Updates to the with-statement:

- New semantics for __exit__() -- it must re-raise the exception
  if type is not None; the with-statement itself doesn't do this.
  (See the updated PEP for motivation.)

- Added context managers to:
  - file
  - thread.LockType
  - threading.{Lock,RLock,Condition,Semaphore,BoundedSemaphore}
  - decimal.Context

- Added contextlib.py, which defines @contextmanager, nested(), closing().

- Unit tests all around; bot no docs yet.

Lib/test/test_contextlib.py

@@ -0,0 +1,240 @@
+"""Unit tests for contextlib.py, and other context managers."""
+
+from __future__ import with_statement
+
+import os
+import decimal
+import tempfile
+import unittest
+import threading
+from contextlib import *  # Tests __all__
+
+class ContextManagerTestCase(unittest.TestCase):
+
+    def test_contextmanager_plain(self):
+        state = []
+        @contextmanager
+        def woohoo():
+            state.append(1)
+            yield 42
+            state.append(999)
+        with woohoo() as x:
+            self.assertEqual(state, [1])
+            self.assertEqual(x, 42)
+            state.append(x)
+        self.assertEqual(state, [1, 42, 999])
+
+    def test_contextmanager_finally(self):
+        state = []
+        @contextmanager
+        def woohoo():
+            state.append(1)
+            try:
+                yield 42
+            finally:
+                state.append(999)
+        try:
+            with woohoo() as x:
+                self.assertEqual(state, [1])
+                self.assertEqual(x, 42)
+                state.append(x)
+                raise ZeroDivisionError()
+        except ZeroDivisionError:
+            pass
+        else:
+            self.fail("Expected ZeroDivisionError")
+        self.assertEqual(state, [1, 42, 999])
+
+    def test_contextmanager_except(self):
+        state = []
+        @contextmanager
+        def woohoo():
+            state.append(1)
+            try:
+                yield 42
+            except ZeroDivisionError, e:
+                state.append(e.args[0])
+                self.assertEqual(state, [1, 42, 999])
+        with woohoo() as x:
+            self.assertEqual(state, [1])
+            self.assertEqual(x, 42)
+            state.append(x)
+            raise ZeroDivisionError(999)
+        self.assertEqual(state, [1, 42, 999])
+
+class NestedTestCase(unittest.TestCase):
+
+    # XXX This needs more work
+
+    def test_nested(self):
+        @contextmanager
+        def a():
+            yield 1
+        @contextmanager
+        def b():
+            yield 2
+        @contextmanager
+        def c():
+            yield 3
+        with nested(a(), b(), c()) as (x, y, z):
+            self.assertEqual(x, 1)
+            self.assertEqual(y, 2)
+            self.assertEqual(z, 3)
+
+    def test_nested_cleanup(self):
+        state = []
+        @contextmanager
+        def a():
+            state.append(1)
+            try:
+                yield 2
+            finally:
+                state.append(3)
+        @contextmanager
+        def b():
+            state.append(4)
+            try:
+                yield 5
+            finally:
+                state.append(6)
+        try:
+            with nested(a(), b()) as (x, y):
+                state.append(x)
+                state.append(y)
+                1/0
+        except ZeroDivisionError:
+            self.assertEqual(state, [1, 4, 2, 5, 6, 3])
+        else:
+            self.fail("Didn't raise ZeroDivisionError")
+
+class ClosingTestCase(unittest.TestCase):
+
+    # XXX This needs more work
+
+    def test_closing(self):
+        state = []
+        class C:
+            def close(self):
+                state.append(1)
+        x = C()
+        self.assertEqual(state, [])
+        with closing(x) as y:
+            self.assertEqual(x, y)
+        self.assertEqual(state, [1])
+
+    def test_closing_error(self):
+        state = []
+        class C:
+            def close(self):
+                state.append(1)
+        x = C()
+        self.assertEqual(state, [])
+        try:
+            with closing(x) as y:
+                self.assertEqual(x, y)
+                1/0
+        except ZeroDivisionError:
+            self.assertEqual(state, [1])
+        else:
+            self.fail("Didn't raise ZeroDivisionError")
+
+class FileContextTestCase(unittest.TestCase):
+
+    def testWithOpen(self):
+        tfn = tempfile.mktemp()
+        try:
+            f = None
+            with open(tfn, "w") as f:
+                self.failIf(f.closed)
+                f.write("Booh\n")
+            self.failUnless(f.closed)
+            f = None
+            try:
+                with open(tfn, "r") as f:
+                    self.failIf(f.closed)
+                    self.assertEqual(f.read(), "Booh\n")
+                    1/0
+            except ZeroDivisionError:
+                self.failUnless(f.closed)
+            else:
+                self.fail("Didn't raise ZeroDivisionError")
+        finally:
+            try:
+                os.remove(tfn)
+            except os.error:
+                pass
+
+class LockContextTestCase(unittest.TestCase):
+
+    def boilerPlate(self, lock, locked):
+        self.failIf(locked())
+        with lock:
+            self.failUnless(locked())
+        self.failIf(locked())
+        try:
+            with lock:
+                self.failUnless(locked())
+                1/0
+        except ZeroDivisionError:
+            self.failIf(locked())
+        else:
+            self.fail("Didn't raise ZeroDivisionError")
+
+    def testWithLock(self):
+        lock = threading.Lock()
+        self.boilerPlate(lock, lock.locked)
+
+    def testWithRLock(self):
+        lock = threading.RLock()
+        self.boilerPlate(lock, lock._is_owned)
+
+    def testWithCondition(self):
+        lock = threading.Condition()
+        def locked():
+            return lock._is_owned()
+        self.boilerPlate(lock, locked)
+
+    def testWithSemaphore(self):
+        lock = threading.Semaphore()
+        def locked():
+            if lock.acquire(False):
+                lock.release()
+                return False
+            else:
+                return True
+        self.boilerPlate(lock, locked)
+
+    def testWithBoundedSemaphore(self):
+        lock = threading.BoundedSemaphore()
+        def locked():
+            if lock.acquire(False):
+                lock.release()
+                return False
+            else:
+                return True
+        self.boilerPlate(lock, locked)
+
+class DecimalContextTestCase(unittest.TestCase):
+
+    # XXX Somebody should write more thorough tests for this
+
+    def testBasic(self):
+        ctx = decimal.getcontext()
+        ctx.prec = save_prec = decimal.ExtendedContext.prec + 5
+        with decimal.ExtendedContext:
+            self.assertEqual(decimal.getcontext().prec,
+                             decimal.ExtendedContext.prec)
+        self.assertEqual(decimal.getcontext().prec, save_prec)
+        try:
+            with decimal.ExtendedContext:
+                self.assertEqual(decimal.getcontext().prec,
+                                 decimal.ExtendedContext.prec)
+                1/0
+        except ZeroDivisionError:
+            self.assertEqual(decimal.getcontext().prec, save_prec)
+        else:
+            self.fail("Didn't raise ZeroDivisionError")
+
+
+if __name__ == "__main__":
+    unittest.main()