What is the output of the following Python snippet, and why does sum equal 11?

The snippet demonstrates how assignment in Python creates references rather than copies. When you write names2 = names1, both variables point to the same list object in memory. Mutating the list through names2, such as names2[0] = 'Bob', also changes names1[0] because they share the same underlying list. In the code, b[0] is 'Bob', so the condition if b[0] in (…) evaluates to True. The next condition if a[1] == 'Alice' is False because a[1] is 'Bear'. Therefore, only the statement sum += 1 executes, adding 1 to the initial sum of 10, resulting in sum = 11. The key takeaway is that assignment copies references, not the actual list contents. To create an independent copy, you would use names2 = names1.copy() or names2 = list(names1). This subtlety is crucial when working with mutable objects in Python, as unintended side effects can arise if you assume a deep copy has been made.