“Deep copy” nested list without using the deepcopy function

Question

I am trying to copy the nested list a, but do not know how to do it without using the copy.deepcopy function.

a = [[1, 2], [3, 4]]

I used:

b = a[:]

and

b = a[:][:]

But they all turn out to be shallow copy.

Any hints?

Answer 1

My entry to simulate copy.deepcopy:

def deepcopy(obj):
    if isinstance(obj, dict):
        return {deepcopy(key): deepcopy(value) for key, value in obj.items()}
    if hasattr(obj, '__iter__'):
        return type(obj)(deepcopy(item) for item in obj)
    return obj

The strategy: iterate across each element of the passed-in object, recursively descending into elements that are also iterable and making new objects of their same type.

I make no claim whatsoever that this is comprehensive or without fault [1] (don't pass in an object that references itself!) but should get you started.

[1] Truly! The point here is to demonstrate, not cover every possible eventuality. The source to copy.deepcopy is 50 lines long and it doesn't handle everything.

Answer 2

You can use a LC if there's but a single level.

b = [x[:] for x in a]

Answer 3

This is a complete cheat - but will work for lists of "primitives" - lists, dicts, strings, numbers:

def cheat_copy(nested_content):
  return eval(repr(nested_content))

There are probably implications to consider for this - and it will not be particularly fast.

Answer 4

I found a way to do it using recursion.

def deep_copy(nested_content):
    if not isinstance(nested_content,list):
        return nested_content
    else:
        holder = []
        for sub_content in nested_content:
            holder.append(deep_copy(sub_content))
        return holder