How do I pass a variable by reference?

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In Python, variables are passed by object reference. This means that when you pass a variable to a function, you are passing a reference to the object that the variable points to. If you modify the object within the function, the changes will be reflected outside the function. However, you cannot reassign a variable to a new object and have that change reflected outside the function without using mutable objects like lists or dictionaries. Here's an example:

python
# Function to modify a variable by reference
def modify_variable(var):
    var += 10  # This modifies the object the variable points to

# Function to modify a list by reference
def modify_list(lst):
    lst.append(42)  # This modifies the list object itself

# Example with an integer (immutable)
num = 5
print("Before modify_variable:", num)
modify_variable(num)
print("After modify_variable:", num)  # num remains unchanged

# Example with a list (mutable)
my_list = [1, 2, 3]
print("Before modify_list:", my_list)
modify_list(my_list)
print("After modify_list:", my_list)  # my_list is modified

In this example:

  • modify_variable takes an integer as an argument, increments it by 10, but the original num variable remains unchanged because integers are immutable in Python.

  • modify_list takes a list as an argument and appends 42 to the list. Since lists are mutable in Python, the original my_list is modified.

So, in Python, you pass variables by reference, but whether the changes made inside a function affect the original variable depends on whether the object is mutable or immutable. Immutable objects, like integers and strings, cannot be modified in place, while mutable objects, like lists and dictionaries, can be modified in place.

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