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The Application class

The Application class in BlackSheep is responsible of handling the application life cicle (start, working state, stop), routing, web requests, exceptions. This page describes details of the Application class:

  • How to handle errors.
  • Application events and life cycle.

Handling errors

BlackSheep catches any unhandled exception that happens during the execution of request handlers, producing a HTTP 500 Internal Server Error response. To see this in practice, start an application like the following:

from blacksheep.server import Application

app = Application()
get = app.router.get

def crash_test():
    raise Exception("Crash test")

And observe how a request to its root produces a response with HTTP status 500, and the text "Internal server error".

Exception details are hidden to the client by default: it would be a security issue if the web application returned error details to the client. However, while developing and occasionally while investigating issues, it is useful to be able to obtain error details directly from the web requests that are failing. To enable error details, update the app declaration as follows:

app = Application(show_error_details=True)

Now the application returns the details of the exception with the full stack trace, serving a page like the following:

Internal server error page

Consider using environmental variables to handle this kind of settings that can vary across environments. For example:

import os
from blacksheep.server import Application

app = Application(show_error_details=bool(os.environ.get("SHOW_ERROR_DETAILS", None)))
get = app.router.get

def crash_test():
    raise Exception("Crash test")

Note: BlackSheep project templates use a library to handle application settings and configuration roots, named roconfiguration.

Configuring exceptions handlers

The BlackSheep Application object has a exception_handlers dictionary that defines how errors should be handled. When an exception happens while handling a web request and reaches the application, the application checks if there is a matching handler for that kind of exception. An exception handler is defined as a function with the following signature:

from blacksheep import Request, Response

async def exception_handler(self, request: Request, exc: Exception) -> Response:

In the exception below

class CustomException(Exception):

async def exception_handler(self, request, exc: CustomException):
    nonlocal app
    assert self is app
    assert isinstance(exc, CustomException)
    return Response(200, content=TextContent('Called'))

# Register the exception handler for the CustomException type:
app.exceptions_handlers[CustomException] = exception_handler

async def home(request):
    # of course, the exception can be risen at any point
    # for example in the business logic layer
    raise CustomException()

Exceptions inheriting from HTTPException are mapped to handlers by their status code, using int keys; while user defined exceptions are mapped to handlers by their type.

Configuring exception handlers using decorators

It is also possible to register exception handlers using decorators, instead of interacting with app.exceptions_handlers dictionary:

class CustomException(Exception):

async def handler_example(self, request, exc: CustomException):

🚀 New in version 1.0.4

Application events

A BlackSheep application exposes three events: on_start, after_start, on_stop.


This event should be used to configure things such as new request handlers, service registered in


This event should be used to configure things that must happen after request handlers are normalized. At this point, the application router contains information about actual routes handled by the web application, and routes can be inspected. For examples, the built-in generation of OpenAPI Documentation generates the API specification file at this point.


This event should be used to fire callbacks that need to happen when the application is stopped. For example, disposing of services that require disposal, such as DB Connection Pools, HTTP Connection Pools.

Application life cycle

Refer to the following diagram to know more about when application events are fired, and the state of the application when they are executed.

App life cycle

Application events example

from blacksheep.server import Application
from blacksheep.server.responses import text, redirect
from blacksheep.messages import Request, Response

app = Application()
get = app.router.get

async def home(request: Request) -> Response:
    return "Example Async"

async def before_start(application: Application) -> None:
    print("Before start")

async def after_start(application: Application) -> None:
    print("After start")

async def on_stop(application: Application) -> None:
    print("On stop")

app.on_start += before_start
app.after_start += after_start
app.on_stop += on_stop

Example: after_start callback to log all routes

To define an after_start callback that logs all routes registered in the application router:

async def after_start_print_routes(application: Application) -> None:

app.after_start += after_start_print_routes


Read about the details of routing in BlackSheep.