Менеджеры контекста

# Что-то типа дескриптора файла

with open('file','r') as myfile:
    for line in myfile:
        print line


# Пользовательский контекстный менеджер

class Trace:
    def message(self,x):
        print x
    def __enter__(self)
        print "__enter__"
        return self
    def __exit__(self,exc_type,exc_value,exc_td)
        if exc_type is None:
            print "Exit normally"
        else:
            print "Exception: ", exc_type
            return False

# call 1
with Trace() as action:
    action.message(1)
    print "reached"

# call 2
with Trace() as action:
    action.message(2)
    raise TypeError
    print "not reached"