[Python] Pointers/references in Python

[Python] Pointers/references in Python

[Python] Pointers/references in Python

Coming from lower-level programming languages like pure C/C++, you may think there are no pointers/references in dynamically-typed higher-level PLs like Lisp or Python.

In fact, there are, just hidden.

Ever tried to create a 2D-array in Python 2/3? Ahh, the classic bug:

>>> a=[[None]*4]*4

>>> a
[[None, None, None, None], [None, None, None, None], [None, None, None, None], [None, None, None, None]]

>>> a[0][0]=1

>>> a
[[1, None, None, None], [1, None, None, None], [1, None, None, None], [1, None, None, None]]

You do not create a 2D-array. You create a 4-item list of None-s and 4-item list of lists. The 'outer' list contains 4 links (or pointers) to the same 4-item list of None-s! They are like 'mirrors'.

So there are in fact only two lists in memory at this moment. Inner list: [None, None, None, None], and after modification: [1, None, None, None]. Outer list: [pointer, pointer, pointer, poiner].

Correct way of creating a 2D-array is creating each element:

a = [[0 for x in range(number_of_rows)] for y in range(number_of_columns)]

Hence you will have 5 lists in memory: 4 'inner' lists and 1 'outer'.

This is why there exist 'shallow copy' and 'deep copy'. These procedures are exist also in Lisp family of PLs.

'Shallow copy' copies only links/pointers. 'Deep copy' recreates all structure recursively, hence, works slow.

See the source code of the Python module that does deep copy, it recreates dictionaries and lists copying all elements one-by-one.

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Update from IRC #python (Libera):

00:50 < SnoopJ> Everything, see also: `lst = [1,2,3]; lst.append(lst)`
00:51 < SnoopJ> Everything, but in fact, in CPython, pretty much everything is PyObject*, so references and even pointers are all over
                Python ;)

Let's see:

>>> lst = [1,2,3]; lst.append(lst)
  
>>> lst
[1, 2, 3, [...]]

>>> lst[0]
1

>>> lst[1]
2

>>> lst[2]
3

>>> lst[3]
[1, 2, 3, [...]]

Note the last line -- it's like recursion. Recursive structure cannot be dumped fully due to infinite loop, you must stop at some point.

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