copy in Python (Deep Copy and Shallow Copy)

In Python, Assignment statements do not copy objects, they create bindings between a target and an object. When we use = operator user thinks that this creates a new object; well, it doesn’t. It only creates a new variable that shares the reference of the original object. Sometimes a user wants to work with mutable objects, in order to do that user looks for a way to create “real copies” or “clones” of these objects. Or, sometimes a user wants copies that user can modify without automatically modifying the original at the same time, in order to do that we create copies of objects.

A copy is sometimes needed so one can change one copy without changing the other. In Python, there are two ways to create copies :

  • Deep copy
  • Shallow copy

In order to make these copy, we use copy module. We use copy module for shallow and deep copy operations. For Example

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# importing copy module
import copy
  
# initializing list 1 
li1 = [1, 2, [3,5], 4]
  
  
# using copy for shallow copy  
li2 = copy.copy(li1) 
  
# using deepcopy for deepcopy  
li3 = copy.deepcopy(li1) 

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In the above code, the copy() returns a shallow copy of list and deepcopy() return a deep copy of list.

Deep copy

Deep

Deep copy is a process in which the copying process occurs recursively. It means first constructing a new collection object and then recursively populating it with copies of the child objects found in the original. In case of deep copy, a copy of object is copied in other object. It means that any changes made to a copy of object do not reflect in the original object. In python, this is implemented using “deepcopy()” function.

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# Python code to demonstrate copy operations
  
# importing "copy" for copy operations
import copy
  
# initializing list 1
li1 = [1, 2, [3,5], 4]
  
# using deepcopy to deep copy 
li2 = copy.deepcopy(li1)
  
# original elements of list
print ("The original elements before deep copying")
for i in range(0,len(li1)):
    print (li1[i],end=" ")
  
print("\r")
  
# adding and element to new list
li2[2][0] = 7
  
# Change is reflected in l2 
print ("The new list of elements after deep copying ")
for i in range(0,len( li1)):
    print (li2[i],end=" ")
  
print("\r")
  
# Change is NOT reflected in original list
# as it is a deep copy
print ("The original elements after deep copying")
for i in range(0,len( li1)):
    print (li1[i],end=" ")

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Output:

The original elements before deep copying
1 2 [3, 5] 4 
The new list of elements after deep copying 
1 2 [7, 5] 4 
The original elements after deep copying
1 2 [3, 5] 4 

In the above example, the change made in the list did not effect in other lists, indicating the list is deep copied.
 

Shallow copy

Shallow

A shallow copy means constructing a new collection object and then populating it with references to the child objects found in the original. The copying process does not recurse and therefore won’t create copies of the child objects themselves. In case of shallow copy, a reference of object is copied in other object. It means that any changes made to a copy of object do reflect in the original object. In python, this is implemented using “copy()” function.

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# Python code to demonstrate copy operations
  
# importing "copy" for copy operations
import copy
  
# initializing list 1
li1 = [1, 2, [3,5], 4]
  
# using copy to shallow copy 
li2 = copy.copy(li1)
  
# original elements of list
print ("The original elements before shallow copying")
for i in range(0,len(li1)):
    print (li1[i],end=" ")
  
print("\r")
  
# adding and element to new list
li2[2][0] = 7
  
# checking if change is reflected
print ("The original elements after shallow copying")
for i in range(0,len( li1)):
    print (li1[i],end=" ")

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Output:

The original elements before shallow copying
1 2 [3, 5] 4 
The original elements after shallow copying
1 2 [7, 5] 4 

In the above example, the change made in the list did effect in other list, indicating the list is shallow copied.

Important Points:
The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

  • A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
  • A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.


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