mutable default arguments

[Arnaud Delobelle]

Hi everyone,

I am an 'amateur' programmer who's recently discovered python, and I
must say it's great.  I had this problem though, it took me the
whole morning to understand what it boils down to (I think).  So here is
my little piece of code:

def foo(l=[]):
	l.append('Spam!')
	print ' '.join(l)

for i in range(10): foo()

And now the output (python 2.2.1):

Spam!
Spam! Spam!
Spam! Spam! Spam!
Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam!
Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam!

Not what I expected (well it is because I made it up just for that, but
not what I would have expected).  Of course this is because argument l
defaults to [] which is mutable and obviously the default value is
stored somewhere once and for all, and not (as seemed natural to me)
dynamically recreated each time.

So my question is: what's the rationale behind it?  I've spent so much
time working out what the problem was that I have no brainpower left,
sorry.

-- 
Arnaud Delobelle
delobelle at blueyonder.co.uk