Does Python Pass Lists by Reference? Exploring Pass by Value and Pass by Reference in Python

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Does Python Pass Lists by Reference?

In the world of programming, passing arguments to functions is a common task. Python, being a versatile language, provides two ways to pass arguments: pass by value and pass by reference.

Pass by Value and Pass by Reference in Python

Python uses a combination of pass by value and pass by reference depending on the type of object being passed. For immutable objects like numbers, strings, and tuples, Python passes the value of the object. This means that any changes made to the object within the function will not affect the original object.

On the other hand, for mutable objects like lists and dictionaries, Python passes the reference to the object. This means that any modifications made to the object within the function will be reflected in the original object.

The Variable is Not the Object

Before diving deeper into pass by value and pass by reference, it's important to understand the concept that the variable is not the object itself. In Python, variables are simply names that reference objects in memory. When an object is assigned to a variable, the variable stores a reference to the object's location in memory.

For example, consider the following code:

x = [1, 2, 3]  # x references a list object
y = x  # y references the same list object as x
y.append(4)  # modify the list object through y
def print_list(obj):
    print(obj)
print_list(x)  # prints [1, 2, 3, 4]

In this example, both x and y reference the same list object. When y.append(4) is called, it modifies the list object through the reference stored in y. As a result, the output of print_list(x) is [1, 2, 3, 4].

What is Pass by Reference In Python?

In pass by reference, the reference to an object is passed to the function. This means that any changes made to the object within the function will affect the original object. In Python, pass by reference is used for mutable objects like lists and dictionaries.

What is Pass by Value In Python?

In pass by value, the value of the object is passed to the function. This means that any changes made to the object within the function will not affect the original object. In Python, pass by value is used for immutable objects like numbers, strings, and tuples.

Pass by Reference In Python Example

Let's take a look at an example to understand pass by reference in Python:

def modify_list(lst):
    lst.append(4)

x = [1, 2, 3]
modify_list(x)
print(x)  # prints [1, 2, 3, 4]

In this example, the function modify_list takes a list as an argument and appends the value 4 to the list. When the function is called with x as the argument, it modifies the list object through the reference passed to the function. As a result, the output is [1, 2, 3, 4].

Pass by Value In Python Example

Now, let's explore an example of pass by value in Python:

def modify_number(num):
    num += 1

x = 1
modify_number(x)
print(x)  # prints 1

In this example, the function modify_number takes a number as an argument and increments its value by 1. However, since numbers are immutable objects in Python, the += operation creates a new object with the updated value. As a result, the original object referenced by x remains unchanged, and the output is 1.

Conclusion

In conclusion, Python uses a combination of pass by value and pass by reference. Immutable objects like numbers, strings, and tuples are passed by value, while mutable objects like lists and dictionaries are passed by reference. Understanding how Python handles pass by value and pass by reference is crucial for writing efficient and bug-free code.