def load_installed_version(self):
        installed_packages = get_installed_pypackages()
        if self.package in installed_packages:
            self.version = "=={}".format(
                installed_packages[self.package].version)
def merge(left, right):
    if left is None:
        return right
    if right is None:
        return left
    if left.prio > right.prio:
        left.right = merge(left.right, right)
        left.update()
        return left
    else:
        right.left = merge(left, right.left)
        right.update()
        return right