Understand Python swapping: why is a, b = b, a not always equivalent to b, a = a, b?

by

From python.org

Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.

…

  • Else: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.

So I interpret that to mean that your assignment

nums[i], nums[nums[i]-1] = nums[nums[i]-1], nums[i]

is roughly equivalent to

tmp = nums[nums[i]-1], nums[i]
nums[i] = tmp[0]
nums[nums[i] - 1] = tmp[1]

(with better error-checking, of course)

whereas the other

nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]

is like

tmp = nums[i], nums[nums[i]-1]
nums[nums[i] - 1] = tmp[0]
nums[i] = tmp[1]

So the right-hand side is evaluated first in both cases. But then the two pieces of the left-hand side are evaluated in order, and the assignments are done immediately after evaluation. Crucially, this means that the second term on the left-hand side is only evaluated after the first assignment is already done. So if you update nums[i] first, then the nums[nums[i] - 1] refers to a different index than if you update nums[i] second.

More Related Contents:

  1. Access multiple elements of list knowing their index
  2. How do you find min and max without using min and max function [duplicate]
  3. Replace list object based on indexes of another list (Python)
  4. Finding the index of an item in a list
  5. User input integer list [duplicate]
  6. how to unpack a tuple in this form (‘abc’, [20, 20])? [closed]
  7. Getting a map() to return a list in Python 3.x
  8. What do ellipsis […] mean in a list?
  9. Are lists thread-safe?
  10. Why is a list comprehension so much faster than appending to a list?
  11. Extending list returns None [duplicate]
  12. In Python, how do I index a list with another list?
  13. How can I generate a list of consecutive numbers?
  14. Python: return the index of the first element of a list which makes a passed function true
  15. Using an index to get an item
  16. Print list of lists in separate lines
  17. Sort list by frequency
  18. Insert element in Python list after every nth element
  19. Finding the indices of matching elements in list in Python
  20. Why do I get “List index out of range” when trying to add consecutive numbers in a list using “for i in list”? [duplicate]
  21. TypeError: ‘float’ object is not subscriptable
  22. Indexing a list with an unique index
  23. How to apply a function to each sublist of a list in python?
  24. Changing one dict changes all dicts in a list of dicts [duplicate]
  25. Appending to list in Python dictionary [duplicate]
  26. Swap values in a tuple/list inside a list in python?
  27. How does str.startswith really work?
  28. How to find the max number(s) in a list with tied numbers
  29. Size of a Python list in memory
  30. Skip elements on a condition based in a list comprehension in python

Leave a Comment