Flask Exception Handlers

Defines an extension for standardising exception handling in HMLR's Flask applications.

Basic Usage

Initialise as any Flask extension:

exception_handlers = ExceptionHandlers()

def register_extensions(app):
    exception_handlers.init_app(app)

Custom Renderers

If you need to change how the errors are returned to the client, use the on_http_error_render, on_application_error_render and on_unhandled_error_render event handlers:

def http_error_renderer(description, code, http_code, e=None):
    return render_template('app/errors/unhandled.html', http_code=http_code), http_code


def unhandled_error_renderer(description, code, http_code, e=None):
    return render_template('app/errors/unhandled.html', http_code=http_code), http_code


def application_error_renderer(description, code, http_code, e=None):
    try:
        return render_template('app/errors/application/{}.html'.format(e.code), description=e.message, code=e.code,
                               http_code=http_code, e=e), http_code
    except TemplateNotFound:
        return render_template('app/errors/application.html', description=e.message, code=e.code, http_code=http_code),
                               http_code


def register_exception_handlers(app):
    handlers = ExceptionHandlers()
    handlers.init_app(app)
    handlers.on_http_error_render = http_error_renderer
    handlers.on_application_error_render = application_error_renderer
    handlers.on_unhandled_error_render = unhandled_error_renderer

Suppressing HTTP Response Codes

Want your app to only send certain HTTP response codes? Supply the list of allowed codes to init_app:

handlers = ExceptionHandlers()
handlers.init_app(app, [403, 404])
# Or...
handlers = ExceptionHandlers(app=app, allow_codes=[418])

(Anything not in the list gets replaced with a 500).

The helper list ALLOWED_UI_RESPONSES (403, 404, 429 and 500) is available:

from landregistry.exceptions import ExceptionHandlers, ALLOWED_UI_RESPONSES

handlers = ExceptionHandlers()
handlers.init_app(app, ALLOWED_UI_RESPONSES)

ApplicationError

Use this class when the application identifies there's been a problem and the client should be informed.

exception ApplicationError(message: str, error_code: str, http_code: int, force_logging: bool)

message The text of the exception message.

error_code Unique identifier for the error. Can be anything, but useful to reference in support documentation or knowledge items.

http_code Defaults to 500. Specifies the HTTP response code to send.

force_logging Defaults to False. Forces logging to Info if True (Debug otherwise). If the http code is 500, this parameter is ignored (and logging is to Error).

Examples:

from landregistry.exceptions import ApplicationError

raise ApplicationError("Critical error", "DB")
# or
raise ApplicationError("Title number invalid", "E102", http_code=400)
# or
raise ApplicationError("Title number invalid", "E102", http_code=400, force_logging=True)