Free Variables

Free variables are identifiers in an expression that are not bound to a specific value or parameter within that expression.

So, if you have:

def make_adder(x):
	def adder(y):
		return x + y
	return adder

• In the adder function, y is a bound variable because it is a parameter.
• But x is a free variable inside adder because it is not defined there, it comes from the surrounding scope of make_adder.