It’s very common to implement RESTful APIs on top of Flask. One of the
first things that developers run into is the realization that the builtin
exceptions are not expressive enough for APIs and that the content type of
text/html
they are emitting is not very useful for API consumers.
The better solution than using abort
to signal errors for invalid API
usage is to implement your own exception type and install an error handler
for it that produces the errors in the format the user is expecting.
Simple Exception Class
The basic idea is to introduce a new exception that can take a proper human readable message, a status code for the error and some optional payload to give more context for the error.
This is a simple example:
from flask import jsonify
class InvalidUsage(Exception):
status_code = 400
def __init__(self, message, status_code=None, payload=None):
Exception.__init__(self)
self.message = message
if status_code is not None:
self.status_code = status_code
self.payload = payload
def to_dict(self):
rv = dict(self.payload or ())
rv['message'] = self.message
return rv
A view can now raise that exception with an error message. Additionally some extra payload can be provided as a dictionary through the [UNKNOWN NODE title_reference] parameter.
Registering an Error Handler
At that point views can raise that error, but it would immediately result in an internal server error. The reason for this is that there is no handler registered for this error class. That however is easy to add:
@app.errorhandler(InvalidUsage)
def handle_invalid_usage(error):
response = jsonify(error.to_dict())
response.status_code = error.status_code
return response
Usage in Views
Here is how a view can use that functionality:
@app.route('/foo')
def get_foo():
raise InvalidUsage('This view is gone', status_code=410)