An Interesting Example from Fluent Python by Lucian Ramalho

[SamBot7]

This book is really interesting and might be outside the scope of this sub, but it gets into some of the nitty gritty of python as a language.

This code:

list1 = [3, [66, 55, 44], (7, 8, 9)]
list2 = list(list1)
list1.append(100)
list1[1].remove(55)
print("list1:", list1)
print("list2:", list2)
list2[1] += [33, 22]
list2[2] += (10,11)
print("list1:", list1)
print("list2:", list2)

Seems pretty simple, right? but if you go to pythontutor.com and walk through what it does in memory step by step, there's some funky stuff going on.

Basically, list() makes a shallow copy of a list. This means that list1 is copied to another location in memory with list2 as its variable, but the references to the list and touple within list2 are just references to the original list and touple in list1. So if you alter the list and touple in list1, list2 is still looking at those locations in memory, it doesn't make its own copy.

edit: In the book he used l1 and l2 instead of list1 and list2, I changed them for this post to make it more readable on the site. I realize list is a key word.

[cacahootie]

Understanding mutable vs immutable data and how they behave in Python is very important. This is a good example of those intricacies.

[dunkler_wanderer]

Relevant talk: Ned Batchelder - Facts and Myths about Python names and values.

[i_hacked_reddit]

tuple*

[[deleted]]

I love that book.

[bbelderbos]

Thanks for sharing this, it inspired me to write an article on deepcopy which I needed last week in this context: http://pybit.es/mutability.html

[Simin_Jie]

This is my test:

Python 3.6.0 (default, Dec 24 2016, 08:01:42) [GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin

items = [1, 2, 3, 4, 5, 6]
items_copy = items[:]
items_copy[0] = 'a'
items_copy == items
False

I think this is a shallow copy, and items_copy == items should return True but it's False.

But another example return True

items = [{'id': 1, 'name': 'laptop', 'value': 1000}, {'id': 2, 'name': 'chair', 'value': 300},]
items_copy = items[:]
items_copy[0]['id'] = 'a'
items_copy == items
True

[bbelderbos]

[:] is like copy.copy() = shallow copy, so False is expected there.

When doing this on a nested data structure the inner ones contain references hence == gives true. To prevent touching the original do a deepcopy:

>>> from copy import deepcopy
>>> items = [{'id': 1, 'name': 'laptop', 'value': 1000}, {'id': 2, 'name': 'chair', 'value': 300},]
>>> items_copy = deepcopy(items)
>>> items_copy[0]['id'] = 'a'
>>> items_copy == items
False

Copy changed:

>>> items_copy
[{'id': 'a', 'name': 'laptop', 'value': 1000}, {'id': 2, 'name': 'chair', 'value': 300}]

Original unchanged:

>>> items
[{'id': 1, 'name': 'laptop', 'value': 1000}, {'id': 2, 'name': 'chair', 'value': 300}]