python adding list inside list unexpected behavior

Question

Python 2.7.3 - Debian 7 - 32 bits

I am trying to add lists(listado) inside a list(tabla), but when tabla is printed all the elements in tabla are the same and besides it is the last list added !!!!

tabla = []
listado = [0,0,0]    
lista_base = range(100)                       

for elemento in lista_base:
    listado[0] = elemento
    listado[1] = elemento+1
    listado[2] = elemento+2
    tabla.append(listado)       # <--- What is wrong here ??
    print(listado)              # <--- This works fine. It is print each *listado*.

print(tabla)

Rohit Jain · Accepted Answer · 2013-08-10 11:08:37Z

1

You are changing the content of same list, and adding a reference to it in your tabla. So, all the lists in the tabla will be same as the last list added.

You should create a new list each time in the loop. Try changing your loop to:

for elemento in lista_base:
    listado = [elemento, elemento+1, elemento+2]
    tabla.append(listado)

answered Aug 10, 2013 at 11:08

Rohit Jain

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Martijn Pieters · Accepted Answer · 2013-08-10 11:32:36Z

You are manipulating the same list over and over again, not a copy.

Create a new list in the loop instead:

for elemento in lista_base:
    listado = [elemento, elemento + 1, elemento + 2]
    tabla.append(listado)

or create a copy of the list:

for elemento in lista_base:
    listado[0] = elemento
    listado[1] = elemento+1
    listado[2] = elemento+2
    tabla.append(listado[:])

where [:] returns a full slice of all the elements. You could also use list(listado) or import the copy module and use copy.copy(listado) to create a copy of the existing list.

Appending a list to another list only adds a reference, and your code thus created many references to the same list that you kept changing in the loop.

You could have seen what was happening had you printed tabla on every loop. Printing listado on every loop iteration only shows that the state of that list was correct for that iteration, not that all the references to that list in tabla were changing along with it:

>>> tabla = []
>>> listado = [0, 0, 0]
>>> for elemento in range(3):
...     listado[0] = elemento
...     listado[1] = elemento+1
...     listado[2] = elemento+2
...     tabla.append(listado)
...     print 'tabla after iteration {}: {!r}'.format(elemento, tabla)
... 
tabla after iteration 0: [[0, 1, 2]]
tabla after iteration 1: [[1, 2, 3], [1, 2, 3]]
tabla after iteration 2: [[2, 3, 4], [2, 3, 4], [2, 3, 4]]

Notice how all tabla lists change together; they are in fact all the same list. If you create a new list instead, things work as expected:

>>> tabla = []
>>> for elemento in range(3):
...     listado = [elemento, elemento + 1, elemento + 2]
...     tabla.append(listado)
...     print 'tabla after iteration {}: {!r}'.format(elemento, tabla)
... 
tabla after iteration 0: [[0, 1, 2]]
tabla after iteration 1: [[0, 1, 2], [1, 2, 3]]
tabla after iteration 2: [[0, 1, 2], [1, 2, 3], [2, 3, 4]]

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