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Python Lists, Tuples and Slicing, what does print(a) print after a = [[1, 2], [3, 4]]?
Block 4, Lists, Tuples and Slicing, question 12 of the Python sample
a = [[1, 2], [3, 4]]
b = a.copy()
b[0].append(99)
print(a)What does this print?
Show the answer and why each option is right or wrong
Answer, C. [[1, 2, 99], [3, 4]]
- A. [[1, 2, 99], [3, 4], [1, 2, 99], [3, 4]]
- Assumes copying the outer list duplicates its inner lists as well, doubling the total number of sublists.
- B. [[1, 2], [3, 4]]
- Assumes copy() makes a fully independent copy at every level, so mutating a shared inner list through b would not be visible through a.
- C. [[1, 2, 99], [3, 4]]
- Correct. copy() only makes a shallow copy, so b holds new outer list slots that still point at the very same inner lists as a, and mutating one of those inner lists through b is visible through a as well.
- D. [[1, 2], [3, 4, 99]]
- Assumes the append lands on the second inner list instead of the first, misreading which sublist index 0 refers to.
A shallow copy duplicates only the outer container, the references it holds still point at the same nested objects as the original. To get inner lists that can be mutated independently, a deep copy is needed instead of a shallow one.
The answer above is what the CPython interpreter printed when the snippet was run, not what someone expected it to do.
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