allow to send empty data values and docs changes

docs/topics/request-response.rst

@@ -520,13 +520,13 @@ dealing with JSON requests.
    remaining arguments are the same as for the :class:`Request` class and are
    not documented here.
 
-   Using the :class:`JSONRequest` will set the `Content-Type` header to `application/json`
-   and `Accept` header to `application/json, text/javascript, */*; q=0.01` 
+   Using the :class:`JSONRequest` will set the ``Content-Type`` header to ``application/json``
+   and ``Accept`` header to ``application/json, text/javascript, */*; q=0.01``
 
    :param data: is any JSON serializable object that needs to be JSON encoded and assigned to body.
       if :attr:`Request.body` argument is provided this parameter will be ignored.
       if :attr:`Request.body` argument is not provided and data argument is provided :attr:`Request.method` will be 
-      set to POST automatically.
+      set to ``'POST'`` automatically.
    :type data: JSON serializable object
 
 JSONRequest usage example

scrapy/http/request/json_request.py

@@ -13,12 +13,14 @@ from scrapy.http.request import Request
 
 class JSONRequest(Request):
     def __init__(self, *args, **kwargs):
-        body_passed = 'body' in kwargs
+        body_passed = kwargs.get('body', None) is not None
         data = kwargs.pop('data', None)
-        if body_passed and data:
+        data_passed = data is not None
+
+        if body_passed and data_passed:
             warnings.warn('Both body and data passed. data will be ignored')
 
-        elif not body_passed and data:
+        elif not body_passed and data_passed:
             kwargs['body'] = json.dumps(data)
 
             if 'method' not in kwargs:

tests/test_http_request.py

@@ -1175,20 +1175,35 @@ class JSONRequestTest(RequestTest):
         self.assertEqual(r3.body, to_bytes(json.dumps(data)))
         self.assertEqual(r3.method, 'POST')
 
+        r4 = self.request_class(url="http://www.example.com/", data=[])
+        self.assertEqual(r4.body, to_bytes(json.dumps([])))
+        self.assertEqual(r4.method, 'POST')
+
         with warnings.catch_warnings(record=True) as _warnings:
-            r4 = self.request_class(url="http://www.example.com/", body=body, data=data)
-            self.assertEqual(r4.body, body)
-            self.assertEqual(r4.method, 'GET')
+            r5 = self.request_class(url="http://www.example.com/", body=body, data=data)
+            self.assertEqual(r5.body, body)
+            self.assertEqual(r5.method, 'GET')
             self.assertEqual(len(_warnings), 1)
             self.assertIn('data will be ignored', str(_warnings[0].message))
 
         with warnings.catch_warnings(record=True) as _warnings:
-            r5 = self.request_class(url="http://www.example.com/", body=b'', data=data)
-            self.assertEqual(r5.body, b'')
-            self.assertEqual(r5.method, 'GET')
+            r6 = self.request_class(url="http://www.example.com/", body=b'', data=data)
+            self.assertEqual(r6.body, b'')
+            self.assertEqual(r6.method, 'GET')
             self.assertEqual(len(_warnings), 1)
             self.assertIn('data will be ignored', str(_warnings[0].message))
 
+        with warnings.catch_warnings(record=True) as _warnings:
+            r7 = self.request_class(url="http://www.example.com/", body=None, data=data)
+            self.assertEqual(r7.body, to_bytes(json.dumps(data)))
+            self.assertEqual(r7.method, 'POST')
+            self.assertEqual(len(_warnings), 0)
+
+        with warnings.catch_warnings(record=True) as _warnings:
+            r8 = self.request_class(url="http://www.example.com/", body=None, data=None)
+            self.assertEqual(r8.method, 'GET')
+            self.assertEqual(len(_warnings), 0)
+
     def tearDown(self):
         warnings.resetwarnings()
         super(JSONRequestTest, self).tearDown()