Hairy Code

Have you ever tried to test POST requests to an API written with Flask? Today I was going through an old code base writing unittests that should have been written eons ago when I ran into a snag. The issue stemmed when I tried manually setting the headers Content-Type.

But before we get to that, let me show you the skeleton of the Flask route I was testing:

@app.route('/someendpoint/' methods=['POST'])
def some_endpoint():
    """API endpoint for submitting data to

    :return: status code 405 - invalid JSON or invalid request type
    :return: status code 400 - unsupported Content-Type or invalid publisher
    :return: status code 201 - successful submission
    """
    # Ensure post's Content-Type is supported
    if request.headers['content-type'] == 'application/json':
        # Ensure data is a valid JSON
        try:
            user_submission = json.loads(request.data)
        except ValueError:
            return Response(status=405)
        ... some magic stuff happens
        if everything_went_well:
            return Response(status=201)
        else:
            return Response(status=405)

    # User submitted an unsupported Content-Type
    else:
        return Response(status=400)

Ignoring the magic, everything seems in order. So, lets go ahead and write a quick unittest that posts an invalid json.

    def test_invalid_JSON(self):
        """Test status code 405 from improper JSON on post to raw"""
        response = self.app.post('/someendpoint',
                                data="This isn't a json... it's a string!")
        self.assertEqual(response.status_code, 405)

Cool, let’s run it!

    Failure
    self.assertEqual(response.status_code, 405)
    AssertionError: 400 != 405

A quick glance at the code and I realize I’ve forgotten to set the headers Content-Type! Wait. How do I set the Content-Type? That’s a good question. After searching around for people who had run into similar problems this is what I came up with:

    def test_invalid_JSON(self):
        """Test status code 405 from improper JSON on post to raw"""
        response = self.app.post('/someendpoint',
                                data="This isn't a json... it's a string!",
                                headers={'content-type':'application/json')
        self.assertEqual(response.status_code, 405)

Let’s try this again.

    Failure
    self.assertEqual(response.status_code, 405)
    AssertionError: 400 != 405

Hmmm, okay. Next, I decided to inspect the request coming into the flask app and found something odd in request.headers:

    Host: localhost
    Content-Length: 10
    Content-Type: 

Why is the Content-Type empty? Another search gave hinted at another possible solution. Why not just build the headers dict inline?

    headers=dict(content-type='application/json') # But that's not right. We can't have '-' in a key.

By this point I’ve become agitated. Neither the Flask docs themselves nor various forums have been of much use. Then I stumbled across this.

Perhaps I missed something in the docs. Either way, I learned that you can hand the Content-Type as a parameter to the post method. It works and it looks much cleaner. Let’s revise my initial unittest accordinlgy:

    def test_invalid_JSON(self):
        """Test status code 405 from improper JSON on post to raw"""
        response = self.app.post('/raw',
                                data="not a json",
                                content_type='application/json')
        self.assertEqual(response.status_code, 405)

And, let’s run this one last time and look at the request as it comes though.

    Tests passed
    Host: localhost
    Content-Length: 10
    Content-Type: application/json

Much better. Now, back to testing!

-H.