What is the difference in python between doing:
a, b = c, max(a, b)
and
a = c
b = max(a, b)
what does having the two variable assignments set on the same line do?
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Alex Riley
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farid99
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1related: http://stackoverflow.com/questions/14836228/is-there-a-standardized-method-to-swap-two-variables-in-python – NightShadeQueen Jul 22 '15 at 14:47
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1also: http://stackoverflow.com/questions/21047524/how-does-swapping-of-members-in-the-python-tuples-a-b-b-a-work-internally – NightShadeQueen Jul 22 '15 at 14:49
2 Answers
8
Your two snippets do different things: try with a, b and c equal to 7, 8 and 9 respectively.
The first snippet sets the three variables to 9, 8 and 9. In other words, max(a, b) is calculated before a is assigned to the value of c. Essentially, all that a, b = c, max(a, b) does is push two values onto the stack; the variables a and b are then assigned to these values when they are popped back off.
On the other hand, running the second snippet sets all three variables to 9. This is because a is set to point to the value of c before the function call max(a, b) is made.
Alex Riley
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How would you rewrite the first snippet in multiple lines of code? Using a "temp" variable to hold a value I'm guessing? – farid99 Jul 23 '15 at 00:46
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@farid99: yes a temp variable would be a good way: `temp = max(a, b)` then `a = c` then `b = temp`. Alternatively, in this case you could just carefully choose the order of the two lines, e.g. `b = max(a, b)` then `a = c` should have the same effect. – Alex Riley Jul 23 '15 at 08:14
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They are different. The second one is like doing
a, b = c, max(c, b)
because you are assigning c to a before doing b = max(a, b). While the first one is using the old value of a to calculate it.
Assem
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