Bug ??

[Eddy Ilg]

Hi,

I have a class and I am trying to set the instance varirable 'variables'
(also tried different names). The variable gets initialized by
default-value parameter of the constructor. When I change the variable and
call the constructor again, the default value changes !!! Is this supposed
to happen? See code and example below:

class url:
def __init__(self,l ink,vars={}):
self.link=link
print "vars:",var s
if hasattr(self,'v ars'):
print "self.variables :",self.variabl es
self.variables= vars
print "self.variables :",self.variabl es

def set_var(self,na me,value):
self.variables[name]=value

[color=blue][color=green][color=darkred]
>>> from generator import url
>>> u=url('image')[/color][/color][/color]
vars: {}
self.variables: {}[color=blue][color=green][color=darkred]
>>> u.set_var('a',5 )
>>> v=url('test')[/color][/color][/color]
vars: {'a': 5}
self.variables: {'a': 5}

See that 'vars' gets the old value of 'variables' passed? How can this be???
I am using python version 2.3.5

Eddy

[Mike Meyer]

Re: Bug ??

"Eddy Ilg" <eddy@fericom.n et> writes:[color=blue]
> Hi,
>
> I have a class and I am trying to set the instance varirable 'variables'
> (also tried different names). The variable gets initialized by
> default-value parameter of the constructor. When I change the variable and
> call the constructor again, the default value changes !!! Is this supposed
> to happen? See code and example below:
>
> class url:
> def __init__(self,l ink,vars={}):
> self.link=link
> print "vars:",var s
> if hasattr(self,'v ars'):
> print "self.variables :",self.variabl es
> self.variables= vars
> print "self.variables :",self.variabl es
>
> def set_var(self,na me,value):
> self.variables[name]=value
>
>[color=green][color=darkred]
>>>> from generator import url
>>>> u=url('image')[/color][/color]
> vars: {}
> self.variables: {}[color=green][color=darkred]
>>>> u.set_var('a',5 )
>>>> v=url('test')[/color][/color]
> vars: {'a': 5}
> self.variables: {'a': 5}
>
> See that 'vars' gets the old value of 'variables' passed? How can this be???
> I am using python version 2.3.5[/color]

This is a FAQ. Default arguments are evaluated when the function is
*defined*, not when it's called. To get the behavior you want, write:

def __init__(self, link, vars = None):
if not vars:
vars = {}
...

<mike
--
Mike Meyer <mwm@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.