Numpy problem: Arrays in a list of dictionaries

[Robert Kern]

ZMY wrote:
> I am new to Numpy/Pylab, and I am trying to construct a list of
> dictionaries with arrays as the items, for example:
> 
>>>> dict = {1: array([2, 3, 4]), 2: ''}
>>>> list1 = []
>>>> for i in range(3): list1.append(dict.copy())
> ...
>>>> list1
> [{1: array([2, 3, 4]), 2: ''}, {1: array([2, 3, 4]), 2: ''}, {1:
> array([2, 3, 4]), 2: ''}]
>>>> list1[0][1][1]=100
>>>> list1
> [{1: array([  2, 100,   4]), 2: ''}, {1: array([  2, 100,   4]), 2:
> ''}, {1: array([  2, 100,   4]), 2: ''}]
> 
>>>> list1[0][2]='Jack'
>>>> list1
> [{1: array([  2, 100,   4]), 2: 'Jack'}, {1: array([  2, 100,   4]),
> 2: ''}, {1: array([  2, 100,   4]), 2: ''}]
> 
> So the strings can be assigned seperately but arrays can not. What is
> the problem here?

When you call dict.copy(), all it does is make a copy of the dictionary; each
copy of the dictionary still refers to the same objects.

list1[i][1] refers to the same array for all i, so when you modify it in-place
like you did, you will see the modifications in every reference to that object.

list[i][2] also referred to the same string for all i until you *replaced* the
entry in the 0'th dictionary.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco