Data replicated in untouched array python

Question

I'm having trouble with working with this array containing a dictionary. I made the code as small as I could so the problem could be seen more easily but I'm working with a bigger array, of course.

I just want to operate with a two copies of the array and change their values to my needs, each of which can be different for the different copy of the array. So, I operate with one of the copy but I just don't understand why I'm getting the operation replicated in the other copy of the array too.

The code:

#set of data
value_indicador = [{'value': 98.0}]
value_indicador_positivo = value_indicador
value_indicador_negativo = value_indicador

for x in range(0, len(value_indicador_negativo)):

    value_indicador_negativo[x]['value'] = value_indicador_negativo[x]['value']*0.95

#what I'd like to get
value_indicador = [{'value': 98.0}]
value_indicador_positivo = [{'value': 98.0}]
value_indicador_negativo = [{'value': 93.1}]

#what I'm actually getting
value_indicador = [{'value': 93.1}]
value_indicador_positivo = [{'value': 93.1}]
value_indicador_negativo = [{'value': 93.1}]

http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list-in-python — Esref, May 12 '16 at 11:35

Matthias Schreiber · Accepted Answer · 2016-05-12T11:52:57.623

2

You didn't create a copy of the dict, just another reference to the same one. To truly copy it you can simply do this:

dict_a = {'value': 98.0}
dict_b = dict_a.copy()

And for a list:

list_a = ['value', 98.0]
list_b = list_a[:]  # This is called slicing, you simply take all the content from the other list

So your example would need to be like this:

value_indicador = [{'value': 98.0}]
value_indicador_positivo = value_indicador
value_indicador_negativo = [d.copy() for d in value_indicador]

edited May 12 '16 at 11:52

answered May 12 '16 at 11:35

Matthias Schreiber

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But it's an array, copy() doesn't work with arrays. Is it the same behaviour with arrays? – Didina Deen May 12 '16 at 11:38
arrays don't exist in Python, the most common similar things are lists and dicts. I added how to copy a list in my answer. – Matthias Schreiber May 12 '16 at 11:39
Ok, I found how, can you answer with the right method for arrays (lists) so I can accept your answer as the right one? Thank you! You showed me the way. – Didina Deen May 12 '16 at 11:40
Perfect! You were quick! – Didina Deen May 12 '16 at 11:41
I would like to link this answer too if it's useful for anyone: [link](http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list-in-python) – Didina Deen May 12 '16 at 11:42
But with copying lists like this, still if you change a dict value inside the list, other dictionaries will also be affected. – Esref May 12 '16 at 11:48
@Esref that is true, I changed my answer to reflect that. – Matthias Schreiber May 12 '16 at 11:53

Data replicated in untouched array python

1 Answers1