Allow 'continue' inside 'try' clause

SF patch 102989 by Thomas Wouters

Doc/ref/ref7.tex

@@ -260,17 +260,19 @@ The \keyword{try}...\keyword{finally} form specifies a `cleanup' handler.  The
 \keyword{try} clause, the exception is temporarily saved, the
 \keyword{finally} clause is executed, and then the saved exception is
 re-raised.  If the \keyword{finally} clause raises another exception or
-executes a \keyword{return}, \keyword{break} or \keyword{continue} statement,
-the saved exception is lost.  The exception information is not
-available to the program during execution of the \keyword{finally}
-clause.
+executes a \keyword{return} or \keyword{break} statement, the saved
+exception is lost.  A \keyword{continue} statement is illegal in the
+\keyword{finally} clause.  (The reason is a problem with the current
+implementation -- thsi restriction may be lifted in the future).  The
+exception information is not available to the program during execution of
+the \keyword{finally} clause.
 \kwindex{finally}
 
-When a \keyword{return} or \keyword{break} statement is executed in the
-\keyword{try} suite of a \keyword{try}...\keyword{finally} statement, the
-\keyword{finally} clause is also executed `on the way out.'  A
-\keyword{continue} statement is illegal in the \keyword{try} clause.  (The
-reason is a problem with the current implementation --- this
+When a \keyword{return}, \keyword{break} or \keyword{continue} statement is
+executed in the \keyword{try} suite of a \keyword{try}...\keyword{finally}
+statement, the \keyword{finally} clause is also executed `on the way out.' A
+\keyword{continue} statement is illegal in the \keyword{finally} clause.
+(The reason is a problem with the current implementation --- this
 restriction may be lifted in the future).
 \stindex{return}
 \stindex{break}

Include/opcode.h

@@ -104,6 +104,7 @@ extern "C" {
 
 #define LOAD_GLOBAL	116	/* Index in name list */
 
+#define CONTINUE_LOOP	119	/* Start of loop (absolute) */
 #define SETUP_LOOP	120	/* Target address (absolute) */
 #define SETUP_EXCEPT	121	/* "" */
 #define SETUP_FINALLY	122	/* "" */

Lib/dis.py

@@ -259,6 +259,7 @@ jrel_op('FOR_LOOP', 114)        # Number of bytes to skip
 
 name_op('LOAD_GLOBAL', 116)     # Index in name list
 
+jabs_op('CONTINUE_LOOP', 119)   # Target address
 jrel_op('SETUP_LOOP', 120)      # Distance to target address
 jrel_op('SETUP_EXCEPT', 121)    # ""
 jrel_op('SETUP_FINALLY', 122)   # ""

Lib/test/output/test_exceptions

@@ -27,11 +27,7 @@ RuntimeError
 (not used any more?)
 spam
 SyntaxError
-'continue' not supported inside 'try' clause
-ok
-'continue' not supported inside 'try' clause
-ok
-'continue' not supported inside 'try' clause
+'continue' not supported inside 'finally' clause
 ok
 'continue' not properly in loop
 ok

Lib/test/output/test_grammar

@@ -33,6 +33,8 @@ pass_stmt
 flow_stmt
 break_stmt
 continue_stmt
+continue + try/except ok
+continue + try/finally ok
 return_stmt
 raise_stmt
 import_stmt

Lib/test/test_exceptions.py

@@ -104,28 +104,11 @@ def ckmsg(src, msg):
 s = '''\
 while 1:
     try:
-        continue
-    except:
         pass
-'''
-ckmsg(s, "'continue' not supported inside 'try' clause")
-s = '''\
-while 1:
-    try:
-        continue
     finally:
-        pass
-'''
-ckmsg(s, "'continue' not supported inside 'try' clause")
-s = '''\
-while 1:
-    try:
-        if 1:
         continue
-    finally:
-        pass
 '''
-ckmsg(s, "'continue' not supported inside 'try' clause")
+ckmsg(s, "'continue' not supported inside 'finally' clause")
 s = '''\
 try:
     continue

Lib/test/test_grammar.py

@@ -349,6 +349,25 @@ print 'continue_stmt' # 'continue'
 i = 1
 while i: i = 0; continue
 
+msg = ""
+while not msg:
+    msg = "continue + try/except ok"
+    try:
+        continue
+        msg = "continue failed to continue inside try"
+    except:
+        msg = "continue inside try called except block"
+print msg
+
+msg = ""
+while not msg:
+    msg = "finally block not called"
+    try:
+        continue
+    finally:
+        msg = "continue + try/finally ok"
+print msg
+    
 print 'return_stmt' # 'return' [testlist]
 def g1(): return
 def g2(): return 1

Python/ceval.c

@@ -322,7 +322,8 @@ enum why_code {
 		WHY_EXCEPTION,	/* Exception occurred */
 		WHY_RERAISE,	/* Exception re-raised by 'finally' */
 		WHY_RETURN,	/* 'return' statement */
-		WHY_BREAK	/* 'break' statement */
+		WHY_BREAK,	/* 'break' statement */
+		WHY_CONTINUE,	/* 'continue' statement */
 };
 
 static enum why_code do_raise(PyObject *, PyObject *, PyObject *);
@@ -1358,6 +1359,11 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 			why = WHY_BREAK;
 			break;
 		
+		case CONTINUE_LOOP:
+			retval = PyInt_FromLong(oparg);
+			why = WHY_CONTINUE;
+			break;
+
 		case RAISE_VARARGS:
 			u = v = w = NULL;
 			switch (oparg) {
@@ -1419,7 +1425,8 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 			v = POP();
 			if (PyInt_Check(v)) {
 				why = (enum why_code) PyInt_AsLong(v);
-				if (why == WHY_RETURN)
+				if (why == WHY_RETURN ||
+				    why == CONTINUE_LOOP)
 					retval = POP();
 			}
 			else if (PyString_Check(v) || PyClass_Check(v)) {
@@ -2110,6 +2117,18 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 
 		while (why != WHY_NOT && f->f_iblock > 0) {
 			PyTryBlock *b = PyFrame_BlockPop(f);
+
+			if (b->b_type == SETUP_LOOP && why == WHY_CONTINUE) {
+				/* For a continue inside a try block,
+				   don't pop the block for the loop. */
+				PyFrame_BlockSetup(f, b->b_type, b->b_level, 
+						   b->b_handler);
+				why = WHY_NOT;
+				JUMPTO(PyInt_AS_LONG(retval));
+				Py_DECREF(retval);
+				break;
+			}
+
 			while (STACK_LEVEL() > b->b_level) {
 				v = POP();
 				Py_XDECREF(v);
@@ -2145,7 +2164,8 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 					PUSH(exc);
 				}
 				else {
-					if (why == WHY_RETURN)
+					if (why == WHY_RETURN ||
+					    why == CONTINUE_LOOP)
 						PUSH(retval);
 					v = PyInt_FromLong((long)why);
 					PUSH(v);

Python/compile.c

@@ -5,7 +5,7 @@
    XXX add __doc__ attribute == co_doc to code object attributes?
    XXX   (it's currently the first item of the co_const tuple)
    XXX Generate simple jump for break/return outside 'try...finally'
-   XXX Allow 'continue' inside try-finally
+   XXX Allow 'continue' inside finally clause of try-finally
    XXX New opcode for loading the initial index for a for loop
    XXX other JAR tricks?
 */
@@ -3247,17 +3247,22 @@ com_continue_stmt(struct compiling *c, node *n)
 	}
 	else {
 		int j;
-		for (j = 0; j <= i; ++j) {
+		for (j = i-1; j >= 0; --j) {
 			if (c->c_block[j] == SETUP_LOOP)
 				break;
 		}
-		if (j < i+1) {
+		if (j >= 0) {
 			/* there is a loop, but something interferes */
-			for (++j; j <= i; ++j) {
-				if (c->c_block[i] == SETUP_EXCEPT
-				    || c->c_block[i] == SETUP_FINALLY) {
+			for (; i > j; --i) {
+				if (c->c_block[i] == SETUP_EXCEPT ||
+				    c->c_block[i] == SETUP_FINALLY) {
+					com_addoparg(c, CONTINUE_LOOP,
+						     c->c_begin);
+					return;
+				}
+				if (c->c_block[i] == END_FINALLY) {
 					com_error(c, PyExc_SyntaxError,
-			       "'continue' not supported inside 'try' clause");
+			  "'continue' not supported inside 'finally' clause");
 			  		return;
 			  	}
 			}