Behind the Underscores EP11: Callable Objects: __call__

When we think of calling something in Python, the first thing that comes to mind is usually a function. But Python gives us the power to go a step further: objects can behave like functions if they implement a special method called __call__. This concept might seem a bit strange at first, but once you understand how it works, you’ll find it incredibly useful, especially for building clean, modular backend systems.

In this blog post, we’ll dive deep into the __call__ method. We'll explore:

• What __call__ really does under the hood
• Why and when you should use it
• The potential pitfalls to watch out for
• Realistic backend examples

Let’s get started.

What is the __call__ Method?

In Python, every function is an object. And just like functions, any object that implements the __call__ method can be used with parentheses to make it behave like a function:

class Greeter:
    def __init__(self, name):
        self.name = name

    def __call__(self, greeting="Hello"):
        return f"{greeting}, {self.name}!"

hello_john = Greeter("John")
print(hello_john())           # Output: Hello, John!
print(hello_john("Hi"))       # Output: Hi, John!

Here, hello_john is an instance of Greeter, but thanks to __call__, we can treat it like a function.

Why Would You Use __call__?

You might wonder: “Why not just use a regular function?”. That’s a fair question. Here are some reasons why using __call__ can be a better choice in certain situations:

1. Encapsulation of State

Classes let you store data, or we say state, and behavior together. With __call__, you can package that logic in a neat callable form.

class Multiplier:
    def __init__(self, factor):
        self.factor = factor

    def __call__(self, number):
        return self.factor * number

triple = Multiplier(3)
print(triple(10))  # Output: 30

2. Cleaner Dependency Injection

In frameworks like FastAPI or Flask, you may want to inject behavior or data as a callable. Making your class instance callable makes the integration seamless.

3. Custom Middleware or Pipelines

If you're building your own middleware system, having callable objects makes it easier to write and chain operations.

When Not to Use __call__

While __call__ can be powerful, it can also lead to confusion or overengineering if used improperly.

• Readability concerns: Readers of your code might not expect that an instance behaves like a function. This can hurt code clarity.
• Debugging difficulty: Errors in a __call__ implementation might be harder to track if your class hides too much logic inside it.
• Unnecessary abstraction: Sometimes a simple function is better. Don’t wrap everything in a class just because you can.

Rule of Thumb

Use __call__ when your object:

• Maintains state that’s useful across calls
• Needs to fit into APIs that expect callables
• Has a behavior that makes sense as a single primary action

Backend Example #1: Request Logger Middleware

Imagine you’re building a custom logging middleware for a web server. You want to log the request method and path.

class RequestLoggerMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] == "http":
            method = scope["method"]
            path = scope["path"]
            print(f"[LOG] {method} request to {path}")

        await self.app(scope, receive, send)

This can be used in a FastAPI or Starlette app like this:

app.add_middleware(RequestLoggerMiddleware)

Because the middleware instance is callable, it fits perfectly into the ASGI lifecycle.

Backend Example #2: Authorization Checker

Let’s say you need to check if a user has a specific role before allowing access to a route.

class RoleChecker:
    def __init__(self, allowed_roles):
        self.allowed_roles = allowed_roles

    def __call__(self, user):
        if user.role not in self.allowed_roles:
            raise PermissionError("Access denied")
        return True

check_admin = RoleChecker(["admin"])

user = User(role="guest")
check_admin(user)  # Raises PermissionError

This is especially useful when you’re designing reusable validation or guard components.

Summary: The Power of __call__

The __call__ method turns your objects into callable powerhouses. It combines the simplicity of functions with the structure of classes, giving you the best of both worlds. But like any powerful tool, it should be used thoughtfully.

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The original post is here.