PEP 343 -- the with-statement.

This was started by Mike Bland and completed by Guido
(with help from Neal).

This still needs a __future__ statement added;
Thomas is working on Michael's patch for that aspect.

There's a small amount of code cleanup and refactoring
in ast.c, compile.c and ceval.c (I fixed the lltrace
behavior when EXT_POP is used -- however I had to make
lltrace a static global).

Doc/lib/asttable.tex

@@ -272,6 +272,11 @@
 \lineiii{}{\member{else_}}{}
 \hline 
 
+\lineiii{With}{\member{expr}}{}
+\lineiii{}{\member{vars&}}{}
+\lineiii{}{\member{body}}{}
+\hline 
+
 \lineiii{Yield}{\member{value}}{}
 \hline 
 

Doc/ref/ref7.tex

@@ -308,6 +308,12 @@ section~\ref{exceptions}, and information on using the \keyword{raise}
 statement to generate exceptions may be found in section~\ref{raise}.
 
 
+\section{The \keyword{with} statement\label{with}}
+\stindex{with}
+
+The \keyword{with} statement specifies 
+
+
 \section{Function definitions\label{function}}
 \indexii{function}{definition}
 \stindex{def}

Grammar/Grammar

@@ -70,7 +70,7 @@ global_stmt: 'global' NAME (',' NAME)*
 exec_stmt: 'exec' expr ['in' test [',' test]]
 assert_stmt: 'assert' test [',' test]
 
-compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef
+compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef
 if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
 while_stmt: 'while' test ':' suite ['else' ':' suite]
 for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
@@ -79,6 +79,8 @@ try_stmt: ('try' ':' suite
 	    ['else' ':' suite]
 	    ['finally' ':' suite] |
 	   'finally' ':' suite))
+with_stmt: 'with' test [ with_var ] ':' suite
+with_var: NAME expr
 # NB compile.c makes sure that the default except clause is last
 except_clause: 'except' [test [',' test]]
 suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT

Include/Python-ast.h

@@ -61,11 +61,11 @@ struct _mod {
 struct _stmt {
         enum { FunctionDef_kind=1, ClassDef_kind=2, Return_kind=3,
                Delete_kind=4, Assign_kind=5, AugAssign_kind=6, Print_kind=7,
-               For_kind=8, While_kind=9, If_kind=10, Raise_kind=11,
+               For_kind=8, While_kind=9, If_kind=10, With_kind=11,
-               TryExcept_kind=12, TryFinally_kind=13, Assert_kind=14,
+               Raise_kind=12, TryExcept_kind=13, TryFinally_kind=14,
-               Import_kind=15, ImportFrom_kind=16, Exec_kind=17,
+               Assert_kind=15, Import_kind=16, ImportFrom_kind=17,
-               Global_kind=18, Expr_kind=19, Pass_kind=20, Break_kind=21,
+               Exec_kind=18, Global_kind=19, Expr_kind=20, Pass_kind=21,
-               Continue_kind=22 } kind;
+               Break_kind=22, Continue_kind=23 } kind;
         union {
                 struct {
                         identifier name;
@@ -124,6 +124,12 @@ struct _stmt {
                         asdl_seq *orelse;
                 } If;
                 
+                struct {
+                        expr_ty context_expr;
+                        expr_ty optional_vars;
+                        asdl_seq *body;
+                } With;
+                
                 struct {
                         expr_ty type;
                         expr_ty inst;
@@ -355,6 +361,8 @@ stmt_ty While(expr_ty test, asdl_seq * body, asdl_seq * orelse, int lineno,
               PyArena *arena);
 stmt_ty If(expr_ty test, asdl_seq * body, asdl_seq * orelse, int lineno,
            PyArena *arena);
+stmt_ty With(expr_ty context_expr, expr_ty optional_vars, asdl_seq * body, int
+             lineno, PyArena *arena);
 stmt_ty Raise(expr_ty type, expr_ty inst, expr_ty tback, int lineno, PyArena
               *arena);
 stmt_ty TryExcept(asdl_seq * body, asdl_seq * handlers, asdl_seq * orelse, int

Include/graminit.h

@@ -38,45 +38,47 @@
 #define while_stmt 293
 #define for_stmt 294
 #define try_stmt 295
-#define except_clause 296
+#define with_stmt 296
-#define suite 297
+#define with_var 297
-#define testlist_safe 298
+#define except_clause 298
-#define old_test 299
+#define suite 299
-#define old_lambdef 300
+#define testlist_safe 300
-#define test 301
+#define old_test 301
-#define or_test 302
+#define old_lambdef 302
-#define and_test 303
+#define test 303
-#define not_test 304
+#define or_test 304
-#define comparison 305
+#define and_test 305
-#define comp_op 306
+#define not_test 306
-#define expr 307
+#define comparison 307
-#define xor_expr 308
+#define comp_op 308
-#define and_expr 309
+#define expr 309
-#define shift_expr 310
+#define xor_expr 310
-#define arith_expr 311
+#define and_expr 311
-#define term 312
+#define shift_expr 312
-#define factor 313
+#define arith_expr 313
-#define power 314
+#define term 314
-#define atom 315
+#define factor 315
-#define listmaker 316
+#define power 316
-#define testlist_gexp 317
+#define atom 317
-#define lambdef 318
+#define listmaker 318
-#define trailer 319
+#define testlist_gexp 319
-#define subscriptlist 320
+#define lambdef 320
-#define subscript 321
+#define trailer 321
-#define sliceop 322
+#define subscriptlist 322
-#define exprlist 323
+#define subscript 323
-#define testlist 324
+#define sliceop 324
-#define dictmaker 325
+#define exprlist 325
-#define classdef 326
+#define testlist 326
-#define arglist 327
+#define dictmaker 327
-#define argument 328
+#define classdef 328
-#define list_iter 329
+#define arglist 329
-#define list_for 330
+#define argument 330
-#define list_if 331
+#define list_iter 331
-#define gen_iter 332
+#define list_for 332
-#define gen_for 333
+#define list_if 333
-#define gen_if 334
+#define gen_iter 334
-#define testlist1 335
+#define gen_for 335
-#define encoding_decl 336
+#define gen_if 336
-#define yield_expr 337
+#define testlist1 337
+#define encoding_decl 338
+#define yield_expr 339

Include/opcode.h

@@ -72,13 +72,12 @@ extern "C" {
 #define INPLACE_XOR	78
 #define INPLACE_OR	79
 #define BREAK_LOOP	80
-
+#define WITH_CLEANUP    81
 #define LOAD_LOCALS	82
 #define RETURN_VALUE	83
 #define IMPORT_STAR	84
 #define EXEC_STMT	85
 #define YIELD_VALUE	86
-
 #define POP_BLOCK	87
 #define END_FINALLY	88
 #define BUILD_CLASS	89

Lib/compiler/ast.py

@@ -553,7 +553,7 @@ class Function(Node):
             self.varargs = 1
         if flags & CO_VARKEYWORDS:
             self.kwargs = 1
-
+    
 
 
     def getChildren(self):
@@ -584,7 +584,7 @@ class GenExpr(Node):
         self.lineno = lineno
         self.argnames = ['[outmost-iterable]']
         self.varargs = self.kwargs = None
-
+    
 
 
     def getChildren(self):
@@ -763,7 +763,7 @@ class Lambda(Node):
             self.varargs = 1
         if flags & CO_VARKEYWORDS:
             self.kwargs = 1
-
+    
 
 
     def getChildren(self):
@@ -1297,6 +1297,31 @@ class While(Node):
     def __repr__(self):
         return "While(%s, %s, %s)" % (repr(self.test), repr(self.body), repr(self.else_))
 
+class With(Node):
+    def __init__(self, expr, vars, body, lineno=None):
+        self.expr = expr
+        self.vars = vars
+        self.body = body
+        self.lineno = lineno
+
+    def getChildren(self):
+        children = []
+        children.append(self.expr)
+        children.append(self.vars)
+        children.append(self.body)
+        return tuple(children)
+
+    def getChildNodes(self):
+        nodelist = []
+        nodelist.append(self.expr)
+        if self.vars is not None:
+            nodelist.append(self.vars)
+        nodelist.append(self.body)
+        return tuple(nodelist)
+
+    def __repr__(self):
+        return "With(%s, %s, %s)" % (repr(self.expr), repr(self.vars), repr(self.body))
+
 class Yield(Node):
     def __init__(self, value, lineno=None):
         self.value = value

Lib/opcode.py

@@ -41,6 +41,7 @@ def jabs_op(name, op):
     hasjabs.append(op)
 
 # Instruction opcodes for compiled code
+# Blank lines correspond to available opcodes
 
 def_op('STOP_CODE', 0)
 def_op('POP_TOP', 1)
@@ -59,7 +60,6 @@ def_op('UNARY_INVERT', 15)
 
 def_op('LIST_APPEND', 18)
 def_op('BINARY_POWER', 19)
-
 def_op('BINARY_MULTIPLY', 20)
 def_op('BINARY_DIVIDE', 21)
 def_op('BINARY_MODULO', 22)
@@ -70,7 +70,6 @@ def_op('BINARY_FLOOR_DIVIDE', 26)
 def_op('BINARY_TRUE_DIVIDE', 27)
 def_op('INPLACE_FLOOR_DIVIDE', 28)
 def_op('INPLACE_TRUE_DIVIDE', 29)
-
 def_op('SLICE+0', 30)
 def_op('SLICE+1', 31)
 def_op('SLICE+2', 32)
@@ -93,7 +92,6 @@ def_op('INPLACE_DIVIDE', 58)
 def_op('INPLACE_MODULO', 59)
 def_op('STORE_SUBSCR', 60)
 def_op('DELETE_SUBSCR', 61)
-
 def_op('BINARY_LSHIFT', 62)
 def_op('BINARY_RSHIFT', 63)
 def_op('BINARY_AND', 64)
@@ -113,13 +111,12 @@ def_op('INPLACE_AND', 77)
 def_op('INPLACE_XOR', 78)
 def_op('INPLACE_OR', 79)
 def_op('BREAK_LOOP', 80)
-
+def_op('WITH_CLEANUP', 81)
 def_op('LOAD_LOCALS', 82)
 def_op('RETURN_VALUE', 83)
 def_op('IMPORT_STAR', 84)
 def_op('EXEC_STMT', 85)
 def_op('YIELD_VALUE', 86)
-
 def_op('POP_BLOCK', 87)
 def_op('END_FINALLY', 88)
 def_op('BUILD_CLASS', 89)
@@ -171,7 +168,6 @@ def_op('RAISE_VARARGS', 130)    # Number of raise arguments (1, 2, or 3)
 def_op('CALL_FUNCTION', 131)    # #args + (#kwargs << 8)
 def_op('MAKE_FUNCTION', 132)    # Number of args with default values
 def_op('BUILD_SLICE', 133)      # Number of items
-
 def_op('MAKE_CLOSURE', 134)
 def_op('LOAD_CLOSURE', 135)
 hasfree.append(135)
@@ -183,7 +179,6 @@ hasfree.append(137)
 def_op('CALL_FUNCTION_VAR', 140)     # #args + (#kwargs << 8)
 def_op('CALL_FUNCTION_KW', 141)      # #args + (#kwargs << 8)
 def_op('CALL_FUNCTION_VAR_KW', 142)  # #args + (#kwargs << 8)
-
 def_op('EXTENDED_ARG', 143)
 EXTENDED_ARG = 143
 

Lib/symbol.py

@@ -50,48 +50,50 @@ if_stmt = 292
 while_stmt = 293
 for_stmt = 294
 try_stmt = 295
-except_clause = 296
+with_stmt = 296
-suite = 297
+with_var = 297
-testlist_safe = 298
+except_clause = 298
-old_test = 299
+suite = 299
-old_lambdef = 300
+testlist_safe = 300
-test = 301
+old_test = 301
-or_test = 302
+old_lambdef = 302
-and_test = 303
+test = 303
-not_test = 304
+or_test = 304
-comparison = 305
+and_test = 305
-comp_op = 306
+not_test = 306
-expr = 307
+comparison = 307
-xor_expr = 308
+comp_op = 308
-and_expr = 309
+expr = 309
-shift_expr = 310
+xor_expr = 310
-arith_expr = 311
+and_expr = 311
-term = 312
+shift_expr = 312
-factor = 313
+arith_expr = 313
-power = 314
+term = 314
-atom = 315
+factor = 315
-listmaker = 316
+power = 316
-testlist_gexp = 317
+atom = 317
-lambdef = 318
+listmaker = 318
-trailer = 319
+testlist_gexp = 319
-subscriptlist = 320
+lambdef = 320
-subscript = 321
+trailer = 321
-sliceop = 322
+subscriptlist = 322
-exprlist = 323
+subscript = 323
-testlist = 324
+sliceop = 324
-dictmaker = 325
+exprlist = 325
-classdef = 326
+testlist = 326
-arglist = 327
+dictmaker = 327
-argument = 328
+classdef = 328
-list_iter = 329
+arglist = 329
-list_for = 330
+argument = 330
-list_if = 331
+list_iter = 331
-gen_iter = 332
+list_for = 332
-gen_for = 333
+list_if = 333
-gen_if = 334
+gen_iter = 334
-testlist1 = 335
+gen_for = 335
-encoding_decl = 336
+gen_if = 336
-yield_expr = 337
+testlist1 = 337
+encoding_decl = 338
+yield_expr = 339
 #--end constants--
 
 sym_name = {}

Lib/test/contextmanager.py

@@ -0,0 +1,34 @@
+class GeneratorContextManager(object):
+    def __init__(self, gen):
+        self.gen = gen
+
+    def __context__(self):
+        return self
+
+    def __enter__(self):
+        try:
+            return self.gen.next()
+        except StopIteration:
+            raise RuntimeError("generator didn't yield")
+
+    def __exit__(self, type, value, traceback):
+        if type is None:
+            try:
+                self.gen.next()
+            except StopIteration:
+                return
+            else:
+                raise RuntimeError("generator didn't stop")
+        else:
+            try:
+                self.gen.throw(type, value, traceback)
+            except (type, StopIteration):
+                return
+            else:
+                raise RuntimeError("generator caught exception")
+
+def contextmanager(func):
+    def helper(*args, **kwds):
+        return GeneratorContextManager(func(*args, **kwds))
+    return helper
+

Lib/test/nested.py

@@ -0,0 +1,41 @@
+import sys
+from collections import deque
+
+
+class nested(object):
+    def __init__(self, *contexts):
+        self.contexts = contexts
+        self.entered = None
+
+    def __context__(self):
+        return self
+
+    def __enter__(self):
+        if self.entered is not None:
+            raise RuntimeError("Context is not reentrant")
+        self.entered = deque()
+        vars = []
+        try:
+            for context in self.contexts:
+                mgr = context.__context__()
+                vars.append(mgr.__enter__())
+                self.entered.appendleft(mgr)
+        except:
+            self.__exit__(*sys.exc_info())
+            raise
+        return vars
+
+    def __exit__(self, *exc_info):
+        # Behave like nested with statements
+        # first in, last out
+        # New exceptions override old ones
+        ex = exc_info
+        for mgr in self.entered:
+            try:
+                mgr.__exit__(*ex)
+            except:
+                ex = sys.exc_info()
+        self.entered = None
+        if ex is not exc_info:
+            raise ex[0], ex[1], ex[2]
+

Lib/test/test_with.py

@@ -0,0 +1,560 @@
+#!/usr/bin/env python
+
+"""Unit tests for the with statement specified in PEP 343."""
+
+__author__ = "Mike Bland"
+__email__ = "mbland at acm dot org"
+
+import unittest
+from test.contextmanager import GeneratorContextManager
+from test.nested import nested
+from test.test_support import run_unittest
+
+
+class MockContextManager(GeneratorContextManager):
+    def __init__(self, gen):
+        GeneratorContextManager.__init__(self, gen)
+        self.context_called = False
+        self.enter_called = False
+        self.exit_called = False
+        self.exit_args = None
+
+    def __context__(self):
+        self.context_called = True
+        return GeneratorContextManager.__context__(self)
+
+    def __enter__(self):
+        self.enter_called = True
+        return GeneratorContextManager.__enter__(self)
+
+    def __exit__(self, type, value, traceback):
+        self.exit_called = True
+        self.exit_args = (type, value, traceback)
+        return GeneratorContextManager.__exit__(self, type, value, traceback)
+
+
+def mock_contextmanager(func):
+    def helper(*args, **kwds):
+        return MockContextManager(func(*args, **kwds))
+    return helper
+
+
+class MockResource(object):
+    def __init__(self):
+        self.yielded = False
+        self.stopped = False
+
+
+@mock_contextmanager
+def mock_contextmanager_generator():
+    mock = MockResource()
+    try:
+        mock.yielded = True
+        yield mock
+    finally:
+        mock.stopped = True
+
+
+class MockNested(nested):
+    def __init__(self, *contexts):
+        nested.__init__(self, *contexts)
+        self.context_called = False
+        self.enter_called = False
+        self.exit_called = False
+        self.exit_args = None
+
+    def __context__(self):
+        self.context_called = True
+        return nested.__context__(self)
+
+    def __enter__(self):
+        self.enter_called = True
+        return nested.__enter__(self)
+
+    def __exit__(self, *exc_info):
+        self.exit_called = True
+        self.exit_args = exc_info
+        return nested.__exit__(self, *exc_info)
+
+ 
+class FailureTestCase(unittest.TestCase):
+    def testNameError(self):
+        def fooNotDeclared():
+            with foo: pass
+        self.assertRaises(NameError, fooNotDeclared)
+
+    def testContextAttributeError(self):
+        class LacksContext(object):
+            def __enter__(self):
+                pass
+
+            def __exit__(self, type, value, traceback):
+                pass
+
+        def fooLacksContext():
+            foo = LacksContext()
+            with foo: pass
+        self.assertRaises(AttributeError, fooLacksContext)
+
+    def testEnterAttributeError(self):
+        class LacksEnter(object):
+            def __context__(self):
+                pass
+
+            def __exit__(self, type, value, traceback):
+                pass
+
+        def fooLacksEnter():
+            foo = LacksEnter()
+            with foo: pass
+        self.assertRaises(AttributeError, fooLacksEnter)
+
+    def testExitAttributeError(self):
+        class LacksExit(object):
+            def __context__(self):
+                pass
+
+            def __enter__(self):
+                pass
+
+        def fooLacksExit():
+            foo = LacksExit()
+            with foo: pass
+        self.assertRaises(AttributeError, fooLacksExit)
+
+    def assertRaisesSyntaxError(self, codestr):
+        def shouldRaiseSyntaxError(s):
+            compile(s, '', 'single')
+        self.assertRaises(SyntaxError, shouldRaiseSyntaxError, codestr)
+
+    def testAssignmentToNoneError(self):
+        self.assertRaisesSyntaxError('with mock as None:\n  pass')
+        self.assertRaisesSyntaxError(
+            'with mock as (None):\n'
+            '  pass')
+
+    def testAssignmentToEmptyTupleError(self):
+        self.assertRaisesSyntaxError(
+            'with mock as ():\n'
+            '  pass')
+
+    def testAssignmentToTupleOnlyContainingNoneError(self):
+        self.assertRaisesSyntaxError('with mock as None,:\n  pass')
+        self.assertRaisesSyntaxError(
+            'with mock as (None,):\n'
+            '  pass')
+
+    def testAssignmentToTupleContainingNoneError(self):
+        self.assertRaisesSyntaxError(
+            'with mock as (foo, None, bar):\n'
+            '  pass')
+
+    def testContextThrows(self):
+        class ContextThrows(object):
+            def __context__(self):
+                raise RuntimeError("Context threw")
+
+        def shouldThrow():
+            ct = ContextThrows()
+            self.foo = None
+            with ct as self.foo:
+                pass
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertEqual(self.foo, None)
+
+    def testEnterThrows(self):
+        class EnterThrows(object):
+            def __context__(self):
+                return self
+
+            def __enter__(self):
+                raise RuntimeError("Context threw")
+
+            def __exit__(self, *args):
+                pass
+
+        def shouldThrow():
+            ct = EnterThrows()
+            self.foo = None
+            with ct as self.foo:
+                pass
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertEqual(self.foo, None)
+
+    def testExitThrows(self):
+        class ExitThrows(object):
+            def __context__(self):
+                return self
+            def __enter__(self):
+                return
+            def __exit__(self, *args):
+                raise RuntimeError(42)
+        def shouldThrow():
+            with ExitThrows():
+                pass
+        self.assertRaises(RuntimeError, shouldThrow)
+
+class ContextmanagerAssertionMixin(object):
+    TEST_EXCEPTION = RuntimeError("test exception")
+
+    def assertInWithManagerInvariants(self, mock_manager):
+        self.assertTrue(mock_manager.context_called)
+        self.assertTrue(mock_manager.enter_called)
+        self.assertFalse(mock_manager.exit_called)
+        self.assertEqual(mock_manager.exit_args, None)
+
+    def assertAfterWithManagerInvariants(self, mock_manager, exit_args):
+        self.assertTrue(mock_manager.context_called)
+        self.assertTrue(mock_manager.enter_called)
+        self.assertTrue(mock_manager.exit_called)
+        self.assertEqual(mock_manager.exit_args, exit_args)
+
+    def assertAfterWithManagerInvariantsNoError(self, mock_manager):
+        self.assertAfterWithManagerInvariants(mock_manager,
+            (None, None, None))
+
+    def assertInWithGeneratorInvariants(self, mock_generator):
+        self.assertTrue(mock_generator.yielded)
+        self.assertFalse(mock_generator.stopped)
+
+    def assertAfterWithGeneratorInvariantsNoError(self, mock_generator):
+        self.assertTrue(mock_generator.yielded)
+        self.assertTrue(mock_generator.stopped)
+
+    def raiseTestException(self):
+        raise self.TEST_EXCEPTION
+
+    def assertAfterWithManagerInvariantsWithError(self, mock_manager):
+        self.assertTrue(mock_manager.context_called)
+        self.assertTrue(mock_manager.enter_called)
+        self.assertTrue(mock_manager.exit_called)
+        self.assertEqual(mock_manager.exit_args[0], RuntimeError)
+        self.assertEqual(mock_manager.exit_args[1], self.TEST_EXCEPTION)
+
+    def assertAfterWithGeneratorInvariantsWithError(self, mock_generator):
+        self.assertTrue(mock_generator.yielded)
+        self.assertTrue(mock_generator.stopped)
+
+
+class NonexceptionalTestCase(unittest.TestCase, ContextmanagerAssertionMixin):
+    def testInlineGeneratorSyntax(self):
+        with mock_contextmanager_generator():
+            pass
+
+    def testUnboundGenerator(self):
+        mock = mock_contextmanager_generator()
+        with mock:
+            pass
+        self.assertAfterWithManagerInvariantsNoError(mock)
+
+    def testInlineGeneratorBoundSyntax(self):
+        with mock_contextmanager_generator() as foo:
+            self.assertInWithGeneratorInvariants(foo)
+        # FIXME: In the future, we'll try to keep the bound names from leaking
+        self.assertAfterWithGeneratorInvariantsNoError(foo)
+
+    def testInlineGeneratorBoundToExistingVariable(self):
+        foo = None
+        with mock_contextmanager_generator() as foo:
+            self.assertInWithGeneratorInvariants(foo)
+        self.assertAfterWithGeneratorInvariantsNoError(foo)
+
+    def testInlineGeneratorBoundToDottedVariable(self):
+        with mock_contextmanager_generator() as self.foo:
+            self.assertInWithGeneratorInvariants(self.foo)
+        self.assertAfterWithGeneratorInvariantsNoError(self.foo)
+
+    def testBoundGenerator(self):
+        mock = mock_contextmanager_generator()
+        with mock as foo:
+            self.assertInWithGeneratorInvariants(foo)
+            self.assertInWithManagerInvariants(mock)
+        self.assertAfterWithGeneratorInvariantsNoError(foo)
+        self.assertAfterWithManagerInvariantsNoError(mock)
+
+    def testNestedSingleStatements(self):
+        mock_a = mock_contextmanager_generator()
+        with mock_a as foo:
+            mock_b = mock_contextmanager_generator()
+            with mock_b as bar:
+                self.assertInWithManagerInvariants(mock_a)
+                self.assertInWithManagerInvariants(mock_b)
+                self.assertInWithGeneratorInvariants(foo)
+                self.assertInWithGeneratorInvariants(bar)
+            self.assertAfterWithManagerInvariantsNoError(mock_b)
+            self.assertAfterWithGeneratorInvariantsNoError(bar)
+            self.assertInWithManagerInvariants(mock_a)
+            self.assertInWithGeneratorInvariants(foo)
+        self.assertAfterWithManagerInvariantsNoError(mock_a)
+        self.assertAfterWithGeneratorInvariantsNoError(foo)
+
+
+class NestedNonexceptionalTestCase(unittest.TestCase,
+    ContextmanagerAssertionMixin):
+    def testSingleArgInlineGeneratorSyntax(self):
+        with nested(mock_contextmanager_generator()):
+            pass
+
+    def testSingleArgUnbound(self):
+        mock_contextmanager = mock_contextmanager_generator()
+        mock_nested = MockNested(mock_contextmanager)
+        with mock_nested:
+            self.assertInWithManagerInvariants(mock_contextmanager)
+            self.assertInWithManagerInvariants(mock_nested)
+        self.assertAfterWithManagerInvariantsNoError(mock_contextmanager)
+        self.assertAfterWithManagerInvariantsNoError(mock_nested)
+
+    def testSingleArgBoundToNonTuple(self):
+        m = mock_contextmanager_generator()
+        # This will bind all the arguments to nested() into a single list
+        # assigned to foo.
+        with nested(m) as foo:
+            self.assertInWithManagerInvariants(m)
+        self.assertAfterWithManagerInvariantsNoError(m)
+
+    def testSingleArgBoundToSingleElementParenthesizedList(self):
+        m = mock_contextmanager_generator()
+        # This will bind all the arguments to nested() into a single list
+        # assigned to foo.
+        # FIXME: what should this do:  with nested(m) as (foo,):
+        with nested(m) as (foo):
+            self.assertInWithManagerInvariants(m)
+        self.assertAfterWithManagerInvariantsNoError(m)
+
+    def testSingleArgBoundToMultipleElementTupleError(self):
+        def shouldThrowValueError():
+            with nested(mock_contextmanager_generator()) as (foo, bar):
+                pass
+        self.assertRaises(ValueError, shouldThrowValueError)
+
+    def testSingleArgUnbound(self):
+        mock_contextmanager = mock_contextmanager_generator()
+        mock_nested = MockNested(mock_contextmanager)
+        with mock_nested:
+            self.assertInWithManagerInvariants(mock_contextmanager)
+            self.assertInWithManagerInvariants(mock_nested)
+        self.assertAfterWithManagerInvariantsNoError(mock_contextmanager)
+        self.assertAfterWithManagerInvariantsNoError(mock_nested)
+
+    def testMultipleArgUnbound(self):
+        m = mock_contextmanager_generator()
+        n = mock_contextmanager_generator()
+        o = mock_contextmanager_generator()
+        mock_nested = MockNested(m, n, o)
+        with mock_nested:
+            self.assertInWithManagerInvariants(m)
+            self.assertInWithManagerInvariants(n)
+            self.assertInWithManagerInvariants(o)
+            self.assertInWithManagerInvariants(mock_nested)
+        self.assertAfterWithManagerInvariantsNoError(m)
+        self.assertAfterWithManagerInvariantsNoError(n)
+        self.assertAfterWithManagerInvariantsNoError(o)
+        self.assertAfterWithManagerInvariantsNoError(mock_nested)
+
+    def testMultipleArgBound(self):
+        mock_nested = MockNested(mock_contextmanager_generator(),
+            mock_contextmanager_generator(), mock_contextmanager_generator())
+        with mock_nested as (m, n, o):
+            self.assertInWithGeneratorInvariants(m)
+            self.assertInWithGeneratorInvariants(n)
+            self.assertInWithGeneratorInvariants(o)
+            self.assertInWithManagerInvariants(mock_nested)
+        self.assertAfterWithGeneratorInvariantsNoError(m)
+        self.assertAfterWithGeneratorInvariantsNoError(n)
+        self.assertAfterWithGeneratorInvariantsNoError(o)
+        self.assertAfterWithManagerInvariantsNoError(mock_nested)
+
+
+class ExceptionalTestCase(unittest.TestCase, ContextmanagerAssertionMixin):
+    def testSingleResource(self):
+        cm = mock_contextmanager_generator()
+        def shouldThrow():
+            with cm as self.resource:
+                self.assertInWithManagerInvariants(cm)
+                self.assertInWithGeneratorInvariants(self.resource)
+                self.raiseTestException()
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertAfterWithManagerInvariantsWithError(cm)
+        self.assertAfterWithGeneratorInvariantsWithError(self.resource)
+
+    def testNestedSingleStatements(self):
+        mock_a = mock_contextmanager_generator()
+        mock_b = mock_contextmanager_generator()
+        def shouldThrow():
+            with mock_a as self.foo:
+                with mock_b as self.bar:
+                    self.assertInWithManagerInvariants(mock_a)
+                    self.assertInWithManagerInvariants(mock_b)
+                    self.assertInWithGeneratorInvariants(self.foo)
+                    self.assertInWithGeneratorInvariants(self.bar)
+                    self.raiseTestException()
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertAfterWithManagerInvariantsWithError(mock_a)
+        self.assertAfterWithManagerInvariantsWithError(mock_b)
+        self.assertAfterWithGeneratorInvariantsWithError(self.foo)
+        self.assertAfterWithGeneratorInvariantsWithError(self.bar)
+
+    def testMultipleResourcesInSingleStatement(self):
+        cm_a = mock_contextmanager_generator()
+        cm_b = mock_contextmanager_generator()
+        mock_nested = MockNested(cm_a, cm_b)
+        def shouldThrow():
+            with mock_nested as (self.resource_a, self.resource_b):
+                self.assertInWithManagerInvariants(cm_a)
+                self.assertInWithManagerInvariants(cm_b)
+                self.assertInWithManagerInvariants(mock_nested)
+                self.assertInWithGeneratorInvariants(self.resource_a)
+                self.assertInWithGeneratorInvariants(self.resource_b)
+                self.raiseTestException()
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertAfterWithManagerInvariantsWithError(cm_a)
+        self.assertAfterWithManagerInvariantsWithError(cm_b)
+        self.assertAfterWithManagerInvariantsWithError(mock_nested)
+        self.assertAfterWithGeneratorInvariantsWithError(self.resource_a)
+        self.assertAfterWithGeneratorInvariantsWithError(self.resource_b)
+
+    def testNestedExceptionBeforeInnerStatement(self):
+        mock_a = mock_contextmanager_generator()
+        mock_b = mock_contextmanager_generator()
+        self.bar = None
+        def shouldThrow():
+            with mock_a as self.foo:
+                self.assertInWithManagerInvariants(mock_a)
+                self.assertInWithGeneratorInvariants(self.foo)
+                self.raiseTestException()
+                with mock_b as self.bar:
+                    pass
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertAfterWithManagerInvariantsWithError(mock_a)
+        self.assertAfterWithGeneratorInvariantsWithError(self.foo)
+
+        # The inner statement stuff should never have been touched
+        self.assertEqual(self.bar, None)
+        self.assertFalse(mock_b.context_called)
+        self.assertFalse(mock_b.enter_called)
+        self.assertFalse(mock_b.exit_called)
+        self.assertEqual(mock_b.exit_args, None)
+
+    def testNestedExceptionAfterInnerStatement(self):
+        mock_a = mock_contextmanager_generator()
+        mock_b = mock_contextmanager_generator()
+        def shouldThrow():
+            with mock_a as self.foo:
+                with mock_b as self.bar:
+                    self.assertInWithManagerInvariants(mock_a)
+                    self.assertInWithManagerInvariants(mock_b)
+                    self.assertInWithGeneratorInvariants(self.foo)
+                    self.assertInWithGeneratorInvariants(self.bar)
+                self.raiseTestException()
+        self.assertRaises(RuntimeError, shouldThrow)
+        self.assertAfterWithManagerInvariantsWithError(mock_a)
+        self.assertAfterWithManagerInvariantsNoError(mock_b)
+        self.assertAfterWithGeneratorInvariantsWithError(self.foo)
+        self.assertAfterWithGeneratorInvariantsNoError(self.bar)
+
+
+class NonLocalFlowControlTestCase(unittest.TestCase):
+
+    def testWithBreak(self):
+        counter = 0
+        while True:
+            counter += 1
+            with mock_contextmanager_generator():
+                counter += 10
+                break
+            counter += 100 # Not reached
+        self.assertEqual(counter, 11)
+
+    def testWithContinue(self):
+        counter = 0
+        while True:
+            counter += 1
+            if counter > 2:
+                break
+            with mock_contextmanager_generator():
+                counter += 10
+                continue
+            counter += 100 # Not reached
+        self.assertEqual(counter, 12)
+
+    def testWithReturn(self):
+        def foo():
+            counter = 0
+            while True:
+                counter += 1
+                with mock_contextmanager_generator():
+                    counter += 10
+                    return counter
+                counter += 100 # Not reached
+        self.assertEqual(foo(), 11)
+
+    def testWithYield(self):
+        def gen():
+            with mock_contextmanager_generator():
+                yield 12
+                yield 13
+        x = list(gen())
+        self.assertEqual(x, [12, 13])
+
+    def testWithRaise(self):
+        counter = 0
+        try:
+            counter += 1
+            with mock_contextmanager_generator():
+                counter += 10
+                raise RuntimeError
+            counter += 100 # Not reached
+        except RuntimeError:
+            self.assertEqual(counter, 11)
+        else:
+            self.fail("Didn't raise RuntimeError")
+
+
+class AssignmentTargetTestCase(unittest.TestCase):
+
+    def testSingleComplexTarget(self):
+        targets = {1: [0, 1, 2]}
+        with mock_contextmanager_generator() as targets[1][0]:
+            self.assertEqual(targets.keys(), [1])
+            self.assertEqual(targets[1][0].__class__, MockResource)
+        with mock_contextmanager_generator() as targets.values()[0][1]:
+            self.assertEqual(targets.keys(), [1])
+            self.assertEqual(targets[1][1].__class__, MockResource)
+        with mock_contextmanager_generator() as targets[2]:
+            keys = targets.keys()
+            keys.sort()
+            self.assertEqual(keys, [1, 2])
+        class C: pass
+        blah = C()
+        with mock_contextmanager_generator() as blah.foo:
+            self.assertEqual(hasattr(blah, "foo"), True)
+
+    def testMultipleComplexTargets(self):
+        class C:
+            def __context__(self): return self
+            def __enter__(self): return 1, 2, 3
+            def __exit__(self, *a): pass
+        targets = {1: [0, 1, 2]}
+        with C() as (targets[1][0], targets[1][1], targets[1][2]):
+            self.assertEqual(targets, {1: [1, 2, 3]})
+        with C() as (targets.values()[0][2], targets.values()[0][1], targets.values()[0][0]):
+            self.assertEqual(targets, {1: [3, 2, 1]})
+        with C() as (targets[1], targets[2], targets[3]):
+            self.assertEqual(targets, {1: 1, 2: 2, 3: 3})
+        class B: pass
+        blah = B()
+        with C() as (blah.one, blah.two, blah.three):
+            self.assertEqual(blah.one, 1)
+            self.assertEqual(blah.two, 2)
+            self.assertEqual(blah.three, 3)
+
+
+def test_main():
+    run_unittest(FailureTestCase, NonexceptionalTestCase,
+                 NestedNonexceptionalTestCase, ExceptionalTestCase,
+                 NonLocalFlowControlTestCase,
+                 AssignmentTargetTestCase)
+
+
+if __name__ == '__main__':
+    test_main()

Misc/ACKS

@@ -62,6 +62,7 @@ Dominic Binks
 Philippe Biondi
 Stuart Bishop
 Roy Bixler
+Mike Bland
 Martin Bless
 Pablo Bleyer
 Erik van Blokland

Misc/NEWS

@@ -19,6 +19,8 @@ Core and builtins
   - dict.__getitem__ now looks for a __missing__ hook before raising
     KeyError.
 
+- PEP 343: with statement implemented.
+
 - Fix the encodings package codec search function to only search
   inside its own package. Fixes problem reported in patch #1433198.
 

Parser/Python.asdl

@@ -25,6 +25,7 @@ module Python
 	      | For(expr target, expr iter, stmt* body, stmt* orelse)
 	      | While(expr test, stmt* body, stmt* orelse)
 	      | If(expr test, stmt* body, stmt* orelse)
+	      | With(expr context_expr, expr? optional_vars, stmt* body)
 
 	      -- 'type' is a bad name
 	      | Raise(expr? type, expr? inst, expr? tback)

Python/Python-ast.c

@@ -84,6 +84,12 @@ char *If_fields[]={
         "body",
         "orelse",
 };
+PyTypeObject *With_type;
+char *With_fields[]={
+        "context_expr",
+        "optional_vars",
+        "body",
+};
 PyTypeObject *Raise_type;
 char *Raise_fields[]={
         "type",
@@ -465,6 +471,8 @@ static int init_types(void)
         if (!While_type) return 0;
         If_type = make_type("If", stmt_type, If_fields, 3);
         if (!If_type) return 0;
+        With_type = make_type("With", stmt_type, With_fields, 3);
+        if (!With_type) return 0;
         Raise_type = make_type("Raise", stmt_type, Raise_fields, 3);
         if (!Raise_type) return 0;
         TryExcept_type = make_type("TryExcept", stmt_type, TryExcept_fields, 3);
@@ -999,6 +1007,29 @@ If(expr_ty test, asdl_seq * body, asdl_seq * orelse, int lineno, PyArena *arena)
         return p;
 }
 
+stmt_ty
+With(expr_ty context_expr, expr_ty optional_vars, asdl_seq * body, int lineno,
+     PyArena *arena)
+{
+        stmt_ty p;
+        if (!context_expr) {
+                PyErr_SetString(PyExc_ValueError,
+                                "field context_expr is required for With");
+                return NULL;
+        }
+        p = (stmt_ty)PyArena_Malloc(arena, sizeof(*p));
+        if (!p) {
+                PyErr_NoMemory();
+                return NULL;
+        }
+        p->kind = With_kind;
+        p->v.With.context_expr = context_expr;
+        p->v.With.optional_vars = optional_vars;
+        p->v.With.body = body;
+        p->lineno = lineno;
+        return p;
+}
+
 stmt_ty
 Raise(expr_ty type, expr_ty inst, expr_ty tback, int lineno, PyArena *arena)
 {
@@ -2062,6 +2093,26 @@ ast2obj_stmt(void* _o)
                         goto failed;
                 Py_DECREF(value);
                 break;
+        case With_kind:
+                result = PyType_GenericNew(With_type, NULL, NULL);
+                if (!result) goto failed;
+                value = ast2obj_expr(o->v.With.context_expr);
+                if (!value) goto failed;
+                if (PyObject_SetAttrString(result, "context_expr", value) == -1)
+                        goto failed;
+                Py_DECREF(value);
+                value = ast2obj_expr(o->v.With.optional_vars);
+                if (!value) goto failed;
+                if (PyObject_SetAttrString(result, "optional_vars", value) ==
+                    -1)
+                        goto failed;
+                Py_DECREF(value);
+                value = ast2obj_list(o->v.With.body, ast2obj_stmt);
+                if (!value) goto failed;
+                if (PyObject_SetAttrString(result, "body", value) == -1)
+                        goto failed;
+                Py_DECREF(value);
+                break;
         case Raise_kind:
                 result = PyType_GenericNew(Raise_type, NULL, NULL);
                 if (!result) goto failed;
@@ -2922,6 +2973,7 @@ init_ast(void)
         if(PyDict_SetItemString(d, "For", (PyObject*)For_type) < 0) return;
         if(PyDict_SetItemString(d, "While", (PyObject*)While_type) < 0) return;
         if(PyDict_SetItemString(d, "If", (PyObject*)If_type) < 0) return;
+        if(PyDict_SetItemString(d, "With", (PyObject*)With_type) < 0) return;
         if(PyDict_SetItemString(d, "Raise", (PyObject*)Raise_type) < 0) return;
         if(PyDict_SetItemString(d, "TryExcept", (PyObject*)TryExcept_type) < 0)
            return;

Python/ast.c

@@ -314,7 +314,7 @@ get_operator(const node *n)
     }
 }
 
-/* Set the context ctx for expr_ty e returning 0 on success, -1 on error.
+/* Set the context ctx for expr_ty e returning 1 on success, 0 on error.
 
    Only sets context for expr kinds that "can appear in assignment context"
    (according to ../Parser/Python.asdl).  For other expr kinds, it sets
@@ -339,7 +339,7 @@ set_context(expr_ty e, expr_context_ty ctx, const node *n)
        a little more complex than necessary as a result.  It also means
        that expressions in an augmented assignment have no context.
        Consider restructuring so that augmented assignment uses
-       set_context(), too
+       set_context(), too.
     */
     assert(ctx != AugStore && ctx != AugLoad);
 
@@ -2713,6 +2713,46 @@ ast_for_try_stmt(struct compiling *c, const node *n)
     return TryFinally(body, finally, LINENO(n), c->c_arena);
 }
 
+static expr_ty
+ast_for_with_var(struct compiling *c, const node *n)
+{
+    REQ(n, with_var);
+    if (strcmp(STR(CHILD(n, 0)), "as") != 0) {
+        ast_error(n, "expected \"with [expr] as [var]\"");
+        return NULL;
+    }
+    return ast_for_expr(c, CHILD(n, 1));
+}
+
+/* with_stmt: 'with' test [ with_var ] ':' suite */
+static stmt_ty
+ast_for_with_stmt(struct compiling *c, const node *n)
+{
+    expr_ty context_expr, optional_vars = NULL;
+    int suite_index = 3;    /* skip 'with', test, and ':' */
+    asdl_seq *suite_seq;
+
+    assert(TYPE(n) == with_stmt);
+    context_expr = ast_for_expr(c, CHILD(n, 1));
+    if (TYPE(CHILD(n, 2)) == with_var) {
+        optional_vars = ast_for_with_var(c, CHILD(n, 2));
+
+        if (!optional_vars) {
+            return NULL;
+        }
+	if (!set_context(optional_vars, Store, n)) {
+	    return NULL;
+	}
+        suite_index = 4;
+    }
+
+    suite_seq = ast_for_suite(c, CHILD(n, suite_index));
+    if (!suite_seq) {
+        return NULL;
+    }
+    return With(context_expr, optional_vars, suite_seq, LINENO(n), c->c_arena);
+}
+
 static stmt_ty
 ast_for_classdef(struct compiling *c, const node *n)
 {
@@ -2813,6 +2853,8 @@ ast_for_stmt(struct compiling *c, const node *n)
                 return ast_for_for_stmt(c, ch);
             case try_stmt:
                 return ast_for_try_stmt(c, ch);
+            case with_stmt:
+                return ast_for_with_stmt(c, ch);
             case funcdef:
                 return ast_for_funcdef(c, ch);
             case classdef:

Python/ceval.c

@@ -97,6 +97,7 @@ static PyObject *load_args(PyObject ***, int);
 #define CALL_FLAG_KW 2
 
 #ifdef LLTRACE
+static int lltrace;
 static int prtrace(PyObject *, char *);
 #endif
 static int call_trace(Py_tracefunc, PyObject *, PyFrameObject *,
@@ -540,9 +541,6 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throw)
 	unsigned char *first_instr;
 	PyObject *names;
 	PyObject *consts;
-#ifdef LLTRACE
-	int lltrace;
-#endif
 #if defined(Py_DEBUG) || defined(LLTRACE)
 	/* Make it easier to find out where we are with a debugger */
 	char *filename;
@@ -661,10 +659,12 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throw)
 #define STACKADJ(n)	{ (void)(BASIC_STACKADJ(n), \
                                lltrace && prtrace(TOP(), "stackadj")); \
                                assert(STACK_LEVEL() <= f->f_stacksize); }
+#define EXT_POP(STACK_POINTER) (lltrace && prtrace(*(STACK_POINTER), "ext_pop"), *--(STACK_POINTER))
 #else
 #define PUSH(v)		BASIC_PUSH(v)
 #define POP()		BASIC_POP()
 #define STACKADJ(n)	BASIC_STACKADJ(n)
+#define EXT_POP(STACK_POINTER) (*--(STACK_POINTER))
 #endif
 
 /* Local variable macros */
@@ -2172,6 +2172,43 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throw)
 					   STACK_LEVEL());
 			continue;
 
+		case WITH_CLEANUP:
+		{
+			/* TOP is the context.__exit__ bound method.
+			   Below that are 1-3 values indicating how/why
+			   we entered the finally clause:
+			   - SECOND = None
+			   - (SECOND, THIRD) = (WHY_RETURN or WHY_CONTINUE), retval
+			   - SECOND = WHY_*; no retval below it
+			   - (SECOND, THIRD, FOURTH) = exc_info()
+			   In the last case, we must call
+			     TOP(SECOND, THIRD, FOURTH)
+			   otherwise we must call
+			     TOP(None, None, None)
+			   but we must preserve the stack entries below TOP.
+			   The code here just sets the stack up for the call;
+			   separate CALL_FUNCTION(3) and POP_TOP opcodes are
+			   emitted by the compiler.
+			*/
+			
+			x = TOP();
+			u = SECOND();
+			if (PyInt_Check(u) || u == Py_None) {
+				u = v = w = Py_None;
+			}
+			else {
+				v = THIRD();
+				w = FOURTH();
+			}
+			Py_INCREF(u);
+			Py_INCREF(v);
+			Py_INCREF(w);
+			PUSH(u);
+			PUSH(v);
+			PUSH(w);
+			break;
+		}
+
 		case CALL_FUNCTION:
 		{
 			PyObject **sp;
@@ -2511,9 +2548,9 @@ fast_yield:
 	return retval;
 }
 
-/* this is gonna seem *real weird*, but if you put some other code between
+/* This is gonna seem *real weird*, but if you put some other code between
    PyEval_EvalFrame() and PyEval_EvalCodeEx() you will need to adjust
-	the test in the if statement in Misc/gdbinit:pystack* */
+   the test in the if statements in Misc/gdbinit (pystack and pystackv). */
 
 PyObject *
 PyEval_EvalCodeEx(PyCodeObject *co, PyObject *globals, PyObject *locals,
@@ -3473,8 +3510,6 @@ PyEval_GetFuncDesc(PyObject *func)
 	}
 }
 
-#define EXT_POP(STACK_POINTER) (*--(STACK_POINTER))
-
 static void
 err_args(PyObject *func, int flags, int nargs)
 {

Python/compile.c

@@ -191,6 +191,8 @@ static void compiler_pop_fblock(struct compiler *, enum fblocktype,
 static int inplace_binop(struct compiler *, operator_ty);
 static int expr_constant(expr_ty e);
 
+static int compiler_with(struct compiler *, stmt_ty);
+
 static PyCodeObject *assemble(struct compiler *, int addNone);
 static PyObject *__doc__;
 
@@ -289,6 +291,7 @@ PyAST_Compile(mod_ty mod, const char *filename, PyCompilerFlags *flags,
 
  error:
 	compiler_free(&c);
+	assert(!PyErr_Occurred());
 	return co;
 }
 
@@ -1157,6 +1160,18 @@ compiler_exit_scope(struct compiler *c)
 
 }
 
+/* Allocate a new "anonymous" local variable.
+   Used by list comprehensions and with statements.
+*/
+
+static PyObject *
+compiler_new_tmpname(struct compiler *c)
+{
+	char tmpname[256];
+	PyOS_snprintf(tmpname, sizeof(tmpname), "_[%d]", ++c->u->u_tmpname);
+	return PyString_FromString(tmpname);
+}
+
 /* Allocate a new block and return a pointer to it.
    Returns NULL on error.
 */
@@ -1360,7 +1375,8 @@ opcode_stack_effect(int opcode, int oparg)
 			return -1;
 		case BREAK_LOOP:
 			return 0;
-
+		case WITH_CLEANUP:
+			return 3;
 		case LOAD_LOCALS:
 			return 1;
 		case RETURN_VALUE:
@@ -2663,6 +2679,8 @@ compiler_visit_stmt(struct compiler *c, stmt_ty s)
 		break;
         case Continue_kind:
 		return compiler_continue(c);
+        case With_kind:
+                return compiler_with(c, s);
 	}
 	return 1;
 }
@@ -3124,7 +3142,6 @@ compiler_listcomp_generator(struct compiler *c, PyObject *tmpname,
 static int
 compiler_listcomp(struct compiler *c, expr_ty e)
 {
-	char tmpname[256];
 	identifier tmp;
         int rc = 0;
 	static identifier append;
@@ -3136,8 +3153,7 @@ compiler_listcomp(struct compiler *c, expr_ty e)
 		if (!append)
 			return 0;
 	}
-	PyOS_snprintf(tmpname, sizeof(tmpname), "_[%d]", ++c->u->u_tmpname);
+	tmp = compiler_new_tmpname(c);
-	tmp = PyString_FromString(tmpname);
 	if (!tmp)
 		return 0;
 	ADDOP_I(c, BUILD_LIST, 0);
@@ -3291,6 +3307,148 @@ expr_constant(expr_ty e)
 	}
 }
 
+/*
+   Implements the with statement from PEP 343.
+
+   The semantics outlined in that PEP are as follows:  
+
+   with EXPR as VAR:
+       BLOCK
+  
+   It is implemented roughly as:
+  
+   context = (EXPR).__context__()
+   exit = context.__exit__  # not calling it
+   value = context.__enter__()
+   try:
+       VAR = value  # if VAR present in the syntax
+       BLOCK
+   finally:
+       if an exception was raised:
+           exc = copy of (exception, instance, traceback)
+       else:
+           exc = (None, None, None)
+       exit(*exc)
+ */
+static int
+compiler_with(struct compiler *c, stmt_ty s)
+{
+    static identifier context_attr, enter_attr, exit_attr;
+    basicblock *block, *finally;
+    identifier tmpexit, tmpvalue = NULL;
+
+    assert(s->kind == With_kind);
+
+    if (!context_attr) {
+        context_attr = PyString_InternFromString("__context__");
+        if (!context_attr)
+            return 0;
+    }
+    if (!enter_attr) {
+        enter_attr = PyString_InternFromString("__enter__");
+        if (!enter_attr)
+            return 0;
+    }
+    if (!exit_attr) {
+        exit_attr = PyString_InternFromString("__exit__");
+        if (!exit_attr)
+            return 0;
+    }
+
+    block = compiler_new_block(c);
+    finally = compiler_new_block(c);
+    if (!block || !finally)
+        return 0;
+
+    /* Create a temporary variable to hold context.__exit__ */
+    tmpexit = compiler_new_tmpname(c);
+    if (tmpexit == NULL)
+        return 0;
+    PyArena_AddPyObject(c->c_arena, tmpexit);
+
+    if (s->v.With.optional_vars) {
+        /* Create a temporary variable to hold context.__enter__().
+	   We need to do this rather than preserving it on the stack
+	   because SETUP_FINALLY remembers the stack level.
+	   We need to do the assignment *inside* the try/finally
+	   so that context.__exit__() is called when the assignment
+	   fails.  But we need to call context.__enter__() *before*
+	   the try/finally so that if it fails we won't call
+	   context.__exit__().
+	*/
+        tmpvalue = compiler_new_tmpname(c);
+	if (tmpvalue == NULL)
+	    return 0;
+	PyArena_AddPyObject(c->c_arena, tmpvalue);
+    }
+
+    /* Evaluate (EXPR).__context__() */
+    VISIT(c, expr, s->v.With.context_expr);
+    ADDOP_O(c, LOAD_ATTR, context_attr, names);
+    ADDOP_I(c, CALL_FUNCTION, 0);
+
+    /* Squirrel away context.__exit__  */
+    ADDOP(c, DUP_TOP);
+    ADDOP_O(c, LOAD_ATTR, exit_attr, names);
+    if (!compiler_nameop(c, tmpexit, Store))
+	return 0;
+
+    /* Call context.__enter__() */
+    ADDOP_O(c, LOAD_ATTR, enter_attr, names);
+    ADDOP_I(c, CALL_FUNCTION, 0);
+
+    if (s->v.With.optional_vars) {
+        /* Store it in tmpvalue */
+        if (!compiler_nameop(c, tmpvalue, Store))
+	    return 0;
+    }
+    else {
+        /* Discard result from context.__enter__() */
+        ADDOP(c, POP_TOP);
+    }
+
+    /* Start the try block */
+    ADDOP_JREL(c, SETUP_FINALLY, finally);
+
+    compiler_use_next_block(c, block);
+    if (!compiler_push_fblock(c, FINALLY_TRY, block)) {
+        return 0;
+    }
+
+    if (s->v.With.optional_vars) {
+        /* Bind saved result of context.__enter__() to VAR */
+        if (!compiler_nameop(c, tmpvalue, Load) ||
+	    !compiler_nameop(c, tmpvalue, Del))
+	  return 0;
+        VISIT(c, expr, s->v.With.optional_vars);
+    }
+
+    /* BLOCK code */
+    VISIT_SEQ(c, stmt, s->v.With.body);
+
+    /* End of try block; start the finally block */
+    ADDOP(c, POP_BLOCK);
+    compiler_pop_fblock(c, FINALLY_TRY, block);
+
+    ADDOP_O(c, LOAD_CONST, Py_None, consts);
+    compiler_use_next_block(c, finally);
+    if (!compiler_push_fblock(c, FINALLY_END, finally))
+        return 0;
+
+    /* Finally block starts; push tmpexit and issue our magic opcode. */
+    if (!compiler_nameop(c, tmpexit, Load) ||
+	!compiler_nameop(c, tmpexit, Del))
+        return 0;
+    ADDOP(c, WITH_CLEANUP);
+    ADDOP_I(c, CALL_FUNCTION, 3);
+    ADDOP(c, POP_TOP);
+
+    /* Finally block ends. */
+    ADDOP(c, END_FINALLY);
+    compiler_pop_fblock(c, FINALLY_END, finally);
+    return 1;
+}
+
 static int
 compiler_visit_expr(struct compiler *c, expr_ty e)
 {

Python/import.c

@@ -54,9 +54,10 @@ extern time_t PyOS_GetLastModificationTime(char *, FILE *);
        Python 2.4b1: 62061
        Python 2.5a0: 62071
        Python 2.5a0: 62081 (ast-branch)
+       Python 2.5a0: 62091 (with)
 .
 */
-#define MAGIC (62081 | ((long)'\r'<<16) | ((long)'\n'<<24))
+#define MAGIC (62091 | ((long)'\r'<<16) | ((long)'\n'<<24))
 
 /* Magic word as global; note that _PyImport_Init() can change the
    value of this global to accommodate for alterations of how the

Python/symtable.c

@@ -890,6 +890,21 @@ error:
 	} \
 }
 
+static int
+symtable_new_tmpname(struct symtable *st)
+{
+	char tmpname[256];
+	identifier tmp;
+
+	PyOS_snprintf(tmpname, sizeof(tmpname), "_[%d]",
+		      ++st->st_cur->ste_tmpname);
+	tmp = PyString_InternFromString(tmpname);
+	if (!symtable_add_def(st, tmp, DEF_LOCAL))
+		return 0;
+	Py_DECREF(tmp);
+	return 1;
+}
+
 static int
 symtable_visit_stmt(struct symtable *st, stmt_ty s)
 {
@@ -1051,6 +1066,17 @@ symtable_visit_stmt(struct symtable *st, stmt_ty s)
         case Continue_kind:
 		/* nothing to do here */
 		break;
+        case With_kind:
+		if (!symtable_new_tmpname(st))
+			return 0;
+                VISIT(st, expr, s->v.With.context_expr);
+                if (s->v.With.optional_vars) {
+			if (!symtable_new_tmpname(st))
+				return 0;
+                        VISIT(st, expr, s->v.With.optional_vars);
+                }
+                VISIT_SEQ(st, stmt, s->v.With.body);
+                break;
 	}
 	return 1;
 }
@@ -1093,26 +1119,16 @@ symtable_visit_expr(struct symtable *st, expr_ty e)
 		VISIT_SEQ(st, expr, e->v.Dict.keys);
 		VISIT_SEQ(st, expr, e->v.Dict.values);
 		break;
-        case ListComp_kind: {
+        case ListComp_kind:
-		char tmpname[256];
+		if (!symtable_new_tmpname(st))
-		identifier tmp;
-
-		PyOS_snprintf(tmpname, sizeof(tmpname), "_[%d]",
-			      ++st->st_cur->ste_tmpname);
-		tmp = PyString_InternFromString(tmpname);
-		if (!symtable_add_def(st, tmp, DEF_LOCAL))
 			return 0;
-		Py_DECREF(tmp);
 		VISIT(st, expr, e->v.ListComp.elt);
 		VISIT_SEQ(st, comprehension, e->v.ListComp.generators);
 		break;
-	}
+        case GeneratorExp_kind:
-        case GeneratorExp_kind: {
+		if (!symtable_visit_genexp(st, e))
-		if (!symtable_visit_genexp(st, e)) {
 			return 0;
-		}
 		break;
-	}
         case Yield_kind:
 		if (e->v.Yield.value)
 			VISIT(st, expr, e->v.Yield.value);

Tools/compiler/ast.txt

@@ -20,6 +20,7 @@ Break:
 Continue: 
 For: assign, list, body, else_&
 While: test, body, else_&
+With: expr, vars&, body
 If: tests!, else_&
 Exec: expr, locals&, globals&
 From: modname*, names*
@@ -42,7 +43,7 @@ AssAttr: expr, attrname*, flags*
 ListComp: expr, quals!
 ListCompFor: assign, list, ifs!
 ListCompIf: test
-GenExpr: code 
+GenExpr: code
 GenExprInner: expr, quals!
 GenExprFor: assign, iter, ifs!
 GenExprIf: test