There are work-arounds, of course, but there are perfectly adequate
work-arounds for the lack of delayed evaluation defaults now, and it
hasn't stopped the complaints.

I'm going to suggest that any syntax should be applied to the formal
parameter name, not the default value. This feels right to me -- we're
saying that it's the formal parameter that is "special" for using
delayed semantics, not that the default object assigned to it is
special. Hence it should be the formal parameter that is tagged, not
the default value.

By analogy with the use of the unary-* operator, I suggest we use a new
unary-operator to indicate the new semantics. Inside the parameter
list, &x means to delay evaluation of the default argument to x to
runtime:

def parrot(a, b, x=[], &y=[], *args, **kwargs):

As a bonus, this will allow for a whole new series of bike-shedding
arguments about which specific operator should be used. *grin*

Tagging a parameter with unary-& but failing to specify a default value
should be a syntax error:

def parrot(&x, &y=[]):

Likewise for unary-& outside of a parameter list.

Bike-shedding away... *wink*