[SciPy-Dev] `a` and `b` attributes of the stats distributions

Ralf Gommers ralf.gommers@gmail....
Mon Jul 8 15:05:37 CDT 2013

On Mon, Jul 8, 2013 at 12:08 AM, <josef.pktd@gmail.com> wrote:

> On Sun, Jul 7, 2013 at 5:18 PM, Ralf Gommers <ralf.gommers@gmail.com>
> wrote:
> >
> > On Sun, Jul 7, 2013 at 8:36 PM, Warren Weckesser
> > <warren.weckesser@gmail.com> wrote:
> >>
> >> All the distributions would be modified to call `self._support(...)` to
> >> get `a` and `b` instead of using the attributes `self.a` and `self.b`.
>  (The
> >> method could be public--I just defaulted to private while working out
> the
> >> appropriate API.)
> >>
> >> It would be nice to get rid of `a` and `b` entirely, but they are
> >> currently public attributes, so I don't think we can simply remove them.
> >
> >
> > Backwards compatibility does seem to be a problem here. Not sure what the
> > best solution is.
> >
> > Another issue is that self.a and self.b are used inside private methods,
> > which themselves may not have the correct info to call support() again.
> > Probably all fixable, but could be some work. The code will also become a
> > little more verbose.
> My preferred solution to the state problem would be to create new
> instances each time, then we don't have to worry about carrying over
> and cleaning up state all the time

If we could start from scratch that would indeed be the way to go. I don't
think it's that much of a problem that we have to go and break all code
using the instances now though. So that leaves us with incremental

> My first impression is that `_support` would not help much, isn't it
> similar to what _argcheck currently does? We still have to store the
> information as attributes somewhere, or you have to recalculate the
> support each time.

I think the latter was the idea - recalculating is cheap.

> And then every piece of code that needs to know a and b needs to find
> it, either you hand it around as additional arguments, or you need
> every function or method to call `_support` which is similar to what
> the public methods currently do with _argcheck.
> (and the private methods do their work after the public methods have
> called _argcheck and set a and b if necessary).
> What would be useful is a public (top level generic) public function
> `support_bounds` (or something like that) that the user can use to get
> the support of the distribution. (but it won't change the internal
> headaches.
> it could just call _argcheck and return a and b, however translated by
> loc and scale, since a and b are the bounds of the standardized
> distribution (loc=0, scale=1)

That could indeed be useful, it's simple to add.

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