Does Python Assignment Create Copies or References to Objects?

Does Python Assign References or Copies?

Problem:

Python's assignment behavior can be confusing when working with objects like dictionaries. Consider the following code:

dict_a = dict_b = dict_c = {}
dict_c['hello'] = 'goodbye'

print(dict_a)
print(dict_b)
print(dict_c)

Expected Output:

{}
{}
{'hello': 'goodbye'}

Actual Output:

{'hello': 'goodbye'}
{'hello': 'goodbye'}
{'hello': 'goodbye'}

Explanation:

Python variables refer to objects in memory. When you assign dict_a = dict_b, you are not copying the dictionary object itself but rather assigning a reference to the same memory address as dict_b. This means that changes made to one dictionary will affect all three variables since they are pointing to the same underlying object.

Solution:

To create a true copy of an object in Python, you need to use the copy or copy.deepcopy functions. The following code will create independent copies of the original dictionary:

dict_a = dict_b.copy()  # Shallow copy
dict_a = copy.deepcopy(dict_b)  # Deep copy

• Shallow copy: Creates a new dictionary that references the same nested objects as the original.
• Deep copy: Creates a new dictionary and recursively copies all nested objects.

By using these functions, you can ensure that changes made to one dictionary do not affect the others.

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