In Python, you can reference a function by using its name without parentheses. This is useful when you want to pass a function as an argument to another function, or when you want to assign a function to a variable.

Here's an example of referencing a function:

def add(a, b):
    return a + b
 
def apply_operation(operation, a, b):
    return operation(a, b)
 
result = apply_operation(add, 4, 2)  # result = 6

In this example, we define a function add() that takes two arguments and returns their sum. We also define a function apply_operation() that takes a function as its first argument, followed by two input values a and b. The apply_operation() function then calls the input function with the input values and returns the result.

Finally, we call the apply_operation() function with add as the input function and input values 4 and 2. This results in 6 being returned, which is the sum of 4 and 2.

Note that when you reference a function without parentheses, you are not actually calling the function. Instead, you are passing a reference to the function itself, which can be stored in a variable, passed as an argument to another function, or used in any other way that you would use a regular variable.