[Python-Dev] Default constructor values (Re: [Python-checkins] python/dist/src/Doc/lib libfuncs.tex,1.134,1.135)

[Guido van Rossum]

> So, I think some use cases do exist for the general-ish rule
> "whenever it makes some sense, a type is callable without arguments
> and when so called returns the one instance of that type which
> evaluates to false".

Right.  The first half of this is probably why I did it in the first
place: I'd like all types to have a constructor callable with no
arguments, and whenever it's possible and reasonable to construct a
valid instance by providing suitable default constructor arguments,
this should be so.  The second half is a pretty weak additional
requirement that I think Raymond H made up when he fixed the bool()
constructor; it just makes sense for containers and sequences to
construct an empty one and for numbers to use zero.

BTW, for your amusement, here's a (theoretical) use case: sometimes
you write some polymorphic code that takes a sequence but would like
to use an empty sequence of the same type.  You can do a[:0] to create
one, but some sequence types (e.g. Numeric arrays) have funny slice
semantics.  Calling type(a)() would make perfect sense for creating an
empty of the same type.  Moreover (and here it gets really theoretical
:-) it's possible to write polymorphic code that accepts either a
sequence or a number as argument, using only + and int* as operators.
In this case, type(a)() is even more attractive for getting the "zero"
of the group.

--Guido van Rossum (home page: http://www.python.org/~guido/)