Tuples in function argument lists

[Steven D'Aprano]

I'm trying to understand the use of tuples in function argument lists.

I did this:

>>> def tester(a, (b,c)):
...     print a, b, c
... 
>>> tester(1, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 1, in tester
TypeError: unpack non-sequence

That was obvious result.

>>> tester(1, (2, 3))
1 2 3
>>> tester('ab', 'ab')
ab a b

And so were those.

Then I tried this:

>>> def tester(a, (b,c)=None):
...     if (b,c) is None:
...             print a, None
...     else:
...             print a, b, c

Needless to say, it did not do what I expected it to do. I didn't expect
it to either :-)

I tried looking at the language reference here:

http://docs.python.org/ref/function.html

but I can't seem to find anything in their that says that tuples-as-args
are legal. Am I misreading the docs, or is this accidental behaviour that
shouldn't be relied on?

Does anyone use this behaviour, and if so, under what circumstances is it
useful?


-- 
Steven