1. Methods: 3 Flavours

This post will through 3 different methods to copy a list and see the differences.

1.1 Assignment:

Code: =
Creates a new variable name, not a new object.
New varaible name references for the same original object.

1.2 Shallow Copy:

Code: copy.copy()
Creates a new top level object
Maintains original references (i.e. not new copies) of same objects

1.3 Deep Copy:

Code: copy.deepcopy()
Creates a new object
Creates new copies recursively of all inner objects

2. Analysis

High-level Steps:

2.1 Create Cat Class and Object
2.2 Create Cat List and append Cat Object milo
2.3 Create Copies Cat List with Each Method
2.4 Are each milo object new or copies of original milo?
2.5 Update milo to Milo
2.6 Results of milo the Different Methods

2.1 Create Cat Class and Object

class Cat():
    def __init__(self, name: str):
        self.name = name
        
    def __repr__(self) -> str:
        return self.name

Milo_obj = Cat(name="milo")
print(f"id: [{id(Milo_obj)}]")

id: [140517113061744]

2.2 Create Cat List and append Cat Object `milo`

import copy
listcats = ['Oreo','Lilo','Wasabi',Milo_obj]
listcats

['Oreo', 'Lilo', 'Wasabi', milo]

2.3 Create Copies of Cat List with Each Method

Assignment = results:

Gives a reference back to same origin object (same id:140517134247168)

Others results (.copy, .deepcopy, :):

create a new (top-level) object (difference ids):
- Note: Only compared ids of outer layer list objects. We need to see the individual items within the list (next section).

listcats_copy_assn      = listcats               
listcats_copy_shallow   = copy.copy(listcats)    
listcats_copy_deep      = copy.deepcopy(listcats)
listcats_copy_slice     = listcats[:]            
print(f"[id:{id(listcats)}] listcats_og: {listcats}")                       
print(f"[id:{id(listcats_copy_assn)}] listcats_cpy_assn: {listcats_copy_assn}")       
print(f"[id:{id(listcats_copy_shallow)}] listcats_cpy_shallow: {listcats_copy_shallow}") 
print(f"[id:{id(listcats_copy_deep)}] listcats_cpy_deep: {listcats_copy_deep}")      
print(f"[id:{id(listcats_copy_slice)}] listcats_cpy_slice: {listcats_copy_slice}")

[id:140517134247168] listcats_og: ['Oreo', 'Lilo', 'Wasabi', milo]
[id:140517134247168] listcats_cpy_assn: ['Oreo', 'Lilo', 'Wasabi', milo]
[id:140517124516096] listcats_cpy_shallow: ['Oreo', 'Lilo', 'Wasabi', milo]
[id:140517110270272] listcats_cpy_deep: ['Oreo', 'Lilo', 'Wasabi', milo]
[id:140517124392064] listcats_cpy_slice: ['Oreo', 'Lilo', 'Wasabi', milo]

2.4 Are each `milo` object new or copies of original?

Shallow copies keeps the same references:

All copy methods (except deepcopy()) have the same milo object as original id: [140517113061744]

Deep copies creates new objects: creates new id’s

print(f"[{id(listcats[3])}]-[milo id] of [listcats_og]: {listcats[3]}")                       
print(f"[{id(listcats_copy_assn[3])}]-[milo id] of [listcats_cpy_assn]: {listcats_copy_assn[3]}")       
print(f"[{id(listcats_copy_shallow[3])}]-[milo id] of [listcats_cpy_shallow]: {listcats_copy_shallow[3]}") 
print(f"[{id(listcats_copy_deep[3])}]-[milo id] of [listcats_cpy_deep]: {listcats_copy_deep[3]}")      
print(f"[{id(listcats_copy_slice[3])}]-[milo id] of [listcats_cpy_slice]: {listcats_copy_slice[3]}")

[140517113061744]-[milo id] of [listcats_og]: milo
[140517113061744]-[milo id] of [listcats_cpy_assn]: milo
[140517113061744]-[milo id] of [listcats_cpy_shallow]: milo
[140517115254800]-[milo id] of [listcats_cpy_deep]: milo
[140517113061744]-[milo id] of [listcats_cpy_slice]: milo

2.5 Update `milo` to `Milo`

Milo_obj.name="Milo"
Milo_obj

Milo

2.6 Results of `milo` the Different Methods

As expected:

The deepcopy list milo has not updated (since its a new object, i.e. new object id: 140517115254800)
All other methods do update original milo since references are maintained, rather than creating new objects (i.e. same object id: 140517113061744)

print(f"[{id(listcats[3])}]-[milo id] of [listcats_og]: {listcats[3]}")                       
print(f"[{id(listcats_copy_assn[3])}]-[milo id] of [listcats_cpy_assn]: {listcats_copy_assn[3]}")       
print(f"[{id(listcats_copy_shallow[3])}]-[milo id] of [listcats_cpy_shallow]: {listcats_copy_shallow[3]}") 
print(f"[{id(listcats_copy_deep[3])}]-[milo id] of [listcats_cpy_deep]: {listcats_copy_deep[3]}")      
print(f"[{id(listcats_copy_slice[3])}]-[milo id] of [listcats_cpy_slice]: {listcats_copy_slice[3]}")

[140517113061744]-[milo id] of [listcats_og]: Milo
[140517113061744]-[milo id] of [listcats_cpy_assn]: Milo
[140517113061744]-[milo id] of [listcats_cpy_shallow]: Milo
[140517115254800]-[milo id] of [listcats_cpy_deep]: milo
[140517113061744]-[milo id] of [listcats_cpy_slice]: Milo