如何检查列表中是否只有两个可以重复的特定元素？

Question

如何检查列表中是否只有两个可以重复的特定元素？

例如：我希望出现在列表中的有效元素只有2个：

1. abstract.f
2. None

如果list包含任何其他元素，则应抛出错误消息。

list1 = ['abstract.f', None, None, 'abstract.f']
# This is a valid list as it doesn't have any other element than 2 of above.

list1 = ['abstract.f', None, 'xyz']
# This is invalid list as it has 'xyz' which is not expected.

Answer 1

您可以使用all输出布尔值：

# Put items considered valid in this list
valid = ['abstract.f', None]

list1 = ['abstract.f', None, None, 'abstract.f']
print(all(el in valid for el in list1)) # True

list1 = ['abstract.f', None, 'xyz']
print(all(el in valid for el in list1)) # False