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,link,vars={}):
self.link=link
print "vars:",vars
if hasattr(self,'vars'):
print "self.variables:",self.variables
self.variables=vars
print "self.variables:",self.variables

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

vars: {}
self.variables: {}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]

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,link,vars={}):
self.link=link
print "vars:",vars
if hasattr(self,'vars'):
print "self.variables:",self.variables
self.variables=vars
print "self.variables:",self.variables

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

vars: {}
self.variables: {}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

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