Add `JSON_STRICT` config option

Allow flask to raise an error rather than return
data that is not JSON specification compliant.

Useful for edge cases such as `NaN` or `Infinity`, for which
the replacement value is application-dependent.

docs/config.rst

@@ -315,6 +315,14 @@ The following configuration values are used internally by Flask:
 
     Default: ``True``
 
+.. py:data:: JSON_STRICT
+
+    Raise an error when encountering values that cannot be serialized in a
+    strictly JSON compliant manner (such as ``NaN`` and ``Infinity``),
+    rather than encoding them as such.
+
+    Default: ``False``
+
 .. py:data:: JSONIFY_PRETTYPRINT_REGULAR
 
     ``jsonify`` responses will be output with newlines, spaces, and indentation

flask/app.py

@@ -304,6 +304,7 @@ class Flask(_PackageBoundObject):
         'PREFERRED_URL_SCHEME':                 'http',
         'JSON_AS_ASCII':                        True,
         'JSON_SORT_KEYS':                       True,
+        'JSON_STRICT':                          False,
         'JSONIFY_PRETTYPRINT_REGULAR':          False,
         'JSONIFY_MIMETYPE':                     'application/json',
         'TEMPLATES_AUTO_RELOAD':                None,

flask/json/__init__.py

@@ -102,6 +102,9 @@ def _dump_arg_defaults(kwargs):
         if not current_app.config['JSON_AS_ASCII']:
             kwargs.setdefault('ensure_ascii', False)
 
+        if current_app.config['JSON_STRICT']:
+            kwargs.setdefault('allow_nan', False)
+
         kwargs.setdefault('sort_keys', current_app.config['JSON_SORT_KEYS'])
     else:
         kwargs.setdefault('sort_keys', True)
@@ -173,6 +176,10 @@ def dumps(obj, **kwargs):
     default which coerce into unicode strings automatically.  That behavior by
     default is controlled by the ``JSON_AS_ASCII`` configuration variable
     and can be overridden by the simplejson ``ensure_ascii`` parameter.
+
+    If the ``JSON_STRICT`` config parameter is set to True, this function will
+    raise a :exc:`ValueError` if it encounters values that cannot be represented in
+    a JSON specification compliant manner.
     """
     _dump_arg_defaults(kwargs)
     encoding = kwargs.pop('encoding', None)

tests/test_basic.py

@@ -1298,6 +1298,14 @@ def test_jsonify_prettyprint(app, req_ctx):
     assert rv.data == pretty_response
 
 
+@pytest.mark.parametrize('test_value', [float('nan'), float('Infinity')])
+def test_json_strict(app, req_ctx, test_value):
+    app.config.update({'JSON_STRICT': True})
+
+    with pytest.raises(ValueError) as e:
+        flask.jsonify(test_value)
+    assert 'not JSON compliant' in str(e.value)
+
 def test_jsonify_mimetype(app, req_ctx):
     app.config.update({"JSONIFY_MIMETYPE": 'application/vnd.api+json'})
     msg = {

tests/test_helpers.py

@@ -155,7 +155,7 @@ class TestJSON(object):
         rv = flask.json.load(out)
         assert rv == test_data
 
-    @pytest.mark.parametrize('test_value', [0, -1, 1, 23, 3.14, 's', "longer string", True, False, None])
+    @pytest.mark.parametrize('test_value', [0, -1, 1, 23, 3.14, float('Infinity'), 's', "longer string", True, False, None])
     def test_jsonify_basic_types(self, test_value, app, client):
         """Test jsonify with basic types."""