single.py

class OnlyOnce(object):
    class __OnlyOnce(object):
        def __init__(self):
            with open('/tmp/test.txt', 'a') as t:
                t.write("new line\n")

        def lines(self):
            with open('/tmp/test.txt', 'r') as t:
                return t.readlines()
    instance = None

    def __new__(cls):
        if OnlyOnce.instance is None:
            OnlyOnce.instance = OnlyOnce.__OnlyOnce()
        return OnlyOnce.instance

    def __getattr__(self, name):
        return getattr(self.instance, name)