[Tutor] functions and default argument

[Alan Gauld]

On 21/10/11 21:40, Albert-Jan Roskam wrote:
> Interesting thread and webpages. Insightful, but is this really used as
> a technique in daily practice?

Yes, one example is where you use it for a counter to
determine how often a function gets called:

def reserveScarceResource(p1,p2,count = [0]):   : count is mutable
     count[0] += 1
     if count[0] > 100:
        raise UsedTooManyException
     # allocate resources

Its not really a hack, it's an implementation feature and it's
a fair impression of a closure in languages like Lisp, which makes it 
useful for certain classes of problem.

It can also be useful in state machines where you want to carry
through a state value across transitions but don't need to retain
it after the state sequence completes.

You don't need it often but its nice to have when you do!

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/