Python - Deep Copy without using copy module

Question

Essentially, the problem was to create a function deepcopy(L) that will return a deep copy of a list L. However, we were told to not use the copy module or any function in it.

I'm a complete beginner in an intro class and am honestly struggling with this one. The only thing we were really told is that we should use recursion to solve the problem but after being stuck for so long I figured I'd ask for help.

Couldn't figure it out in class and didn't really get a clear answer and now the professor is moving on so ANY help would be greatly appreciated. Thanks.

• user2357112: We answer specific questions here. "ANY help would be greatly appreciated" isn't a question. You need to have a conversation with someone, and Stack Overflow isn't the place to do that.
• PM 2Ring: If you want help with this you need to show us the code you've written so far and explain what it's doing wrong.
• notanexpert: Sorry, this is the first time I've used the site. Truthfully, I don't even really know where to begin to make this work so I don't really have code written.
• PM 2Ring: Do you have any experience writing recursive functions? Complete beginner aren't usually expected to write recursive code... This task isn't hard, but there are a few pitfalls. In particular, you need to be careful with strings.
• notanexpert: Yeah, thats what everyone keeps telling me. The professor is purposely going much harder on us than you would expect in an intro class because he thinks this is the only way we'll learn anything. However it's like learning the spanish alphabet and then being told to go write an essay.

Answer 1

Maybe, you are looking for something like this:

def mydeepcopy(L):
 if isinstance(L, list):
 ret = []
 for i in L:
 ret.append(mydeepcopy(i))
 elif isinstance(L, (int, float, type(None), str, bool)):
 ret = L
 else:
 raise ValueError("Unexpected type for mydeepcopy function")
 return ret
lst = [ [1,2,3], [4,5,6], [7,8,9] ]
copy = mydeepcopy(lst)
print("Original Data: %s" % (lst))
print("Original IDs: %s = %s" % (id(lst),[ id(i) for i in lst ]))
print("Copied Data: %s" % (copy))
print("Copied IDs: %s = %s" % (id(copy),[ id(i) for i in copy ]))

Possible Output:

Original Data: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Original IDs: 140637274065288 = [140637274064712, 140637274064776, 140637274065352]
Copied Data: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Copied IDs: 140637274065480 = [140637274065416, 140637274065160, 140637274065096]

Answer 2

Deepcopy can be done using this one line method,

def deep_copy(source_list):
 dest_list = source_list[:]
 return dest_list

It gives you the smaller function and reduce the processing time.

Answer 3

I would like to pinpoint the fact that performing

dest_list = source_list[:]

does NOT create a deepcopy.

To prove this try performing the following operations:

>>> list1 = [[1,2,3], [4,5,6], [7,8,9]]
>>> list2 = list1[:]
>>> id(list1) == id(list2)
False
>>> id(list1[0]) == id(list2[0])
True

As the nested objects are just a copy of the reference, this operation classifies as shallowcopy

Answer 4

It is a single line answer:

newList = [L[i] for i in range(len(L))]

Answer 5

The answer above by Aaron is correct, except that each list in a list comprehension also needs to be a copy. So,

newList = [L[i].copy() for i in range(len(L))]