[Tutor] beginner: using optional agument in __init__ breaks my code

[Danny Yoo]

>> The values of optional arguments are only once evaluated (when Python 
>> reads the definition). If you place there mutable objects like e.g. a 
>> list most of the time the effect you see is not what you want. So you 
>> have to write it a bit different.
>>
>>      def __init__(self, q = None):
>>          if not q: q = []
>>          self.queue = q
>>
>> Now you get a fresh list for each instance.
>>
> Thank you very much. I will use your code as a "recipe", while I still 
> try to understand the mechanism and the reasons behind it. For me this 
> feels odd.


Hi Barbara,

It is odd.  *grin*

There were at least two possible design choices to Python's behavior here. 
On every __init__, should Python re-evaluate the default argument 
expression?  Or should that value be fixed?

The designers of Python chose to the former, which leads to slight 
weirdness like this.