Python : How to copy a dictionary | Shallow Copy vs Deep Copy

Dictionaries are Python’s implementation of an associative array data structure. A dictionary is a grouping of key-value pairs. Each key pair is represented by a key pair and its associated value.

A dictionary is defined by a list of key value pairs enclosed in curly braces and separated by a comma. The value of each key is separated by the column ‘:’.

It is not possible to sort a dictionary solely to obtain a representation of the sorted dictionary. Dictionary entries are ordered by default, but other data types, such as lists and tuples, are not. As a result, you’ll need an ordered data form like a list—most likely a list of tuples.

Examples:

Input:

dictionary = {'this': 200, 'is':100, 'BTechGeeks':300}

Output:

Old dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}
New dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}

Copy a Dictionary:

There are several ways to copy a dictionary some of them are:

Using loop and copying element by element
Using = operator
Using copy() to create a shallow copy of dictionary
Using copy.deepcopy() to copy a dictionary

Method #1:Using loop and copying element by element

This technique involves traversing the entire dictionary and copying each and every element pointed by the key to a previously declared new dictionary.

Approach:

Take a empty new dictionary.
Traverse the old dictionary using for loop.
Copy the key and value of old dictionary element by element.

Below is the implementation:

# given dictionary
dictionary = {'this': 200, 'is': 100, 'BTechGeeks': 300}
# Take a empty dictionary
newdict = {}
# Traverse the old dictionary
for key in dictionary:
    newdict[key] = dictionary[key]
# print old dictionary
print("Old dictionary : ", dictionary)
# print new dictionary
print("New dictionary : ", newdict)

Output:

Old dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}
New dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}

Method #2: Using = operator

We can simply copy a dictionary using = operator.

Below is the implementation:

# given dictionary
dictionary = {'this': 200, 'is': 100, 'BTechGeeks': 300}
# copying dictionary to new dictionary
newdict = dictionary
# print old dictionary
print("Old dictionary : ", dictionary)
# print new dictionary
print("New dictionary : ", newdict)

Output:

Old dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}
New dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}

Explanation:

We begin by creating a dictionary dictionary. And then use the code line newlist=dictionary to copy it to a new object dict2.
This procedure copies each object’s references from dictionary to the new dictionary newdict.
As a consequence, changing any aspect of newdict will change dictionary and vice versa.
The above code shows that when we update any(iterable or non-iterable) object in newdict, the same change is seen in dictionary.

Method #3: Using copy() to create a shallow copy of dictionary

A shallow copy is when a new dictionary object is formed and references to existing dictionary objects are inserted into it.

In Python, the dictionary copy() method creates a shallow copy of the given dictionary. It’s identical to what we saw before when copying elements by traversing through a dictionary.

Below is the implementation:

# given dictionary
dictionary = {'this': 200, 'is': 100, 'BTechGeeks': 300}
# copying dictionary to new dictionary using copy
newdict = dictionary.copy()
# updating new dictionary
newdict['hello'] = 400
# print old dictionary
print("Old dictionary : ", dictionary)
# print new dictionary
print("New dictionary : ", newdict)

Output:

Old dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}
New dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300, 'hello': 400}

Explanation:

We initialize the dictionary dictionary with any values. To make a shallow copy, use the copy() method on it.
We update the new elements and see the corresponding shift in the original dictionary after the copy is made.
Changes in non-iterable elements of newdict have no impact on the original dictionary, just as they do with the element-by-element copying technique.
The change is reflected in the given dictionary, dictionary, for iterable ones like lists.

Method #4: Using copy.deepcopy() to copy a dictionary

In Python, the deepcopy() method is part of the copy module. It creates a new dictionary from the passed dictionary’s elements. This method recursively copies all of the elements of the given dictionary.

Below is the implementation:

# importing copy
import copy
# given dictionary
dictionary = {'this': 200, 'is': 100, 'BTechGeeks': 300}
# copying dictionary to new dictionary using deepcopy
newdict = copy.deepcopy(dictionary)
# updating new dictionary
newdict['hello'] = 400
# print old dictionary
print("Old dictionary : ", dictionary)
# print new dictionary
print("New dictionary : ", newdict)

Output:

Old dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300}
New dictionary :  {'this': 200, 'is': 100, 'BTechGeeks': 300, 'hello': 400}

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