Anthony <anthonysar...@gmail.com> added the comment: To clarify, A major assumption I'm making is that the default is empty, or the "copying" is handled as some separately coupled concept. A motivation here is wanting to do operations like .append() that depend on the object existing.
On Wed, Nov 13, 2019 at 1:04 PM Anthony Sarkis <anthonysar...@gmail.com> wrote: > Thanks for adding it, I read it now. > > And sorry to back track a moment - I love the idea of @dataclass and I can > only imagine how must work it was to implement as I am only a beginner. > > I'm looking at this primarily from the narrow view point of a user - not > so much understanding the backdrop of how it can actually work. > But I still think this is worth considering as I think this is common > pattern based on this logic: > > When using the default construction method it seems reasonable to pass a > default, such as a List. > Because > def __init__(x=[]): > self.x = x > Is the cleanest way I can think of to write that. > In this new context, if x: list = [] is the cleanest way to write it. > Therefore it should do that, however it is actually implemented. > > Let's imagine for a moment that @dataclass becomes the default way of > constructing classes in python. > Do we really want such a common case to require such verbose language? > > Looking at Guido's first comment on this, I think that detection mechanism > is what I would expect to happen. To illustrate verbosely: > https://gist.github.com/swirlingsand/d59d01ef79c5ee93f430ed324199bc65 > I clearly misunderstood the way classes get passed by default. > I continue to believe that defining it in the argument should be equal to > defining it after. (if there are no other context items present). > > From a beginners perspective here, It would appear that in a sense the > default init is actually "silently failing" in that the expected (because > of the two instances not being equal to each other as shown in L10 above) > isolation is not happening. > In a sense then, @dataclass is turning that silent "failure" into a > ValueError which is actually better. > > What about something like this: > https://gist.github.com/swirlingsand/2494bc482902fada2248698f7b8af61e > If the common case is for it to be None, then this doesn't fire so there's > no cost. > If it's a common default then it handles it as expected. > If it's not found it raises a ValueError like before, so there's no loss > or harm. > A handful of defaults here may cover 80% of cases and the 20% can continue > to be handled by ValueError > In a project with 40 classes with 10 defaults per class that's 400 lines > of code that look like: > [] > instead of > field(default_factory=lambda: []) > (or {} etc.) > > Issue #3 has many comments around copying, but that's not my concern, I'm > just talking about the defaults where the attribute is not provided at all > (ie is None). > > I did the above example in regular python since I don't know enough about > how @dataclass is implemented, but it seems reasonable that if it can work > in normal python there should be a way to say augment operation() on line > 21 with field() > > > > > > > On Wed, Nov 13, 2019 at 10:11 AM Vedran Čačić <rep...@bugs.python.org> > wrote: > >> >> Vedran Čačić <ved...@gmail.com> added the comment: >> >> Have you read https://github.com/ericvsmith/dataclasses/issues/3? >> >> ---------- >> nosy: +veky >> >> _______________________________________ >> Python tracker <rep...@bugs.python.org> >> <https://bugs.python.org/issue38758> >> _______________________________________ >> > ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38758> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
