Dictionary of lists strange behaviour

[Jean-Michel Pichavant]

Ciccio wrote:
> Hi all,
>
> hope you can help me understanding why the following happens:
>
> In [213]: g = {'a': ['a1','a2'], 'b':['b1','b2']}
> In [214]: rg = dict.fromkeys(g.keys(),[])
> In [215]: rg
> Out[215]: {'a': [], 'b': []}
> In [216]: rg['a'].append('x')
> In [217]: rg
> Out[217]: {'a': ['x'], 'b': ['x']}
>
> What I meant was appending 'x' to the list pointed by the key 'a' in 
> the dictionary 'rg'. Why rg['b'] is written too?
>
> Thanks.

rg = dict.fromkeys(g.keys(),[])

you are intialising the content with the same object [].

write instead

for key in g:
    rg[key] = [] # python create a new list everytime it hits this line

For the same reason you never assign an empty list to default parameters 
value:

In [37]: def a(p=[]):
   ....:     return p
   ....:

In [38]: a1 = a()

In [39]: a2 = a()

In [40]: id(a1) ; id(a2)
Out[40]: 161119884
Out[40]: 161119884

Jean-Michel