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Python Alias, Shallow and Deep copy

4 min read

Index

  1. Alias
  2. Shallow copy vs Deep copy
  3. Shallow copy vs Deep copy (unmutable objects)
  4. Shallow copy vs Deep copy (mutable objects)

Alias

  • The assignment operator in python:
    • Doesn’t create a copy of the objects
    • It makes bindings between a variable and a object

Partial example:

a = [1,2,3]
b = a # y is an alias of x
 
a.append(4)
#a.out = [1,2,3,4]
#b.out = [1,2,3,4]
 
print(a is b) #Output: True
 
a = 1
#a.out = 1
#b.out = [1,2,3,4]
 
print(a is b) #Output: False

Shallow copy vs Deep copy

Shallow copy: Create a new object and insert references into the new object pointed at those found in the original.

Deep copy: Create a new object, and then recursively insert redundant copies of the objects found in the original.

Shallow copy vs Deep copy (unmutable objects)

Oss:

There isn’t any difference between alice, shallow copy and deep copy of an unmutable object

Unmutable not compound object

import copy
 
x = "hello world"
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
#test is x?
print(alias is x)   #Output: True
print(shallow is x) #Output: True
print(deep is x)    #Output: True
 
# x edit test
x += '!' # you can't modify an unmutable object, 
          # so the new x is a different object than the old x
print(alias is x)   #Output: False
print(shallow is x) #Output: False
print(deep is x)    #Output: False

Unmutable object compound by unmutable objects

import copy
 
x = ((1), 2)
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
#test is X?
print(alias is x)   #Output: True
print(shallow is x) #Output: True
print(deep is x)    #Output: True
 
# x[1] += 1 -> error
#you can't use an assignment operator on a tuple item 
#so it's impossible to edit it

Unmutable object compound by un mutable objects:

import copy
 
x = ([1], {3})
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
#test is x?
print(alias is x)   #Output: True
print(shallow is x) #Output: True
print(deep is x)    #Output: False
 
# test is x[0]
print(alias[0] is x[0])   #Output: True
print(shallow[0] is x[0]) #Output: True
print(deep[0] is x[0])    #Output: False
 
#x item edit tedt
x[0].append(2), 
print(alias)   #Output: ([1, 2], {3})
print(shallow) #Output: ([1, 2], {3})
print(deep)    #Output: ([1], {3})


Shallow copy vs Deep copy (mutable objects)

Mutable not compound object

import copy
 
x = [1,1]
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
# test is x?
print(alias is x)   #Output: True
print(shallow is x) #Output: False
print(deep is x)    #Output: False
 
# test is x[1]
print(alias[1] is x[1])   #Output: True
print(shallow[1] is x[1]) #Output: True
print(deep[1] is x[1])    #Output: True
 
# x edit test
x.append(3)
print(alias)    #Output: [1,1,3]
print(shallow)  #Output: [1,1]
print(deep)     #Output: [1,1]
 
# x item edit test
x[1] += 1
print(alias)    #Output: [1,2,3]
print(shallow)  #Output: [1,1]
print(deep)     #Output: [1,1]
 
# test is x[1]
print(alias[1] is x[1])   #Output: True
print(shallow[1] is x[1]) #Output: False
print(deep[1] is x[1])    #Output: False

Mutable object compound by unmutable objects

import copy
 
x = ["hello", 1] 
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
# test is x ?
print(alias is x)   #Output: True
print(shallow is x) #Output: False
print(deep is x)    #Output: False
 
# test is x[0]
print(alias[0] is x[0])   #Output: True
print(shallow[0] is x[0]) #Output: True
print(deep[0] is x[0])    #Output: True
 
# test is x[1]
print(alias[1] is x[1])   #Output: True
print(shallow[1] is x[1]) #Output: True
print(deep[1] is x[1])    #Output: True
 
# x item edit test
x[0] += "!"     # you cant't edit a string so the result is a new sting
print(alias)    #Output: ['hello!', 1]
print(shallow)  #Output: ['hello', 1]
print(deep)     #Output: ['hello', 1]
 
# test is x[0]
print(alias[0] is x[0])   #Output: True
print(shallow[0] is x[0]) #Output: False
print(deep[0] is x[0])    #Output: False

Mutable object compound by un mutable objects

import copy
 
x = [[1], [2]]
 
alias = x
shallow = copy.copy(x)
deep = copy.deepcopy(x)
 
# test is x ?
print(alias is x)   #Output: True
print(shallow is x) #Output: False
print(deep is x)    #Output: False
 
# test is x[0]
print(alias[0] is x[0])   #Output: True
print(shallow[0] is x[0]) #Output: True
print(deep[0] is x[0])    #Output: False
 
# x edit test
x.append(4)
x[0].append(2)
x[1][0] += 1
print(alias)    #Output: [[1, 2], [3], 4]
print(shallow)  #Output: [[1, 2], [3]]
print(deep)     #Output: [[1], [2]]

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