Python Lists: Pass by Value or Pass by Reference?

Python: Passing Lists by Value vs. Reference

In Python, understanding variable assignments is crucial. When assigning lists, the default behavior is pass-by-reference, meaning both the original and assigned variables share the same underlying object in memory. This can lead to unexpected modifications, as demonstrated below:

a = ['help', 'copyright', 'credits', 'license']
b = a
b.append('XYZ')
print(b)  # Output: ['help', 'copyright', 'credits', 'license', 'XYZ']
print(a)  # Output: ['help', 'copyright', 'credits', 'license', 'XYZ']

In this example, appending 'XYZ' to 'b' also affects 'a' because they reference the same object in memory. To avoid this behavior and create a genuine copy of the list, you must use Python's slicing assignment:

b = a[:]

This operation creates a new list object in memory, independent of the original list. As a result, any modifications made to 'b' will not affect 'a'.

In summary, understanding Python's pass-by-reference mechanism is essential for working with mutable objects such as lists. By using slicing assignment, you can create copies of lists and ensure their values remain unaffected by subsequent modifications.

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