On 3/4/19 5:11 PM, Steven D'Aprano wrote:
On Mon, Mar 04, 2019 at 11:56:54AM -0600, Dan Sommers wrote:
On 3/4/19 10:44 AM, Steven D'Aprano wrote:
If you know ahead of time which order you want, you can simply reverse it:
# prefs = site_defaults + user_defaults + document_prefs prefs = dict(ChainMap(document_prefs, user_defaults, site_defaults))
but that seems a little awkward to me, and reads backwards. I'm used to thinking reading left-to-right, not right-to-left.
I read that as use document preferences first, then user defaults, then site defautls, exactly as I'd explain the functionality to someone else.
If you explained it to me like that, with the term "use", I'd think that the same feature would be done three times: once with document prefs, then with user defaults, then site defaults.
Clearly that's not what you mean, so I'd then have to guess what you meant by "use", since you don't actually mean use. That would leave me trying to guess whether you meant that *site defaults* overrode document prefs or the other way.
I don't like guessing, so I'd probably explicitly ask: "Wait, I'm confused, which wins? It sounds like site defaults wins, surely that's not what you meant."
You're right: "use" is the wrong word. Perhaps "prefer" is more appropriate. To answer the question of which wins: the first one in the list [document, user, site] that contains a given preference in question. Users don't see dictionary updates; they see collections of preferences in order of priority.
Documentation is hard. :-)
So maybe we're agreeing: if you think in terms of updating a dictionary of preferences, then maybe it reads backwards, but if you think of implementing features, then adding dictionaries of preferences reads backwards.
Do you think "last seen wins" is backwards for dict.update() or for command line options?
As a user, "last seen wins" is clearly superior for command line options. As a programmer, because object methods operate on their underlying object, it's pretty obvious that d1.update(d2) starts with d1 and applies the changes expressed in d2, which is effectively "last seen wins."
If I resist the temptation to guess in the face of ambiguity, though, I don't think that d1 + d2 is any less ambiguous than a hypothetical dict_update(d1, d2) function. When I see a + operator, I certainly don't think of one operand or the other winning.