Python - Pass by value or by reference

Python uses a mechanism, which is known as Call by Object Reference or Call by assignment. When an immutable arguments like integers, strings or tuples are passed to a function, the passing is like call-by-value because the immutable objects can not changed by passing to a function as arguments. Whereas When a mutable arguments like lists or dictionaries are passed to a function, the passing is like call-by-reference. Any changes performed on mutable arguments inside the function is also reflected outside the function.

Example: Call-by-value

In the example below, string x is passed to the function test. As string is a immutable object, the passing acts like call-by-value. Inside the function string x is changed which is not reflected outside the function.

def test(str):
  str = str + " World"
  return str

x = "Hello"
print(test(x))
print(x)

The above code will give the following output:

Hello World
Hello

Example: Call-by-reference

In the example below, list MyList is passed to the function test. As list is a mutable object, the passing acts like call-by-reference. Inside the function list MyList is changed which is also reflected outside the function.

def test(x):
  x.append(50)
  return x

x = [10, 20]
print(test(x))
print(x)

The above code will give the following output:

[10, 20, 50]
[10, 20, 50]