Why does changing list 'y' in Python also change list 'x'?

Example

In this article, we will see if if you change a list, say list y, it will also change list x. To do this, let's first look at an example with two lists and try to use append() and print for output -

x = []
y = x
print("Value of y = ",y)
print("Value of x = ",x)
y.append(25)

print("\nAfter changing...")
print("Value of y = ",y)
print("Value of x = ",x)

Output

('Value of y = ', [])
('Value of x = ', [])

After changing...
('Value of y = ', [25])
('Value of x = ', [25])

Above, we saw that updating list y will also change list x.

We saw the above results because −

• Lists are mutable, i.e. you can change their contents.
• The variable is the name pointing to the object.

Above, writing y = x does not create a copy of the list. It creates a new variable y that refers to the same object as x refers to. This means there is only one object, the list, and both x and y refer to it.

When append() is called, the content of the variable object changes from [] to [25]. Since both variables point to the same object, the modified value can be accessed using either variable [25].

Suppose we assign an immutable object to x, then x and y will no longer be equal. This is because integers are immutable -

The Chinese translation of

Example

is:

Example

# ints are immutable
x = 5
y = x
print("Value of y = ",y)
print("Value of x = ",x)

# We are creating a new object
x = x + 1
print("\nAfter changing...")
print("Value of y = ",y)
print("Value of x = ",x)

Output

('Value of y = ', 5)
('Value of x = ', 5)

After changing...
('Value of y = ', 5)
('Value of x = ', 6)

When we write x = x 1, we are not changing int 5 by increasing its value. Instead, we are creating a new object (int 6) and assigning it to x. After this assignment, we have two objects (int 6 and 5) and two variables referencing them.

therefore,

• mutable object − If we have a mutable object, such as a list, dictionary, set, etc., we can use some specific operations to change it, and all variables that reference it will see to this change.

• Immutable Object − If we have an immutable object like str, int, tuple, etc., all variables referencing it will always see the same value, but convert that value Operations on new values ​​always return a new object.

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