errorhandler.rst

Custom Error Handling

While chalice middleware allow for catching of user defined errors, exceptions raised by a third party library can't be seen by the middleware and chalice will set the response without giving the middleware a chance to see the exception. These error handlers will only by used in the case of 'http' event, as the middleware for other types of events can catch other exceptions (see :ref:`middleware-error-handling-rest`).

In the case where you want to return your own Response for those exceptions you can register an error handler to intercept the exception.

Below is an example of Chalice error hanler:

from chalice import Chalice, Response
from thirdparty import func, ThirdPartyError

app = Chalice(app_name='demo-errorhandler')

@app.error(ThirdPartyError)
def my_error_handler(error: Exception):
    app.log.error("ThirdPartyError was raised")
    return Response(
        body=e.__class__.__name__,
        status_code=500
    )

@app.route('/')
def index():
    return func()

In this example, our error handler is registered to catch ThirdPartyError raised in a http event. In this case, if func were to raise a ThirdPartyError, my_error_handler will be called and our custom Response will be returned.

Writing Error Handlers

Error handlers must adhere to these requirements:

• Must be a callable object that accepts one parameter. It will be of Exception type.
• Must return a response. If the response is not of chalice.Response type, Chalice will either try to call the next error handler registered for the same error or in the event where no handlers return a valid response, Chalice will return a chalice.ChaliceViewError.

Error Propagation

Cahlice will propagatet the error through all registered handlers until a valid response is returned. If no handlers return a valid response, chalice will handle the error as if no handlers were registered.

@app.error(ThirdPartyError)
def my_error_handler_1(error: Exception):
    if error.message == '1':
        return Response(
            body='Error 1 was raised',
            status_code=200
        )

@app.error(ThirdPartyError)
def my_error_handler_2(error: Exception):
    if error.message == '2':
        return Response(
            body='Error 2 was raised',
            status_code=400
        )

@app.route('/1')
def index():
    # The response from `my_error_handler_1` will be returned
    raise ThirdPartyError('1')

@app.route('/2')
def index():
    # The response from `my_error_handler_2` will be returned
    raise ThirdPartyError('2')

@app.route('/3')
def index():
    # A ChaliceViewError will be returned
    raise ThirdPartyError('3')