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. :-)
Sorry.
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.
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