variable & scoping question.

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Tue Aug 11 08:51:28 CEST 2009


On Mon, 10 Aug 2009 08:46:17 -0700, Cornelius Keller wrote:

> On 10 Aug., 17:12, "Diez B. Roggisch" <de... at nospam.web.de> wrote:
>> Cornelius Keller wrote:
> [snip]
>>
>> http://effbot.org/zone/default-values.htm
>>
>> Diez
> 
> Ok thank you.
> I' understand now why.
> I still think this is very confusing, because default values don't
> behave like most people would expect without reading the docs.


Really? How do you expect the default value to behave in this example?

>>> import time
>>> def test(x=time.time()):
...     print x
...
>>>
>>> test()
1249972984.33
>>> time.sleep(30)
>>> test()
1249972984.33

You get the same default object each time you call the function, NOT a 
fresh one created. I'm sure I'd be terribly confused if Python re-
evaluated the default value each time I called the function.

There's no difference between this and the case x=[], except that lists 
are mutable and floats aren't. You get the same default list each time, 
it just has different stuff in it. The alternative would be to get a 
different list each time, and that would require re-evaluating the 
default each time the function was called, which is horrible.




-- 
Steven



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