Python - What is the difference between shallow copy, deepcopy and normal assignment operation?

╰+攻爆jí腚メ 2022-05-29 07:47 344阅读 0赞

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Normal assignment operations will simply point the new variable towards the existing object. The docs explain the difference between shallow and deep copies:

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

  • A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
  • A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

Here’s a little demonstration:

  1. import copy
  3. a = [1, 2, 3]
  4. b = [4, 5, 6]
  5. c = [a, b]

Using normal assignment operatings to copy:

  1. d = c
  3. print id(c) == id(d)          # True - d is the same object as c
  4. print id(c[0]) == id(d[0])    # True - d[0] is the same object as c[0]

Using a shallow copy:

  1. d = copy.copy(c)
  3. print id(c) == id(d)          # False - d is now a new object
  4. print id(c[0]) == id(d[0])    # True - d[0] is the same object as c[0]

Using a deep copy:

  1. d = copy.deepcopy(c)
  3. print id(c) == id(d)          # False - d is now a new object
  4. print id(c[0]) == id(d[0])    # False - d[0] is now a new object

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