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Why did changing list ‘y’ also change list ‘x’ in Python?
Example
In this article, we will see if you will change a list, let’s say List y will also change list x. For this, let us first see an example with two lists and try to append() and print −
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 change list x as well.
We saw the above result because −
- Lists are mutable i.e. you can change their content.
- Variables are names that refer to objects.
Above, writing y = x doesn’t create a copy of the list. It create a new variable y that refers to the same object x refers to. This means that there is only one object i.e. the List, and both x and y refer to it.
When the append() is called, the content of the mutable object changed from [] to [25]. Since both the variables refer to the same object, using either name accesses the modified value [25].
Let’s say if we instead assign an immutable object to x, the x and y are not equal anymore. This is because integers are immutable −
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 mutating the int 5 by incrementing its value. Instead, we are creating a new object (int 6) and assigning it to x. After this assignment we have two objects (ints 6 & 5) and two variables referring them.
Therefore,
Mutable Object − If we have a mutable object, such as list, dict, set, etc., we can use some specific operations to mutate it and all the variables that refer to it will see the change.
Immutable Object − If we have an immutable object, such as str, int, tuple, etc., all the variables referring to it will always see the same value, but operations that transform that value into a new value always return a new object.
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