Avoid shared mutable default arguments

The problem

Implement independent_batches(first, second). Call a nested add_batch function twice without providing its optional accumulator. Return the two independent collected lists.

Function: def independent_batches(first,second):

Example input (a JSON list of arguments): [[1, 2], [3]]
Expected output: [[1, 2], [3]]

Inputs satisfy the problem statement. Sequences contain at most 200 elements unless a smaller bound is stated. Use JSON values and the supplied function signature.

Hints

1. Use None as the default accumulator and allocate a list inside the call.
2. Check the supplied examples, empty/boundary cases and the stated answer policy before optimizing.

Worked answer

Try it yourself first. When you are ready, open the answer below.

def independent_batches(first,second):
    def add_batch(values,acc=None):
        if acc is None:acc=[]
        acc.extend(values);return acc
    return [add_batch(first),add_batch(second)]

Why it works

Use None as the default accumulator and allocate a list inside the call. A mutable default object is created once at function definition, so subsequent independent calls would otherwise share it.

Complexity: O(n+m) time / O(n+m) output space.

Time: O(n+m) time · Space: O(n+m) output space