🔗 References vs Values

In Python, assignment does not copy. It binds a name to an object.

This can be surprising with mutable objects:

a = [1, 2]
b = a
b.append(3)

print(a)  # [1, 2, 3]

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✅ Shallow vs Deep Copy

• Shallow copy copies the container, but keeps references to the same inner objects.
• Deep copy recursively copies inner objects too.

import copy

nested = [[1], [2]]
shallow = copy.copy(nested)
deep = copy.deepcopy(nested)

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✅ When to Copy

Copy when:

• you want to prevent accidental mutation via aliases
• you need an independent version to modify

Avoid copying when:

• the object is large and you only need read access
• you can use an immutable type (tuple, frozenset, etc.)

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🔍 Key Takeaways

• Python variables hold references.
• Mutations affect all references to the same object.
• Use shallow/deep copies intentionally.

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