5.7 Passing Lists to Functions

In the last chapter, we mentioned that all objects are passed by reference and demonstrated passing an immutable object as a function argument. Here, we discuss references further by examining what happens when a program passes a mutable list object to a function.

Passing an Entire List to a Function

Consider the function modify_elements, which receives a reference to a list and multiplies each of the list’s element values by 2:


In [1]: def modify_elements(items):
   ...:     """"Multiplies all element values in items by 2."""
   ...:     for i in range(len(items)):
   ...:          items[i] *= 2
   ...:
In [2]: numbers = [10, 3, 7, 1, 9]

In [3]: modify_elements(numbers)

In [4]: numbers
Out[4]: [20, 6, 14, 2, 18]

Function modify_elements’ items parameter receives a reference to the original list, so the statement in the loop’s suite modifies each element in the original list object.

Passing a Tuple to a Function

When you pass a tuple to a function, attempting to modify the tuple’s immutable elements results in a TypeError:

In [5]: numbers_tuple = (10, 20, 30)

In [6]: numbers_tuple
Out[6]: (10, 20, 30)

In [7]: modify_elements(numbers_tuple)
-------------------------------------------------------------------------
TypeError                               Traceback (most recent call last)
<ipython-input-27-9339741cd595> in <module>()
----> 1 modify_elements(numbers_tuple)

<ipython-input-25-27acb8f8f44c> in modify_elements(items)
      2    """"Multiplies all element values in items by 2."""
      3    for i in range(len(items)):
----> 4        items[i] *= 2
      5
      6

TypeError: 'tuple' object does not support item assignment

Recall that tuples may contain mutable objects, such as lists. Those objects still can be modified when a tuple is passed to a function.

A Note Regarding Tracebacks

The previous traceback shows the two snippets that led to the TypeError. The first is snippet [7]’s function call. The second is snippet [1]’s function definition. Line numbers precede each snippet’s code. We’ve demonstrated mostly single-line snippets. When an exception occurs in such a snippet, it’s always preceded by ----> 1, indicating that line 1 (the snippet’s only line) caused the exception. Multiline snippets like the definition of modify_elements show consecutive line numbers starting at 1. The notation ----> 4 above indicates that the exception occurred in line 4 of modify_elements. No matter how long the traceback is, the last line of code with ----> caused the exception.

Self Check

(True/False) You cannot modify a list’s contents when you pass it to a function.
Answer: False. When you pass a list (a mutable object) to a function, the function receives a reference to the original list object and can use that reference to modify the original list’s contents.
(True/False) Tuples can contain lists and other mutable objects. Those mutable objects can be modified when a tuple is passed to a function.
Answer: True.