Bug in python!? persistent value of an optional parameter in function!

[Steven D'Aprano]

On Thu, 08 Mar 2007 03:14:53 +0000, John Nagle wrote:

> Paul Rubin wrote:
>> "C Barr Leigh" <cpblPublic at gmail.com> writes:
>> 
>>>Help! Have I found a serious bug?
>>>This seems like highly undesired behaviour to me. From the program
>>>below, I get output:
>> 
>> 
>> It is intentional, not a bug, see the docs.  Whether it's desirable is
>> a different question.
> 
>     True.  It would make sense to disallow mutable values as
> initial values for optional arguments.  The present behavior
> is silly.

It's just *different*, not silly.

It's also quite useful in some circumstances, e.g. cheap caching without
using a global variable.

def factorial(n, _cache={}):
    try:
       return _cache[n]
    except KeyError:
       # fall back to slow calculation
       if n <= 1:
           result = 1
       else:
           result = factorial(n-1)*n
       _cache[n] = result
       return result

There are other ways of implementing caches, but this is quick and easy
and works well for many functions.


-- 
Steven D'Aprano