Allow dictionary return values as JSON

This supports an increasingly common usecase whereby JSON is the
primary response (rather than a templated string). Given Flask has a
short syntax for HTML reponses, it seems fitting that it should also
do so for JSON responses. In practice it allows,

     @app.route("/")
     def index():
         return {
             "api_stuff": "values",
         }

CHANGES.rst

@@ -52,6 +52,10 @@ Unreleased
 -   Add an ``--extra-files`` option to the ``flask run`` CLI command to
     specify extra files that will trigger the reloader on change.
     :issue:`2897`
+-   Allow returning a dictionary from a view function. Similar to how
+    returning a string will produce a ``text/html`` response, returning
+    a dict will call ``jsonify`` to produce a ``application/json``
+    response. :pr:`3111`
 
 .. _#2935: https://github.com/pallets/flask/issues/2935
 .. _#2957: https://github.com/pallets/flask/issues/2957

docs/quickstart.rst

@@ -679,23 +679,26 @@ See :ref:`error-handlers` for more details.
 About Responses
 ---------------
 
-The return value from a view function is automatically converted into a
-response object for you.  If the return value is a string it's converted
-into a response object with the string as response body, a ``200 OK``
-status code and a :mimetype:`text/html` mimetype.
-The logic that Flask applies to converting return values into
-response objects is as follows:
+The return value from a view function is automatically converted into
+a response object for you. If the return value is a string it's
+converted into a response object with the string as response body, a
+``200 OK`` status code and a :mimetype:`text/html` mimetype. If the
+return value is a dict, ``jsonify`` is called to produce a response.
+The logic that Flask applies to converting return values into response
+objects is as follows:
 
 1.  If a response object of the correct type is returned it's directly
     returned from the view.
-2.  If it's a string, a response object is created with that data and the
-    default parameters.
-3.  If a tuple is returned the items in the tuple can provide extra information.
-    Such tuples have to be in the form ``(response, status, headers)``,
-    ``(response, headers)`` or ``(response, status)`` where at least one item
-    has to be in the tuple. The ``status`` value will override the status code
-    and ``headers`` can be a list or dictionary of additional header values.
-4.  If none of that works, Flask will assume the return value is a
+2.  If it's a string, a response object is created with that data and
+    the default parameters.
+3.  If it's a dict, a response object is created using ``jsonify``.
+4.  If a tuple is returned the items in the tuple can provide extra
+    information. Such tuples have to be in the form
+    ``(response, status)``, ``(response, headers)``, or
+    ``(response, status, headers)``. The ``status`` value will override
+    the status code and ``headers`` can be a list or dictionary of
+    additional header values.
+5.  If none of that works, Flask will assume the return value is a
     valid WSGI application and convert that into a response object.
 
 If you want to get hold of the resulting response object inside the view

flask/app.py

@@ -44,6 +44,7 @@ from .helpers import (
     url_for,
     get_load_dotenv,
 )
+from .json import jsonify
 from .logging import create_logger
 from .sessions import SecureCookieSessionInterface
 from .signals import (
@@ -2001,6 +2002,9 @@ class Flask(_PackageBoundObject):
             ``bytes`` (``str`` in Python 2)
                 A response object is created with the bytes as the body.
 
+            ``dict``
+                A dictionary that will be jsonify'd before being returned.
+
             ``tuple``
                 Either ``(body, status, headers)``, ``(body, status)``, or
                 ``(body, headers)``, where ``body`` is any of the other types
@@ -2064,6 +2068,8 @@ class Flask(_PackageBoundObject):
                 # special logic
                 rv = self.response_class(rv, status=status, headers=headers)
                 status = headers = None
+            elif isinstance(rv, dict):
+                rv = jsonify(rv)
             else:
                 # evaluate a WSGI callable, or coerce a different response
                 # class to the correct type

tests/test_basic.py

@@ -1147,8 +1147,12 @@ def test_response_types(app, client):
     def from_wsgi():
         return NotFound()
 
-    assert client.get("/text").data == u"Hällo Wörld".encode("utf-8")
-    assert client.get("/bytes").data == u"Hällo Wörld".encode("utf-8")
+    @app.route('/dict')
+    def from_dict():
+        return {"foo": "bar"}, 201
+
+    assert client.get('/text').data == u'Hällo Wörld'.encode('utf-8')
+    assert client.get('/bytes').data == u'Hällo Wörld'.encode('utf-8')
 
     rv = client.get("/full_tuple")
     assert rv.data == b"Meh"
@@ -1181,6 +1185,10 @@ def test_response_types(app, client):
     assert b"Not Found" in rv.data
     assert rv.status_code == 404
 
+    rv = client.get('/dict')
+    assert rv.json == {"foo": "bar"}
+    assert rv.status_code == 201
+
 
 def test_response_type_errors():
     app = flask.Flask(__name__)