Changed the dict implementation to take "string shortcuts" only when

keys are true strings -- no subclasses need apply.  This may be debatable.

The problem is that a str subclass may very well want to override __eq__
and/or __hash__ (see the new example of case-insensitive strings in
test_descr), but go-fast shortcuts for strings are ubiquitous in our dicts
(and subclass overrides aren't even looked for then).  Another go-fast
reason for the change is that PyCheck_StringExact() is a quicker test
than PyCheck_String(), and we make such a test on virtually every access
to every dict.

OTOH, a str subclass may also be perfectly happy using the base str eq
and hash, and this change slows them a lot.  But those cases are still
hypothetical, while Python's own reliance on true-string dicts is not.

Lib/test/test_descr.py

@@ -1533,8 +1533,8 @@ def inherits():
     verify(str(s) == base)
     verify(str(s).__class__ is str)
     verify(hash(s) == hash(base))
-    verify({s: 1}[base] == 1)
-    verify({base: 1}[s] == 1)
+    #XXX verify({s: 1}[base] == 1)
+    #XXX verify({base: 1}[s] == 1)
     verify((s + "").__class__ is str)
     verify(s + "" == base)
     verify(("" + s).__class__ is str)
@@ -1758,6 +1758,39 @@ f = t(%r, 'w')  # rexec can't catch this by itself
         except:
             pass
 
+def str_subclass_as_dict_key():
+    if verbose:
+        print "Testing a str subclass used as dict key .."
+
+    class cistr(str):
+        """Sublcass of str that computes __eq__ case-insensitively.
+
+        Also computes a hash code of the string in canonical form.
+        """
+
+        def __init__(self, value):
+            self.canonical = value.lower()
+            self.hashcode = hash(self.canonical)
+
+        def __eq__(self, other):
+            if not isinstance(other, cistr):
+                other = cistr(other)
+            return self.canonical == other.canonical
+
+        def __hash__(self):
+            return self.hashcode
+
+    verify('aBc' == cistr('ABC') == 'abc')
+    verify(str(cistr('ABC')) == 'ABC')
+
+    d = {cistr('one'): 1, cistr('two'): 2, cistr('tHree'): 3}
+    verify(d[cistr('one')] == 1)
+    verify(d[cistr('tWo')] == 2)
+    verify(d[cistr('THrEE')] == 3)
+    verify(cistr('ONe') in d)
+    verify(d.get(cistr('thrEE')) == 3)
+
+
 def all():
     lists()
     dicts()
@@ -1794,6 +1827,7 @@ def all():
     inherits()
     keywords()
     restricted()
+    str_subclass_as_dict_key()
 
 all()
 

Objects/dictobject.c

@@ -298,8 +298,8 @@ Done:
  * means we don't need to go through PyObject_Compare(); we can always use
  * _PyString_Eq directly.
  *
- * This really only becomes meaningful if proper error handling in lookdict()
- * is too expensive.
+ * This is valuable because the general-case error handling in lookdict() is
+ * expensive, and dicts with pure-string keys are very common.
  */
 static dictentry *
 lookdict_string(dictobject *mp, PyObject *key, register long hash)
@@ -311,8 +311,11 @@ lookdict_string(dictobject *mp, PyObject *key, register long hash)
 	dictentry *ep0 = mp->ma_table;
 	register dictentry *ep;
 
-	/* make sure this function doesn't have to handle non-string keys */
-	if (!PyString_Check(key)) {
+	/* Make sure this function doesn't have to handle non-string keys,
+	   including subclasses of str; e.g., one reason to subclass
+	   strings is to override __eq__, and for speed we don't cater to
+	   that here. */
+	if (!PyString_CheckExact(key)) {
 #ifdef SHOW_CONVERSION_COUNTS
 		++converted;
 #endif
@@ -478,7 +481,7 @@ PyDict_GetItem(PyObject *op, PyObject *key)
 		return NULL;
 	}
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -510,7 +513,7 @@ PyDict_SetItem(register PyObject *op, PyObject *key, PyObject *value)
 	}
 	mp = (dictobject *)op;
 #ifdef CACHE_HASH
-	if (PyString_Check(key)) {
+	if (PyString_CheckExact(key)) {
 #ifdef INTERN_STRINGS
 		if (((PyStringObject *)key)->ob_sinterned != NULL) {
 			key = ((PyStringObject *)key)->ob_sinterned;
@@ -562,7 +565,7 @@ PyDict_DelItem(PyObject *op, PyObject *key)
 		return -1;
 	}
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -842,7 +845,7 @@ dict_subscript(dictobject *mp, register PyObject *key)
 	long hash;
 	assert(mp->ma_table != NULL);
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -1358,7 +1361,7 @@ dict_has_key(register dictobject *mp, PyObject *key)
 	long hash;
 	register long ok;
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -1382,7 +1385,7 @@ dict_get(register dictobject *mp, PyObject *args)
 		return NULL;
 
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -1411,7 +1414,7 @@ dict_setdefault(register dictobject *mp, PyObject *args)
 		return NULL;
 
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{
@@ -1647,7 +1650,7 @@ dict_contains(dictobject *mp, PyObject *key)
 	long hash;
 
 #ifdef CACHE_HASH
-	if (!PyString_Check(key) ||
+	if (!PyString_CheckExact(key) ||
 	    (hash = ((PyStringObject *) key)->ob_shash) == -1)
 #endif
 	{