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Python __call__ Method Demystified: Effortlessly Use and Understand


Python’s __call__() Method: Creating Callable Instances

In Python, a callable is any object that can be called using a pair of parentheses and optional arguments. This includes functions, classes, and methods. Additionally, you can create your own custom classes that produce callable instances by adding the .__call__() special method. In this tutorial, we will explore how to create and use callable instances in Python.

Understanding Callable Objects in Python

A callable in Python refers to any object that can be called using parentheses and arguments, if required. Here are some examples of callables in Python:

  • Built-in functions and classes
  • User-defined functions created with the def keyword
  • Anonymous functions created with the lambda keyword
  • Constructors of custom classes
  • Instance, class, and static methods
  • Instances of classes that implement the .__call__() method
  • Closures

Checking Whether an Object is Callable

You can check if an object is callable using the callable() function. It returns True if the object is callable and False otherwise. For example:

def greet():
class MyClass:
def __call__(self):
print(callable(greet)) # True
print(callable(MyClass())) # True

Creating Callable Instances with .__call__() in Python

To create a callable instance, you can add the .__call__() method to your custom class. This method will be called when the instance is invoked as a function. Here’s an example:

class Counter:
def __init__(self):
self.count = 0
def __call__(self):
self.count += 1
return self.count
counter = Counter()
print(counter()) # 1
print(counter()) # 2

In this example, the Counter class implements the .__call__() method, which increments the count attribute each time the instance is called. The instance counter can now be treated as a function and called multiple times, maintaining its state.

Understanding the Difference: .__init__() vs .__call__()

The .__init__() method is called when an object is instantiated, while the .__call__() method is called when the object is invoked as a function. The .__init__() method initializes the object’s attributes, whereas the .__call__() method defines the behavior of the object when it is called. Both methods serve different purposes.

Putting Python’s .__call__() into Action

Let’s explore some real-world examples of using callable instances to solve different problems.

Writing Stateful Callables

You can use callable instances to create stateful objects. For example, consider a Counter class that keeps track of the number of times it has been called:

class Counter:
def __init__(self):
self.count = 0
def __call__(self):
self.count += 1
return self.count
counter = Counter()
print(counter()) # 1
print(counter()) # 2

The Counter instance keeps track of its state across multiple function calls.

Caching Computed Values

Callable instances can be useful for caching expensive computations. Suppose you have a function that calculates the square of a number, but you want to cache the result to avoid redundant calculations:

class SquareCalculator:
def __init__(self):
self.cache = {}
def __call__(self, num):
if num not in self.cache:
self.cache[num] = num * num
return self.cache[num]
calculator = SquareCalculator()
print(calculator(5)) # 25 (computed and cached)
print(calculator(5)) # 25 (retrieved from cache)

The SquareCalculator instance caches the computed squares of numbers to optimize performance. If the same number is passed to the instance again, it retrieves the result from the cache instead of recomputing it.

Creating Clear and Convenient APIs

You can use callable instances to create clear and convenient APIs for your classes. For example, consider a Database class that provides a fluent API for querying data:

class Database:
def __call__(self, query):
# Execute the query
db = Database()
db("SELECT * FROM users").limit(10).order_by("name").execute()

The Database instance is callable and allows chaining of methods to construct a query in a readable and fluent manner.

Exploring Advanced Use Cases of .__call__()

Apart from the examples mentioned above, callable instances can be used in various advanced scenarios, such as:

  • Writing class-based decorators
  • Implementing the strategy design pattern


In this tutorial, we explored the concept of callable objects in Python and learned how to create callable instances using the .__call__() method. We also understood the difference between .__init__() and .__call__() methods and saw how to put Python’s .__call__() into action by solving real-world problems. Creating and using callable instances can be a powerful technique to enhance the flexibility and convenience of your code in Python.