很简单，您添加了多余的/多余的花括号，请尝试以下操作:

It's simple, you have added extra/redundant curly braces, try this:

self.collection.find_and_modify(query={"recid":recid}, 
                                update={"$set": {"creation_date": str(datetime.now())}})

UPD(解释，假设您使用的是python> = 2.7):

UPD (explanation, assuming you are on python>=2.7):

发生错误是因为python认为您正在尝试使用{}表示法进行设置:

The error occurs because python thinks you are trying to make a set with {} notation:

集合类是使用字典实现的.因此， 集合元素的要求与字典的要求相同 钥匙;也就是说，该元素同时定义了__eq __()和__hash __().

The set classes are implemented using dictionaries. Accordingly, the requirements for set elements are the same as those for dictionary keys; namely, that the element defines both __eq__() and __hash__().

换句话说，集合中的元素应该是可散列的: int，string.并且您要向其传递dict，它不能被散列并且不能作为集合的元素.

In other words, elements of a set should be hashable: e.g. int, string. And you are passing a dict to it, which is not hashable and cannot be an element of a set.

此外，请参见以下示例:

Also, see this example:

>>> {{}}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'dict'

希望有帮助.