutable set is not unheard of.
+    def __init__(self, iterable=None):
+        """Construct an immutable set from an optional iterable."""
         self._hashcode = None
-        self._data = data = {}
-        # I don't know a faster way to do this in pure Python.
-        # Custom code written in C only did it 65% faster,
-        # preallocating the dict to len(seq); without
-        # preallocation it was only 25% faster.  So the speed of
-        # this Python code is respectable.  Just copying True into
-        # a local variable is responsible for a 7-8% speedup.
-        value = True
-        # XXX Should this perhaps look for _as_immutable?
-        # XXX If so, should use self.update(seq).
-        # XXX (Well, ImmutableSet doesn't have update(); the base
-        # XXX class could have _update() which does this though, and
-        # XXX we could use that here and in Set.update().)
-        for key in seq:
-            data[key] = value
+        self._data = {}
+        if iterable is not None:
+            self._update(iterable)
 
     def __hash__(self):
         if self._hashcode is None:
@@ -321,15 +321,16 @@ class Set(BaseSet):
 
     # BaseSet + operations requiring mutability; no hashing
 
-    def __init__(self, seq=None):
-        """Construct an immutable set from a sequence."""
-        self._data = data = {}
-        if seq is not None:
-            value = True
-            # XXX Should this perhaps look for _as_immutable?
-            # XXX If so, should use self.update(seq).
-            for key in seq:
-                data[key] = value
+    def __init__(self, iterable=None):
+        """Construct a set from an optional iterable."""
+        self._data = {}
+        if iterable is not None:
+            self._update(iterable)
+
+    def __hash__(self):
+        """A Set cannot be hashed."""
+        # We inherit object.__hash__, so we must deny this explicitly
+        raise TypeError, "Can't hash a Set, only an ImmutableSet."
 
     # In-place union, intersection, differences
 
@@ -380,16 +381,7 @@ class Set(BaseSet):
 
     def update(self, iterable):
         """Add all values from an iterable (such as a list or file)."""
-        data = self._data
-        value = True
-        for element in iterable:
-            try:
-                data[element] = value
-            except TypeError:
-                transform = getattr(element, "_as_temporary_immutable", None)
-                if transform is None:
-                    raise # re-raise the TypeError exception we caught
-                data[transform()] = value
+        self._update(iterable)
 
     def clear(self):
         """Remove all elements from this set."""
@@ -405,7 +397,7 @@ class Set(BaseSet):
         try:
             self._data[element] = True
         except TypeError:
-            transform = getattr(element, "_as_temporary_immutable", None)
+            transform = getattr(element, "_as_immutable", None)
             if transform is None:
                 raise # re-raise the TypeError exception we caught
             self._data[transform()] = True
@@ -418,7 +410,7 @@ class Set(BaseSet):
         try:
             del self._data[element]
         except TypeError:
-            transform = getattr(element, "_as_temporary_immutable", None)
+            transform = getattr(element, "_as_temporarily_immutable", None)
             if transform is None:
                 raise # re-raise the TypeError exception we caught
             del self._data[transform()]