Python Closures vs. Nested Functions: What\'s the Difference?

Closure vs. Nested Functions in Python: A Clarification

Python features a concept known as "nested functions," often mistaken for closures. To address this confusion, let's delve into the true nature of closures within Python.

A closure is defined as a function that maintains access to local variables from an enclosing scope, even after that scope has concluded. In Python, a closure arises when a nested function references local variables from the enclosing function. These variables remain accessible outside the enclosing scope due to the nested function's access to them.

Consider the following example:

def make_printer(msg):
    def printer():
        print(msg)
    return printer

printer = make_printer('Foo!')
printer()

In this scenario, the function printer is a closure because it references the local variable msg from the enclosing function make_printer. Even after make_printer concludes, msg remains accessible to printer.

However, not all nested functions are closures. For a nested function to qualify as a closure, it must meet these criteria:

• Access local variables of the enclosing scope
• Reference these variables when executed outside the scope

If a nested function does not meet these requirements, it is not considered a closure but rather an ordinary nested function.

For example, consider the following code:

def make_printer(msg):
    def printer(msg=msg):
        print(msg)
    return printer

printer = make_printer("Foo!")
printer()  # Output: Foo!

Here, the nested function printer is not a closure because it doesn't reference the msg from the enclosing function. Instead, it assigns the default parameter a value, eliminating the need for external access to msg.

Therefore, to distinguish between closures and nested functions in Python, it's crucial to consider whether or not the nested function maintains access to local variables from the enclosing scope even after the latter has completed execution.

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