Guido van Rossum wrote:
> Here's a related but more complicated wish: define a function in such
> a way that certain parameters *must* be passed as keywords, *without*
> using *args or **kwds. This may require a new syntactic crutch.
Perhaps:
def func(x, *args=(), foo=1): ...
Well, that doesn't really relate at all to what = means in that
position. "def func(x, *None, foo=1)"? "def func(x, **, foo=1)"... oh,
well why not just "def func(x, &optional foo=1)".
Maybe related is a way of telling if the keyword was passed at all. In
other words, a replacement for:
sentinal = []
def func(x, y=sentinal):
if y is sentinal:
# calculate from x
Or, without any danger of a misused sentinal:
def func(x, *args, **kw):
if args:
assert not kw
assert len(args) == 1
y = args[0]
elif kw:
y = kw.pop('y')
assert not kw
else:
# calculate from x
But the second gives a ruined signature, and a proper implementation has
error messages that aren't assertion errors, and so would take much
longer to write. But I can't even think of stupid syntax for the
latter; or I guess I can't think of useful semantics. The best I can
think of (ignoring syntax):
def func(x, [y=sentinal]):
try:
y
except NameError:
# calculate from x
... which is annoying, but maybe better than the alternatives. Well,
I'm not sure if it is actually better than just using a sentinal, which
generally works just fine.
--
Ian Bicking / [EMAIL PROTECTED] / http://blog.ianbicking.org
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