On Wednesday 30 March 2016 14:38, Random832 wrote: > On Tue, Mar 29, 2016, at 20:56, Chris Angelico wrote: >> The map contract is this: >> >> x = StrangeDict() >> x[123] = 456 >> ... >> assert x[123] == 456 >> >> Your mapping does violate the map contract. > > So, you can put *anything* in that "..."?
Yes, we're all very impressed that you spotted the trivial and obvious loophole that changing a key:value will change the key:value that you just changed *wink* but that doesn't really move the discussion anywhere. This is not an argument about dicts being mutable, because clearly they aren't. This is an argument about key:value pairs being stable. "Stable" doesn't mean "immutable". If you change the value associated with a key directly, then it will change. That's the whole point. But if you change *one* key, the relationship between *other* keys and their values shouldn't change. Given a surjection (many-to-one mapping) between keys and values in a mapping, we expect that changing the mapping of one key will not affect other keys. To be pedantic, by "change" I mean deleting the key (and, if necessary, value) or reassigning a new value to the key. To be even more pedantic, mutations to the value *do not count*. Specifically, insertions and deletions to the mapping never affect the existing keys. But, critically, insertions and deletions to a sequence do sometimes affect the index of existing items. So while we can say that there is a surjective function that maps the index of a sequence to an item, but that relationship fails to meet the requirements for it to be a mapping type like a dict. https://en.wikipedia.org/wiki/Bijection,_injection_and_surjection If somebody wants to insist that this is a kind of mapping, I can't disagree, but it isn't useful as a mapping type. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
