Python: Regular Assignment, Shallow copy & Deep Copy

Regular Assignment, Shallow copy & Deep Copy in Python

Regular Assignment, Shallow copy & Deep Copy in Python

There are three ways to copy objects in Python: assignment, shallow copy & deep copy.

1. Regular assignment points the new variable towards the original object. Let's use the builtin id() function to prove this. The id() function returns the object's memory address.

objOne = [1, 2, 3]
objTwo = [4, 5, 6]
objThree = [objOne, objTwo]

objFour = objThree

print( id(objThree) == id(objFour) )			# True as objThree & objFour are the same object
print( id(objThree[0]) == id(objFour[0]) )		# True as objThree[0] & objFour[0] are the same object

2. Shallow copy
   

   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.

   Python Documentation

import copy

objFour = copy.copy(objThree)

print( id(objThree) == id(objFour) )          # False as objFour is a new object
print( id(objThree[0]) == id(objFour[0]) )    # True as objFour[0] is the same object as objThree[0]

3. Deep Copy

objFour = copy.deepcopy(objThree)

print( id(objThree) == id(objFour) )          # False as objFour is a new object
print( id(objThree[0]) == id(objFour[0]) )    # False as objFour[0] is a new object

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See also:

• Python 101
• Object Oriented Python
• Design Patterns in Python
• 50+ Handy Standard Library Modules
• 60+ Handy Third Library Modules
• 50+ Tips & Tricks for Python Developers
• 50+ Know-How(s) Every Pythonista Must Know