Chapter 6: Problem 2

Consider these three statements. Do they change the printed value of A? $$ \begin{aligned} &A=[\text { "spam" }] \\ &B=A \\ &B[0]=\text { "shrubbery" } \end{aligned} $$

Short Answer

Expert verified

Yes, the printed value of A changes to ['shrubbery'].

Step by step solution

Initialize List A

The first statement initializes a list, A, with one element: 'spam'. So, A = ['spam'].

Assign List A to B

The second statement assigns the list A to variable B. Since lists are mutable and assignments in Python are references, B now refers to the same list as A. So, B = A.

Modify Element in List B

The third statement modifies the first element of list B to 'shrubbery'. Since B is just a reference to A, changing B[0] also changes A[0]. So, A now becomes ['shrubbery'].'

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

list assignment

Let’s dive deeper into list assignment in Python. When you assign one variable to another, you are not creating a separate copy of the list. Instead, both variables point to the same memory location where the list is stored. This is important to understand because any changes made to the list through one variable will be reflected when accessing the list through the other variable.

So in our exercise:

The first statement creates a list A = ['spam'].
The second statement, B = A, makes B refer to the same list as A.
At this point, any changes to the list through B will also appear in A and vice versa.

This behavior can be tricky at first, but it is fundamental for efficient memory management and performance in Python.

mutable objects

In Python, some objects are mutable, meaning you can change their content without changing their identity. Lists are a prime example of mutable objects. Understanding mutability is key to comprehending how Python handles complex data structures.

Mutable objects, like lists, allow in-place modifications, which means that the object retains its ID (identity) even if you alter its contents. In the given exercise:

When you modify the content of B using B[0] = 'shrubbery', you are changing the list in-place.
Since A and B reference the same list, A now reflects this change and becomes ['shrubbery'].

This is different from immutable objects, like strings and tuples, where modifying the object actually creates a new instance with a new ID.

reference behavior

Understanding reference behavior is crucial for working with mutable objects like lists in Python. When you assign one variable to another and both are mutable objects, you are creating a reference, not a copy.

Let’s break it down using our example:

Initially, A is set as A = ['spam'].
When you assign B = A, both A and B refer to the same list in memory.
By modifying the list through B, such as with B[0] = 'shrubbery', you’re essentially editing the shared list.

Due to this reference behavior, any changes made via one reference are instantly visible to any other reference to the same object. This concept is pivotal for managing data and ensuring that your programs behave as expected, especially when dealing with complex data structures.

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Consider these three statements. Do they change the printed value of A? $$ \begin{aligned} &A=[\text { "spam" }] \\ &B=A \\ &B[0]=\text { "shrubbery" } \end{aligned} $$

Short Answer

Step by step solution

Initialize List A

Assign List A to B

Modify Element in List B

Key Concepts

list assignment

mutable objects

reference behavior

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