API in Flask--returns JSON but HTML exceptions break my JSON client

python html json api flask

14,519

Solution 1

You should define HTTP error handlers in flask.

A simple JSON returing 404 handler might look something like this:

@app.errorhandler(404)
def page_not_found(e):
    return flask.jsonify(error=404, text=str(e)), 404

With this you will be able to check for data.error on the client and if it exists you can get the error text with data.text (the error passed as e is werkzeug.exceptions.NotFound whose string representation is "404: Not Found").

Solution 2

Making the traceback available to the JSON client has the potential to disclose sensitive information.

My advice is:

turn debug off
install a log aggregation tool like sentry
make the error 500 page for this application return a generic error in json format

The 500 page could look like:

{ "error": "500 - internal server error" }

Solution 3

The code below should do the trick. So the idea is to catch any exception that might have been raised, get the exception details formatted as a string using the traceback module and then return that as valid json. I would recommend putting a bunch of except statements with the main types of errors you expect to happen and a more readable error message. Then you can have one last except as a catch all in case something strange and unexpected happens.

import traceback

@app.route('/route1')
def api_route1():
    if user_id in request.args: 
        try:
            k1 = request.args['user_id']
            return flask.jsonify(recs=some_function(k1))
        except:
            return flask.jsonify(exception=traceback.format_exc())
    else:
        return flask.jsonify(exception="no valid user_id supplied")

14,519

Author by

Deepak Mathpal

i build Systems & Tools for Analysis, Prediction, Visualization, & Simulation. i also design, code, and deploy complete distributed and (horizontally) scalable Machine Learning-based applications (e.g., anti-fraud filter, recommendation engine, monitoring/anomaly detectors), often in the service layer decoupled from the main app. Techniques: Machine Learning decision tree (CART/C4.5) & random forest deep learning (multi-layer perceptron) support vector machine (SVM/SVR) kNN/kdtree probabilistic graphical models (eg, Bayesian Net, Markov Random Field) - ***Dimension Reduction Techniques*** - spectral decomposition (PCA & kPCA, kLDA) - Kohonen Map (self-organizing map) - ***ETL pipelines*** - akka stream - Apache Spark - Kafka/Zookeeper - ***Social Network Analysis & Visualization*** - using graph theoretic techniques for - community detection - identify members essential for network health/growth - identify nascent sub-communities - particular fluency in *NetworkX*, *GraphViz* - ***Analysis & Modeling of Time Series*** - decomposition - forecasting - anomaly detection - ***Optimization*** - combinatorial optimization - csp - ***Numerical Methods*** - matrix decomposition - monte carlo techniques - Gaussian quadrature, - finite difference methods - ***Persistence*** - redis - postgres - ***Geo-Spatial Data Modeling, Persistence, & Computation*** - postgis (storage, query, computation) toolchain: scala apache spark apache kafka akka & akka-stream R python NumPy + SciPy + Matplotlib + pandas git (& gitHub) travis ci no recruiters

Updated on June 05, 2022

Comments

Deepak Mathpal almost 2 years
exceptions returned in HTML break my JSON client. I want to jsonify this output.

More detail: i have a view function which an endpoint of this api app.

As you can see, this function returns the result in json.
```
@app.route('/route1')
def api_route1():
    if user_id in request.args: 
        k1 = request.args['user_id']
        return flask.jsonify(recs=some_function(k1))
    else:
        return "no valid user_id supplied"
```
The problem, unhandled exception are in HTML, e.g.,
```
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 
    Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
    <head>
        <title>TypeError: 'NoneType' object is not iterable // Werkzeug Debugger</title>
        <link rel="stylesheet" 
            href="?__debugger__=yes&amp;cmd=resource&amp;f=style.css" 
            type="text/css">
```
This breaks my json client. The HTML format is clearly a default, but i don't know how to opt out of it and specify jsonified exceptions (and ideally jsonify anything returned even headers).

I suspect what i need is somewhere in the excellent Flask documentation, but i can't find it.
sberry over 11 years

This is what the OP should do.
Anurag over 11 years

even the else part can have jsonify, this will again return html when user_id is not in arguments.
Marwan Alsabbagh over 11 years

@Codeanu good suggestion, I updated the answer with what you suggested.
Michael almost 2 years

Does this mean I need to create an error handler per non-success http status code (5xx and 4xx)? There must be a better way.