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This chapter is from the book

This chapter is from the book

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_elementsitems 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-7-9339741cd595> in <module>()
----> 1 modify_elements(numbers_tuple)

<ipython-input-1-27acb8f8f44c> 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.

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