What is anonymous function? Explain closure of function with example.

An anonymous function is a function that is defined without a name. While normal functions are defined using the 'def' keyword in Python, anonymous functions are defined using the 'lambda' keyword. Hence, anonymous functions are also called lambda functions.

Here is a simple example of an anonymous function that adds two numbers:

sum = lambda a, b: a + b
print(sum(3, 4))  # Output: 7

A closure in Python is a tuple of variables that are created by a function whenever it is called. These variables will still exist after the function has finished executing, which is why they are called 'closures'. The function can then use these 'closed over' variables whenever it is called again.

Here is an example of a closure in Python:

def outer_function(x):
    def inner_function(y):
        return x + y
    return inner_function

closure = outer_function(10)
print(closure(5))  # Output: 15

In this example, outer_function is called with x = 10. It defines inner_function and then returns it. The returned function is a closure that 'closes over' the variable x, which remains available to it. When closure is later called with y = 5, it is able to access x and add it to y.