Python feature request : operator for function composition

[Kay Schluehr]

On 3 Feb., 10:55, Arnaud Delobelle <arno... at googlemail.com> wrote:
> On Feb 3, 9:43 am, Kay Schluehr <kay.schlu... at gmx.net> wrote:
>
>
>
> > On 3 Feb., 10:13, Arnaud Delobelle <arno... at googlemail.com> wrote:
>
> > > On Feb 3, 5:09 am, Kay Schluehr <kay.schlu... at gmx.net> wrote:
>
> > > > As you know, there is no operator for function composition in Python.
> > > > When you have two functions F and G and  want to express the
> > > > composition F o G you have to create a new closure
>
> > > > lambda *args, **kwd: F (G (*args, **kwd))
>
> > > > or you write a composition in functional style
>
> > > > compose( F, G )
>
> > > > None of these solutions is particular terse.
>
> > > > Proposal
> > > > -------------
> > > > I want to propose overloading the logical __and__ operator i.e. '&'
> > > > for functions s.t. F & G means "compose(F,G)". This suggestion is non-
> > > > conflicting since & is not used as an operator in combination with
> > > > function objects yet: given a function object F and an arbitrary
> > > > object X the combination F & X raises a TypeError when evaluated.
>
> > > > Alternatives
> > > > -----------------
> > > > One could use other unused operators like F << G or F * G to write
> > > > terse function composition expressions. I' m not sure which one is
> > > > best and would use the proposed & if no one presents arguments against
> > > > it.
>
> > > What about other callable objects?
>
> > > --
> > > Arnaud
>
> > Supporting general callables would be very fine. I thing a callable
> > ABC with two protocols named __call__ and __compose__ would be most
> > adequate. I'm just not sure if Python 2.X requires a more ad hoc
> > implementation for builtin callable types? On the level of user
> > defined classes a bundling between __call__ and __compose__ shall
> > remain optional ( unlike e.g. the dependency between __iter__ and
> > __next__ for iterables ).
>
> This would conflict with the fact that the operators you suggest for
> composition can already be overloaded for other purposes.
>
> --
> Arnaud

True. Extending a callable ABC by __add__ shall be sufficient and does
not cause ambiguities, Just one tiny teardrop for __add__ not being
very telling. Otherwise the intended semantics might be constrained to
builtin function/method/... types and classes that derive from a
callable ABCs. This is sufficient for covering the intended purpose of
function composition and does not exclude close relatives ( methods,
function-like objects ). How someone assigns semantics to a general
__call__ on his own classes is up to him.