class Plist:
    def __init__(self, path=None, domain=None):
        self._domain = None
        self._path = None
        self.domain = domain
        self.path = path

    @property
    def domain(self):
        return self._domain

    @domain.setter
    def domain(self, domain):
        if not domain and not self.path:
            raise ValueError("A: you must provide either the 'domain' or 'path' argument")
        self._domain = domain

    @property
    def path(self):
        return self._path

    @path.setter
    def path(self, path):
        if not self.domain and not path:
            raise ValueError("B: you must provide either the 'domain' or 'path' argument")
        self._path = path

    def __repr__(self):
        return 'Plist(path={}, domain={})'.format(repr(self.path), repr(self.domain))

print(Plist(path='~/Library/Preferences/com.apple.finder.plist'))
print(Plist(domain='com.apple.finder'))