Source: 📖 Effective Python item 38

Using __call__ to define behaviour when an object is called

The built-in method __call__ can be defined within a class to make instances of that class callable, executing specified behaviour each time a call to the object is made.

Class MyObject:
	def __init__(self):
		self.times_called = 0
	
	def __call__(self):
		self.times_called += 1
		print("Called object!")

The above class can be used to create objects which can be called within function hooks, for example. Each time this object is called it will print "Called object!" and increment the self.times_called counter by 1 to keep track of the number of times this object has been called.