Edited for clarity:
Why do the list items not swap?
# python 3.5
lst = [0, 1]
lookup = {0: 1, 1: 0}
lst[0], lst[lookup[lst[0]]] = lst[lookup[lst[0]]], lst[0]
# lst is still unchanged; why aren't items 0 and 1 not swapped
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max
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This runs fine in Python 3.5. What's the problem? – Vasili Syrakis Jan 04 '17 at 23:53
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See also http://stackoverflow.com/questions/8725673/multiple-assignment-and-evaluation-order-in-python – Eli Bendersky Jan 04 '17 at 23:56
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@VasiliSyrakis Sorry, rewrote for clarity. – max Jan 05 '17 at 00:05
2 Answers
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The rhs is evaluated before the lhs. That's not the problem.
The problem is how the lhs is evaluated. You can prove this to yourself by running this statement:
lst[0], lst[lookup[lst[0]]] = 1, 0
Note that the assigning to lst[0] occurs before the evaluation of lst[lookup[lst[0]]]. So the lst[0] in that complex expression is the new value, not the old value.
Breaking it down:
lst[0], lst[lookup[lst[0]]] = 1, 0
=> lst[0] = 1; lst[lookup[lst[0]]] = 0
=> lst[0] = 1; lst[lookup[1]] = 0
=> lst[0] = 1; lst[0] = 0
So the final result appears unchanged.
Robᵩ
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Because:
lst[0], lst[lookup[lst[0]]] = lst[lookup[lst[0]]], lst[0]
is equivalent to:
lst[0]= lst[lookup[lst[0]]]
lst[lookup[lst[0]]] = lst[0]
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