Using dicts and lists as default arguments of functions

[MRAB]

Johan wrote:
> Dear all,
> 
> Considering this test program:
> 
> def tst(a={}):
>     print 1, a
>     a['1'] = 1
>     print 2, a
>     del a
> 
> def tstb(a=[]):
>     print 1, a
>     a.append(1)
>     print 2, a
>     del a
> 
[snip]
Do this instead:

def tst(a=None):
     if a is None:
         a = {}
     print 1, a
     a['1'] = 1
     print 2, a

def tstb(a=None):
     if a is None:
         a = []
     print 1, a
     a.append(1)
     print 2, a


It's all explained here:

     http://effbot.org/zone/default-values.htm