How can I filter by key, or keys, a query in Python for Google App Engine?

Question

I have a query and I can apply filters on them without any problem. This works fine:

query.filter('foo =', 'bar')

But what if I want to filter my query by key or a list of keys?

I have them as Key() property or as a string and by trying something like this, it didn't work:

query.filter('key =', 'some_key')        #no success
query.filter('key IN', ['key1', 'key2']) #no success