On Tue, Aug 5, 2014 at 12:54 AM, Łukasz Rekucki <[email protected]> wrote:
> On 4 August 2014 16:14, Daniel Pyrathon <[email protected]> wrote: > > Hi All, > > > > This has been resolved by using the ImmutableList datastructure > > > > > https://github.com/PirosB3/django/blob/soc2014_meta_refactor_upgrade_flags_get_field_tree/django/db/models/options.py#L629 > > > > But why? What's the benefit over using a tuple? ImmutableList is not > even a list, because it inherits from tuple. > Communication. >From a purist theoretical perspective, there shouldn't be any argument - the data we're talking about is a list. Lists have homogeneous elements; Tuples have heterogeneous elements, but have *positional* homogeneity. A "Point" is a tuple, because element 0 of the tuple "means" the x coordinate. A Database row is a tuple - The first element is the primary key (an integer), second is the "name" column (a string), and so on. A tuple is *not* "just an immutable list". >From a pragmatic perspective, tuples are faster to work with, because they aren't carrying around all the baggage needed to support mutability. And, in this case, there is a risk of an end-user accidentally mutating the result of get_fields(), which, due to cache-related optimizations, means there's a risk of side effects. So - in this case, at a theoretical level, we are dealing with an list of homogeneous objects (fields) that must be either immutable, or copied every time it is used. Copying is a bit expensive (not *completely* prohibitive, but noticeable), so that means we need to look to immutability. At a theoretical I'm not wild about the fact that ImmutableList subclassing tuple - it should be subclassing *list* - but I'm willing to defer to pragmatism. Given that we're dealing with a relative internal detail that most users won't be exposed to, I'm willing to hold my nose and accept optimised solutions over theoretically pure solutions in the interests of *all* users having better performance. Yours, Russ Magee %-) -- You received this message because you are subscribed to the Google Groups "Django developers" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/CAJxq8490O7-J5sAQ7xK9b4FmkQHrqGQPN56wRuDpzoh7pWHbkA%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
