[Python-ideas] Maybe/Option builtin

[Amber Yust]

If you care about none as a value, you specify a different default.

NO_VALUE = object()
foo = bar.get("baz", NO_VALUE)
if foo is NO_VALUE:
    #....
On May 21, 2014 5:12 PM, "Devin Jeanpierre" <jeanpierreda at gmail.com> wrote:

> On Wed, May 21, 2014 at 4:50 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> > On Wed, May 21, 2014 at 10:49:36AM -0400, mulhern wrote:
> >> I feel that a Maybe/Option type, analogous to the types found in
> Haskell or
> >> OCaml would actually be useful in Python. The value analogous to the
> None
> >> constructor should be Python's None.
> >>
> >> Obviously, it wouldn't give the type-checking benefits that it gives in
> >> statically checked languages, but every use of a Maybe object as if it
> were
> >> the contained object would give an error, alerting the user to the fact
> >> that None is a possibility and allowing them to address the problem
> sooner
> >> rather than later.
> >
> > Since this is a Python list, you shouldn't take it for granted that we
> > will be familiar with Haskell or Ocaml, nor expect us to go off and
> > study those languages well enough to understand what a Maybe object is
> > used for or how it will fit into Python's execution model.
> >
> > I'm no expect on either of those two languages, but it seems to me that
> > that the only point of Maybe is to allow the static type checker to
> > distinguish between (for example) "this function returns a string" and
> > "this function returns either a string or None". Since Python doesn't do
> > static, compile-time type checks, I'm having difficulty in seeing what
> > would be the point of Maybe in Python.
>
> Python has no way to represent "I have a value, and that value is None".
>
> e.g. what is the difference between x.get('a') and x.get('b') for x ==
> {'a': None} ?
>
> Otherwise, agree 100% . And anyway, retrofitting this into Python
> can't really work.
>
> -- Devin
>
> > As I said, I'm not an expert in Haskell, but I don't think this proposal
> > is helpful, and certainly not helpful enough to make it a built-in or
> > standard part of the language. If I have missed some benefit of Maybe,
> > please explain how it would apply in Python terms.
> >
> >
> >
> >> I feel that it would be kind of tricky to implement it as a class.
> >> Something like:
> >>
> >> class Maybe(object):
> >>     def __init__(self, value=None):
> >>         self.value = value
> >>     def value(self):
> >>         return self.value
> >>
> >> is a start but I'm not able to see how to make
> >>
> >> if Maybe():
> >>     print("nothing") # never prints
> >
> > In Python 2, define __nonzero__. In Python 3, define __bool__.
> >
> > https://docs.python.org/2/reference/datamodel.html#object.__nonzero__
> > https://docs.python.org/3/reference/datamodel.html#object.__bool__
> >
> >
> >> but
> >>
> >> if Maybe({}):
> >>     print("yes a value") #always prints
> >>
> >> which is definitely the desired behaviour.
> >
> > Not to me it isn't. This goes against the standard Python convention
> > that empty containers are falsey. Since Maybe({}) wraps an empty dict,
> > it should be considered a false value.
> >
> >
> >
> >
> > --
> > Steven
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