Python objects confusion: a=b, modify b and a changes! [duplicate]

by

What you are experiencing is the concept of references. All objects in Python have a reference and when you assign one to two names a and b, this results in both a and b pointing to the same object.

>>> a = range(3)
>>> b = a                     # same object
>>> b.append(3)
>>> a, b                      # same contents
([0, 1, 2, 3], [0, 1, 2, 3])

With lists, you can make create a new list b that is a copy of another a using b = a[:].

>>> a = range(3)
>>> b = a[:]                  # make b a new copy of a
>>> b.append(3)
>>> a, b                      # a is left unchanged
([0, 1, 2], [0, 1, 2, 3])

For a more general solution for any object, use the copy module. A shallow copy will copy the references stored within the object you’re copying, whereas a deep copy will recursively make new copies of all objects.

>>> a = [range(2), range(3)]
>>> b = copy.copy(a)          # shallow copy of a, equivalent to a[:]
>>> b[0] = range(4)
>>> a, b                      # setting an element of b leaves a unchanged
([[0, 1], [0, 1, 2]], [[0, 1, 2, 3], [0, 1, 2]])
>>> b[1].append(3)
>>> a, b                      # modifying an element of b modifies the element in a
([[0, 1], [0, 1, 2, 3]], [[0, 1, 2, 3], [0, 1, 2, 3]])

>>> a = [range(2), range(3)]
>>> b = copy.deepcopy(a)      # deep recursive copy of a
>>> b[1].append(3)
>>> a, b                      # modifying anything in b leaves a unchanged
([[0, 1], [0, 1, 2]], [[0, 1], [0, 1, 2, 3]])

More Related Contents:

  1. If two variables point to the same object, why doesn’t reassigning one variable affect the other?
  2. Retrieving the list of references to an object in Python
  3. How to add numbers inside a variable? [closed]
  4. How to use variables from a function in a different function
  5. Is there a difference between “==” and “is”?
  6. How can you dynamically create variables? [duplicate]
  7. What is the purpose of the single underscore “_” variable in Python?
  8. How do I put a variable’s value inside a string?
  9. Getting the name of a variable as a string
  10. How to use a variable inside a regular expression?
  11. How do I check if a variable exists?
  12. Are Python variables pointers? Or else, what are they?
  13. Checking multiple values for a variable [duplicate]
  14. How to get the original variable name of variable passed to a function
  15. How to create a reference to a variable in python?
  16. Importing variables from another file?
  17. How would I access variables from one class to another?
  18. Variable assignment and modification (in python) [duplicate]
  19. How can I print variable and string on same line in Python?
  20. What is the naming convention in Python for variable and function?
  21. Dynamic variable in Python [duplicate]
  22. How do I save and restore multiple variables in python?
  23. Setting variables with exec inside a function
  24. pyodbc – How to perform a select statement using a variable for a parameter
  25. Best way to retrieve variable values from a text file?
  26. How do I create a multiline Python string with inline variables?
  27. How to obtain values of request variables using Python and Flask [duplicate]
  28. Only add to a dict if a condition is met
  29. Is there a generic way for a function to reference itself?
  30. Using a loop in Python to name variables [duplicate]

Leave a Comment