with EXPR as VAR:
    BLOCK

mgr = (EXPR)
exit = type(mgr).__exit__  # Not calling it yet
value = type(mgr).__enter__(mgr)
exc = True
try:
    try:
        VAR = value  # Only if "as VAR" is present
        BLOCK
    except:
        # The exceptional case is handled here
        exc = False
        if not exit(mgr, *sys.exc_info()):
            raise
        # The exception is swallowed if exit() returns true
finally:
    # The normal and non-local-goto cases are handled here
    if exc:
        exit(mgr, None, None, None)

f = open('/tmp/tmp.txt')
try:
    for line in f.readlines():
        print(line)
finally:
    f.close()

class opened(object):

    def __init__(self, name):
        self.handle = open(name)

    def __enter__(self):
        return self.handle

    def __exit__(self, type, value, trackback):
        self.handle.close()

with opened('/tmp/a.txt') as f:
    for line in f.readlines():
        print(line)

from contextlib import contextmanager

@contextmanager
def opened(name):
    f = open(name)
    try:
        yield f
    finally:
        f.close()

with opened('/tmp/a.txt') as f:
    for line in f.readlines():
        print(line)

@contextmanager
    def locked(lock):
        lock.acquire()
        try:
            yield
        finally:
            lock.release()

    with locked(myLock):
        # Code here executes with myLock held.  The lock is
        # guaranteed to be released when the block is left (even
        # if via return or by an uncaught exception).

@contextmanager
        def transaction(db):
            db.begin()
            try:
                yield None
            except:
                db.rollback()
                raise
            else:
                db.commit()

@contextmanager
def stdout_redirected(new_stdout):
    save_stdout = sys.stdout
    sys.stdout = new_stdout
    try:
        yield None
    finally:
        sys.stdout = save_stdout

with opened(filename, "w") as f:
    with stdout_redirected(f):
        print "Hello world"