TLDR: The abbreviation mutates the LHS, while the long version assigns a newly created object to it. >>> orig = [1,2,3] >>> L1 = orig >>> L2 = [7,8] >>

TLDR: The abbreviation mutates the LHS, while the long version assigns a newly created object to it. >>> orig = [1,2,3] >>> L1 = orig >>> L2 = [7,8] >>> L1 = L1 + L2 >>> orig [1, 2, 3] >>> orig = [1,2,3] >>> L1 = orig >>> L2 = [7,8] >>> L1 += L2 >>> orig [1, 2, 3, 7, 8]