Iterable Unpacking Evaluation Order

Question

I recently answered a question where a user was having trouble because they were appending a multi-dimensional array to another array, and it was brought to my attention in my a

Answer 1

The right side of the = is always evaluated first, in this case it is packing a tuple. That tuple is then unpacked when interpreting the left hand side. The left and right sides do not share knowledge of variables. The RHS becomes a value and then the LHS uses that value to assign to the variables (labels).

In your example the values of x and y are determined after the RHS is evaluated. The unpacking then occurs left to right, so that board[x][y] has valid indices.

Switching the order demonstrates the unpacking sequence:

>>> board[x][y], x, y = move[2], move[0], move[1]
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-a984ef3168f8> in <module>()
----> 1 board[x][y], x, y = move[2], move[0], move[1]    
NameError: name 'x' is not defined