How to pass a list element as reference?

Question

I am passing a single element of a list to a function. I want to modify that element, and therefore, the list itself.

def ModList(element):
    element = 'TWO'

l = list();
l.append('one')
l.append('two')
l.append('three')
print l
ModList(l[1])
print l

But this method does not modify the list. It's like the element is passed by value. The output is:

['one','two','three']
['one','two','three']

I want that the second element of the list after the function call to be 'TWO':

['one','TWO','three']

Is this possible?

Answer 1

The explanations already here are correct. However, since I have wanted to abuse python in a similar fashion, I will submit this method as a workaround.

Calling a specific element from a list directly returns a copy of the value at that element in the list. Even copying a sublist of a list returns a new reference to an array containing copies of the values. Consider this example:

>>> a = [1, 2, 3, 4]
>>> b = a[2]
>>> b
3
>>> c = a[2:3]
>>> c
[3]
>>> b=5
>>> c[0]=6
>>> a
[1, 2, 3, 4]

Neither b , a value only copy, nor c , a sublist copied from a , is able to change values in a . There is no link, despite their common origin.

However, numpy arrays use a "raw-er" memory allocation and allow views of data to be returned. A view allows data to be represented in a different way while maintaining the association with the original data. A working example is therefore

>>> import numpy as np
>>> a = np.array([1, 2, 3, 4])
>>> a
array([1, 2, 3, 4])
>>> b = a[2]
>>> b
3
>>> b=5
>>> a
array([1, 2, 3, 4])
>>> c = a[2:3]
>>> c
array([3])
>>> c[0]=6
>>> a
array([1, 2, 6, 4])
>>> 

While extracting a single element still copies by value only, maintaining an array view of element 2 is referenced to the original element 2 of a (although it is now element 0 of c ), and the change made to c 's value changes a as well.

Numpy ndarray s have many different types, including a generic object type. This means that you can maintain this "by-reference" behavior for almost any type of data, not only numerical values.

Answer 2

Python is a pass by value language hence you can't change the value by assignment in the function ModList . What you could do instead though is pass the list and index into ModList and then modify the element that way

def ModList(theList, theIndex) :
  theList[theIndex] = 'TWO'

ModList(l, 1)

Answer 3

Python doesn't do pass by reference . Just do it explicitly:

l[1] = ModList(l[1])

Also, since this only changes one element, I'd suggest that ModList is a confusing name.

Answer 4

In many cases you can also consider to let the function both modify and return the modified list. This makes the caller code more readable:

def ModList(theList, theIndex) :
          theList[theIndex] = 'TWO'
    return theList

l = ModList(l, 1)