Add DEBUG_API_REQUESTS Anymail setting to dump API communications.

Optionally dump API requests and responses to stdout, to simplify
debugging of the raw API communications. Currently implemented only
for Requests-based backends.

This (undocumented) setting can log things like API keys, so is not
appropriate for use in production.

tests/test_base_backends.py

@@ -1,7 +1,8 @@
-from django.test import override_settings
+from django.test import override_settings, SimpleTestCase
 
 from anymail.backends.base_requests import AnymailRequestsBackend, RequestsPayload
 from anymail.message import AnymailMessage, AnymailRecipientStatus
+from tests.utils import AnymailTestMixin
 
 from .mock_requests_backend import RequestsBackendMockAPITestCase
 
@@ -10,12 +11,14 @@ class MinimalRequestsBackend(AnymailRequestsBackend):
     """(useful only for these tests)"""
 
     esp_name = "Example"
+    api_url = "https://httpbin.org/post"  # helpful echoback endpoint for live testing
 
     def __init__(self, **kwargs):
-        super(MinimalRequestsBackend, self).__init__("https://esp.example.com/api/", **kwargs)
+        super(MinimalRequestsBackend, self).__init__(self.api_url, **kwargs)
 
     def build_message_payload(self, message, defaults):
-        return MinimalRequestsPayload(message, defaults, self)
+        _payload_init = getattr(message, "_payload_init", {})
+        return MinimalRequestsPayload(message, defaults, self, **_payload_init)
 
     def parse_recipient_status(self, response, payload, message):
         return {'to@example.com': AnymailRecipientStatus('message-id', 'sent')}
@@ -49,7 +52,7 @@ class RequestsBackendBaseTestCase(RequestsBackendMockAPITestCase):
     def test_minimal_requests_backend(self):
         """Make sure the testing backend defined above actually works"""
         self.message.send()
-        self.assert_esp_called("https://esp.example.com/api/")
+        self.assert_esp_called("https://httpbin.org/post")
 
     def test_timeout_default(self):
         """All requests have a 30 second default timeout"""
@@ -63,3 +66,44 @@ class RequestsBackendBaseTestCase(RequestsBackendMockAPITestCase):
         self.message.send()
         timeout = self.get_api_call_arg('timeout')
         self.assertEqual(timeout, 5)
+
+
+@override_settings(EMAIL_BACKEND='tests.test_base_backends.MinimalRequestsBackend')
+class RequestsBackendLiveTestCase(SimpleTestCase, AnymailTestMixin):
+    @override_settings(ANYMAIL_DEBUG_API_REQUESTS=True)
+    def test_debug_logging(self):
+        message = AnymailMessage('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
+        message._payload_init = dict(
+            data="Request body",
+            headers={
+                "Content-Type": "text/plain",
+                "Accept": "application/json",
+            },
+        )
+        with self.assertPrints("===== Anymail API request") as outbuf:
+            message.send()
+
+        # Header order and response data vary to much to do a full comparison, but make sure
+        # that the output contains some expected pieces of the request and the response"
+        output = outbuf.getvalue()
+        self.assertIn("\nPOST https://httpbin.org/post\n", output)
+        self.assertIn("\nUser-Agent: django-anymail/", output)
+        self.assertIn("\nAccept: application/json\n", output)
+        self.assertIn("\nContent-Type: text/plain\n", output)  # request
+        self.assertIn("\n\nRequest body\n", output)
+        self.assertIn("\n----- Response\n", output)
+        self.assertIn("\nHTTP 200 OK\n", output)
+        self.assertIn("\nContent-Type: application/json\n", output)  # response
+
+    def test_no_debug_logging(self):
+        # Make sure it doesn't output anything when DEBUG_API_REQUESTS is not set
+        message = AnymailMessage('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
+        message._payload_init = dict(
+            data="Request body",
+            headers={
+                "Content-Type": "text/plain",
+                "Accept": "application/json",
+            },
+        )
+        with self.assertPrints("", match="equal"):
+            message.send()