[Tutor] The trap of the year

[bob gailer]

On 1/25/2011 3:41 PM, Karim wrote:
>
> Hello All,
>
> Just to share on rageous bug I encounter where 2 lists which "should" 
> to be different (same id) because hold by different instances of the 
> same class are not in fact DIFFERENT, see below:
>
> >>> class Device():
> ...     def __init__(self, parameters=[]):
> ...         self.parameters = parameters
> ...     def param(self):
> ...         print(id(self.parameters))
> ...
> >>> a=Device()
> >>> b=Device()
> >>> a.param()
> 140559202956568
> >>> b.param()
> 140559202956568
>

This is not a bug. It is intentional behavior which is documented in the 
Python Language Reference.
Under 7.6 Function definitions you will find:

*Default parameter values are evaluated when the function definition is 
executed.* This means that the expression is evaluated once, when the 
function is defined, and that that same "pre-computed" value is used for 
each call. This is especially important to understand when a default 
parameter is a mutable object, such as a list or a dictionary: if the 
function modifies the object (e.g. by appending an item to a list), the 
default value is in effect modified. This is generally not what was 
intended. A way around this is to use None as the default, and 
explicitly test for it in the body of the function, e.g.:

-- 
Bob Gailer
919-636-4239
Chapel Hill NC

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