'Returns the Python command, which may vary depending
on the version of Python and how it was installed.
In general, Python 2 comes with ``python`` and ``python2`` commands,
while Python 3 only comes with a ``python3`` command.
:returns: The Python command
:rtype: Executable'

@property
def command(self):

    if (self.spec.satisfies('@:2') and os.path.exists(os.path.join(self.prefix.bin, 'python2'))):
        command = 'python2'
    elif (self.spec.satisfies('@3:') and os.path.exists(os.path.join(self.prefix.bin, 'python3'))):
        command = 'python3'
    elif os.path.exists(os.path.join(self.prefix.bin, 'python')):
        command = 'python'
    else:
        msg = 'Unable    to    locate    {0}    command    in    {1}'
        raise RuntimeError(msg.format(self.name, self.prefix.bin))
    path = os.path.realpath(os.path.join(self.prefix.bin, command))
    return Executable(path)
