38016226...@gmail.com writes: > L=[2,1] > L[0],L[L[0]-1]=L[L[0]-1],L[0] > > The L doesn't change. Can someone provide me the detail procedure of > this expression?
From: https://docs.python.org/3/reference/simple_stmts.html#grammar-token-assignment_stmt | Although the definition of assignment implies that overlaps between the | left-hand side and the right-hand side are ‘simultaneous’ (for example | a, b = b, a swaps two variables), overlaps within the collection of | assigned-to variables occur left-to-right, sometimes resulting in | confusion. For instance, the following program prints [0, 2]: | | x = [0, 1] | i = 0 | i, x[i] = 1, 2 # i is updated, then x[i] is updated | print(x) In your case: - L[0] get 1 (from L[L[0]-1]) - L[L[0]-1] (from lhs) is now L[0], which gets 2 (from L[0] in rhs) Since both operations happen in sequence (not simultaneously), you just overwrite the first element twice. Congratulations, a very good corner case (you can't call it a bug, since it conforms to the definition of the language). -- Alain. -- https://mail.python.org/mailman/listinfo/python-list
