Function changes list values and not variable values in Python [duplicate]

by

Python variables contain pointers, or references, to objects. All values (even integers) are objects, and assignment changes the variable to point to a different object. It does not store a new value in the variable, it changes the variable to refer or point to a different object. For this reason many people say that Python doesn’t have “variables,” it has “names,” and the = operation doesn’t “assign a value to a variable,” but rather “binds a name to an object.”

In plusOne you are modifying (or “mutating”) the contents of y but never change what y itself refers to. It stays pointing to the same list, the one you passed in to the function. The global variable y and the local variable y refer to the same list, so the changes are visible using either variable. Since you changed the contents of the object that was passed in, there is actually no reason to return y (in fact, returning None is what Python itself does for operations like this that modify a list “in place” — values are returned by operations that create new objects rather than mutating existing ones).

In plusOne2 you are changing the local variable a to refer to a different integer object, 3. (“Binding the name a to the object 3.”) The global variable a is not changed by this and continues to point to 2.

If you don’t want to change a list passed in, make a copy of it and change that. Then your function should return the new list since it’s one of those operations that creates a new object, and the new object will be lost if you don’t return it. You can do this as the first line of the function: x = x[:] for example (as others have pointed out). Or, if it might be useful to have the function called either way, you can have the caller pass in x[:] if he wants a copy made.

More Related Contents:

  1. How to add numbers inside a variable? [closed]
  2. Trying to use a returned value without the other parts of a function [closed]
  3. Check if input is empty
  4. How to use variables from a function in a different function
  5. List of lists changes reflected across sublists unexpectedly
  6. What is the naming convention in Python for variable and function names?
  7. How to get the original variable name of variable passed to a function
  8. Apply function to each element of a list
  9. Python append() vs. + operator on lists, why do these give different results?
  10. Python list doesn’t reflect variable change
  11. Calling variable defined inside one function from another function
  12. Variable assignment and modification (in python) [duplicate]
  13. What is the naming convention in Python for variable and function?
  14. Dynamic variable in Python [duplicate]
  15. How to call function that takes an argument in a Django template?
  16. Generating sublists using multiplication ( * ) unexpected behavior [duplicate]
  17. Setting variables with exec inside a function
  18. Print list of lists in separate lines
  19. Assigning values to variables in a list using a loop
  20. Python passing list as argument
  21. How to convert nested list of lists into a list of tuples in python 3.3?
  22. Flatten an irregular (arbitrarily nested) list of lists
  23. Python: function and variable with the same name
  24. How to assign each element of a list to a separate variable? [duplicate]
  25. How to apply a function to each sublist of a list in python?
  26. Check if all values in list are greater than a certain number
  27. Extract first item of each sublist in Python
  28. Sort python list by function
  29. Correct Use Of Global Variables In Python 3
  30. Store functions in list and call them later [duplicate]

Leave a Comment