Is Python call by reference or call by value

Python utilizes a system, which is known as “Call by Object Reference” or “Call by assignment”. In the event that you pass arguments like whole numbers, strings or tuples to a function, the passing is like call-by-value because you can not change the value of the immutable objects being passed to the function. Whereas passing mutable objects can be considered as call by reference because when their values are changed inside the function, then it will also be reflected outside the function.
Example 1:

Python3

# Python code to demonstrate
# call by value


string = "Geeks"


def test(string):

    string = "GeeksforGeeks"
    print("Inside Function:", string)

# Driver's code
test(string)
print("Outside Function:", string)

Output

Inside Function: GeeksforGeeks
Outside Function: Geeks

Example 2

Python3

# Python code to demonstrate
# call by reference


def add_more(list):
    list.append(50)
    print("Inside Function", list)

# Driver's code
mylist = [10,20,30,40]

add_more(mylist)
print("Outside Function:", mylist)

Output 

Inside Function [10, 20, 30, 40, 50]
Outside Function: [10, 20, 30, 40, 50]

Binding Names to Objects

In python, each variable to which we assign a value/container is treated as an object. When we are assigning a value to a variable, we are actually binding a name to an object.

Python3

a = "first"
b = "first"


# Returns the actual location
# where the variable is stored
print(id(a))

# Returns the actual location
# where the variable is stored
print(id(b))

# Returns true if both the variables
# are stored in same location
print(a is b)

Output

110001234557894
110001234557894
True

Now, let’s try and understand this better with another example.
Example 2:

Python3

a = [10, 20, 30]
b = [10, 20, 30]

# return the location
# where the variable
# is stored
print(id(a))

# return the location
# where the variable
# is stored
print(id(b))

# returns false if the
# location is not same
print(a is b)

Output

541190289536222
541190288737777
False

The output of the above two examples are different because the list is mutable and the string is immutable. An immutable variable cannot be changed once created. If we wish to change an immutable variable, such as a string, we must create a new instance and bind the variable to the new instance. Whereas, mutable variable can be changed in place.
Example 3:

Python3

def foo(a):

    # A new variable is assigned
    # for the new string
    a = "new value"
    print("Inside Function:", a)


# Driver's code
string = "old value"
foo(string)

print("Outside Function:", string)

Output:

Inside Function: new value
Outside Function: old value

In the above example, a string which is an immutable type of object is passed as argument to the function foo. Within the scope of the given function foo, a= “new value” has been bounded to the same object that string has been bound outside. Within the scope of the function foo, we modify “old value”` to “new value”. Once we leave the scope of function foo , a=”new value” is no longer in the name space, and the value that string refers to was never changed.
Example 4: Now, let us look at how mutable variable is passed into the function.

Python3

def foo(a):
    a[0] = "Nothing"

# Driver' code
bar = ['Hi', 'how', 'are', 'you', 'doing']
foo(bar)
print(bar)

Output:

['Nothing, 'how', 'are', 'you', 'doing']

When we pass a mutable variable into the function foo and modify it to some other name the function foo still points to that object and continue to point to that object during its execution.