Code by Inyeol Lee, submitted to SF bug 595350, to implement

the string/unicode method .replace() with a zero-lengt first argument.
Inyeol contributed tests for this too.

Lib/test/string_tests.py

@@ -202,6 +202,10 @@ def run_method_tests(test):
     test('replace', 'one!two!three!', 'one@two@three@', '!', '@')
     test('replace', 'one!two!three!', 'one!two!three!', 'x', '@')
     test('replace', 'one!two!three!', 'one!two!three!', 'x', '@', 2)
+    test('replace', 'abc', '-a-b-c-', '', '-')
+    test('replace', 'abc', '-a-b-c', '', '-', 3)
+    test('replace', 'abc', 'abc', '', '-', 0)
+    test('replace', '', '', '', '')
     # Next three for SF bug 422088: [OSF1 alpha] string.replace(); died with
     # MemoryError due to empty result (platform malloc issue when requesting
     # 0 bytes).

Lib/test/test_descr.py

@@ -2996,10 +2996,6 @@ def strops():
     except ValueError: pass
     else: raise TestFailed, "''.rindex('5') doesn't raise ValueError"
 
-    try: ''.replace('', '')
-    except ValueError: pass
-    else: raise TestFailed, "''.replace('', '') doesn't raise ValueError"
-
     try: '%(n)s' % None
     except TypeError: pass
     else: raise TestFailed, "'%(n)s' % None doesn't raise TypeError"

Lib/test/test_unicode.py

@@ -210,12 +210,10 @@ test('replace', u'one!two!three!', u'one!two!three!', u'!', u'@', 0)
 test('replace', u'one!two!three!', u'one@two@three@', u'!', u'@')
 test('replace', u'one!two!three!', u'one!two!three!', u'x', u'@')
 test('replace', u'one!two!three!', u'one!two!three!', u'x', u'@', 2)
-try:
-    u"abc".replace(u"", u"x")
-except ValueError:
-    pass
-else:
-    raise TestFailed, "u.replace('', ...) should raise ValueError"
+test('replace', u'abc', u'-a-b-c-', u'', u'-')
+test('replace', u'abc', u'-a-b-c', u'', u'-', 3)
+test('replace', u'abc', u'abc', u'', u'-', 0)
+test('replace', u'', u'', u'', u'')
 
 test('startswith', u'hello', True, u'he')
 test('startswith', u'hello', True, u'hello')

Objects/stringobject.c

@@ -2215,11 +2215,11 @@ mymemreplace(const char *str, int len,		/* input string */
 	char *new_s;
 	int nfound, offset, new_len;
 
-	if (len == 0 || pat_len > len)
+	if (len == 0 || (pat_len == 0 && sub_len == 0) || pat_len > len)
 		goto return_same;
 
 	/* find length of output string */
-	nfound = mymemcnt(str, len, pat, pat_len);
+	nfound = (pat_len > 0) ? mymemcnt(str, len, pat, pat_len) : len + 1;
 	if (count < 0)
 		count = INT_MAX;
 	else if (nfound > count)
@@ -2242,7 +2242,8 @@ mymemreplace(const char *str, int len,		/* input string */
 			return NULL;
 		out_s = new_s;
 
-		for (; count > 0 && len > 0; --count) {
+		if (pat_len > 0) {
+			for (; nfound > 0; --nfound) {
 				/* find index of next instance of pattern */
 				offset = mymemfind(str, len, pat, pat_len);
 				if (offset == -1)
@@ -2262,6 +2263,18 @@ mymemreplace(const char *str, int len,		/* input string */
 			if (len > 0)
 				memcpy(new_s, str, len);
 		}
+		else {
+			for (;;++str, --len) {
+				memcpy(new_s, sub, sub_len);
+				new_s += sub_len;
+				if (--nfound <= 0) {
+					memcpy(new_s, str, len);
+					break;
+				}
+				*new_s++ = *str;
+			}
+		}
+	}
 	*out_len = new_len;
 	return out_s;
 
@@ -2317,10 +2330,6 @@ string_replace(PyStringObject *self, PyObject *args)
 	else if (PyObject_AsCharBuffer(replobj, &repl, &repl_len))
 		return NULL;
 
-	if (sub_len <= 0) {
-		PyErr_SetString(PyExc_ValueError, "empty pattern string");
-		return NULL;
-	}
 	new_s = mymemreplace(str,len,sub,sub_len,repl,repl_len,count,&out_len);
 	if (new_s == NULL) {
 		PyErr_NoMemory();

Objects/unicodeobject.c

@@ -3494,11 +3494,6 @@ PyObject *replace(PyUnicodeObject *self,
 {
     PyUnicodeObject *u;
 
-    if (str1->length == 0) {
-	PyErr_SetString(PyExc_ValueError, "empty pattern string");
-	return NULL;
-    }
-
     if (maxcount < 0)
 	maxcount = INT_MAX;
 
@@ -3549,6 +3544,7 @@ PyObject *replace(PyUnicodeObject *self,
             if (u) {
                 i = 0;
                 p = u->str;
+                if (str1->length > 0) {
                     while (i <= self->length - str1->length)
                         if (Py_UNICODE_MATCH(self, i, str1)) {
                             /* replace string segment */
@@ -3562,6 +3558,16 @@ PyObject *replace(PyUnicodeObject *self,
                             }
                         } else
                             *p++ = self->str[i++];
+                } else {
+                    while (n > 0) {
+                        Py_UNICODE_COPY(p, str2->str, str2->length);
+                        p += str2->length;
+                        if (--n <= 0)
+                            break;
+                        *p++ = self->str[i++];
+                    }
+                    Py_UNICODE_COPY(p, self->str+i, self->length-i);
+                }
             }
         }
     }