[Python-ideas] Default arguments in Python - the return

[Stephen J. Turnbull]

Terry Reedy writes:

 > Given how often this issue is resuscitated, I am will consider raising 
 > my personal vote from -1 to -0.

 > Do focus on practical issues, properly qualified statements, and how a 
 > proposal would improve Python.

One thing I would like to see addressed is use-cases where the
*calling* syntax *should* use default arguments.[1]  In the case of
the original example, the empty list, I think that requiring the
argument, and simply writing "foo([])" instead of "foo()" is better,
on two counts: EIBTI, and TOOWTDI.  And it's certainly not an
expensive adjustment.

In a more complicated case, it seems to me that defining (and naming)
a separate function would often be preferable to defining a
complicated default, or explicitly calling a function in the actual
argument.  Ie, rather than

    def consume_integers(ints=fibonacci_generator()):
        for i in ints:
            # suite and termination condition

why not

    def consume_integers(ints):
        for i in ints:
            # suite and termination condition

    def consume_fibonacci():
        consume_integers(fibonacci_generator())

or

    def consume_integers_internal(ints):
        for i in ints:
            # suite and termination condition

    def consume_integers():
        consume_integers_internal(fibonacci_generator())

depending on how frequent or intuitive the "default" Fibonacci
sequence is?  IMO, for both above use-cases EIBTI applies as an
argument that those are preferable to a complex, dynamically-
evaluated default, and for the second TOOWTDI also applies.

Footnotes: 
[1]  In the particular cases being advocated as support for dynamic
evaluation of default arguments, not in general.  It is clear to me
that having defaults for rarely used option arguments is a good thing,
and I think that is a sufficient justification for Python to support
default arguments.