Unit tests with nested objects in the request using flask

Question

How can one unit test a REST API endpoint written in flask that accepts a nested dictionary object for the request body?

Here is an example using flask and webargs for input validation,

from flask import Flask
from webargs import fields
from webargs.flaskparser import use_args

app = Flask(__name__)

hello_args = { 
    'a': fields.Nested({'name' : fields.Str()})
}

@app.route('/', methods=['POST'])
@use_args(hello_args)
def index(args):
    return 'Hello ' + str(args)


def test_app():
    app.config['TESTING'] = True
    test_app = app.test_client(use_cookies=False)
    test_app.post(data={"a": {"name": "Alice"}})


if __name__ == '__main__':
    app.run()

which works correctly when using this enpoint directly,

% curl -H "Content-Type: application/json" -X POST \
       -d '{"a":{"name": "Alice"}}' http://localhost:5000  

Hello {'a': {'name': 'Alice'}}%

however raises an exception in werkzeug.test.EnvironBuilder when it is called inside unit-tests,

nosetests /tmp/test.py                                                      
E
======================================================================
ERROR: test.test_app
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib64/python3.4/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/tmp/test.py", line 26, in test_app
    test_app.post(data={"a": {"name": "Alice"}})
  File "/home/rth/.local/lib64/python3.4/site-packages/werkzeug/test.py", line 788, in post
    return self.open(*args, **kw)
  File "/home/rth/.local/lib64/python3.4/site-packages/flask/testing.py", line 103, in open
    builder = make_test_environ_builder(self.application, *args, **kwargs)
  File "/home/rth/.local/lib64/python3.4/site-packages/flask/testing.py", line 34, in make_test_environ_builder
    return EnvironBuilder(path, base_url, *args, **kwargs)
  File "/home/rth/.local/lib64/python3.4/site-packages/werkzeug/test.py", line 338, in __init__
    self._add_file_from_data(key, value)
  File "/home/rth/.local/lib64/python3.4/site-packages/werkzeug/test.py", line 355, in _add_file_from_data
    self.files.add_file(key, **value)
TypeError: add_file() got multiple values for argument 'name'

----------------------------------------------------------------------
Ran 1 test in 0.011s

FAILED (errors=1)

this uses Python 3.5, flask 0.12 and webargs 1.5.2.

Also submitted an issue at https://github.com/pallets/flask/issues/2176

Answer 1

It appears that despite the use of webargs, the input data must still be serialized and content_type explicitly specified for the this to work. In particular, replacing

test_app.post(data={"a": {"name": "Alice"}})

with

test_app.post(data=json.dumps({"a": {"name": "Alice"}}),
            content_type='application/json')

fixed this problem (see also related SO answers here).