data validation when creating an object

[Rita]

Thanks everyone for the replies.




On Thu, Jan 16, 2014 at 1:36 AM, Cameron Simpson <cs at zip.com.au> wrote:

> On 16Jan2014 15:53, Ben Finney <ben+python at benfinney.id.au> wrote:
> > Roy Smith <roy at panix.com> writes:
> > >  Ben Finney <ben+python at benfinney.id.au> wrote:
> > > > Who says it's frowned on to do work in the initialiser? Where are
> they
> > > > saying it? That seems over-broad, I'd like to read the context of
> that
> > > > advice.
> > >
> > > There are some people who advocate that C++ constructors should not do
> > > a lot of work and/or should be incapable of throwing exceptions. The
> > > pros and cons of that argument are largely C++ specific. […]
> >
> > Even simpler: They are mistaken in what the constructor is named, in
> > Python.
> > Python classes have the constructor, ‘__new__’. I would agree with
> > advice not to do anything but allocate the resources for a new instance
> > in the constructor. [...]
> >
> > Python instances have an initialiser, ‘__init__’. That function is for
> > setting up the specific instance for later use. This is commonly
> > over-ridden and many classes define a custom initialiser, which normally
> > does some amount of work.
> >
> > I don't think ‘__init__’ is subject to the conventions of a constructor,
> > because *‘__init__’ is not a constructor*.
>
> 99% of the time this distinction is moot. When I call ClassName(blah,...),
> both the constructor and initialiser are called.
>
> Informally, there's a rule of thumb that making an object (allocate,
> construct and initialise) shouldn't be needlessly expensive. Beyond
> that, what happens depends on the use patterns.
>
> This rule of thumb will be what Rita's encountered, perhaps stated
> without any qualification regarding what's appropriate.
>
> Cheers,
> --
> Cameron Simpson <cs at zip.com.au>
>
> The problem with keeping an open mind is that my ideas all tend to fall
> out...
>         - Bill Garrett <garrett at cs.unc.edu>
> --
> https://mail.python.org/mailman/listinfo/python-list
>



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