'Operating system of the target machine.
The default value is ``linux``.
Allowed values are listed in :attr:`pwnlib.context.ContextType.oses`.
Examples:
>>> context.os = \'linux\'
>>> context.os = \'foobar\' #doctest: +ELLIPSIS
Traceback (most recent call last):
AttributeError: os must be one of [\'android\', \'cgc\', \'freebsd\', \'linux\', \'windows\']'

@_validator
def os(self, os):

    os = os.lower()
    if (os not in ContextType.oses):
        raise AttributeError(('os    must    be    one    of    %r' % ContextType.oses))
    return os
