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Python Alias, Shallow and Deep copy

Jan 18, 20254 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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