Update BlockAttachment to not send invalid JSON due to fields attribu…

…te (#473)

slack/web/classes/attachments.py

@@ -211,9 +211,14 @@ def __init__(self, *, blocks: List[Block], color: Optional[str] = None):
         super().__init__(text="", color=color)
         self.blocks = list(blocks)
 
+    @JsonValidator("fields attribute cannot be populated on BlockAttachment")
+    def fields_attribute_absent(self):
+        return not self.fields
+
     def to_dict(self) -> dict:
         json = super().to_dict()
         json.update({"blocks": extract_json(self.blocks)})
+        del json["fields"]  # cannot supply fields and blocks at the same time
         return json
 
 

tests/web/classes/test_attachments.py

@@ -2,8 +2,10 @@
 
 from slack.errors import SlackObjectFormationError
 from slack.web.classes.actions import ActionButton, ActionLinkButton
+from slack.web.classes.blocks import SectionBlock, ImageBlock
 from slack.web.classes.attachments import (
     Attachment,
+    BlockAttachment,
     AttachmentField,
     InteractiveAttachment,
 )
@@ -206,3 +208,16 @@ def test_actions_length(self):
             InteractiveAttachment(
                 text="some text", callback_id="abc123", actions=actions
             ).to_dict(),
+
+
+class BlockAttachmentTests(unittest.TestCase):
+    def test_basic_json(self):
+        blocks = [
+            SectionBlock(text="Some text"),
+            ImageBlock(image_url="image.jpg", alt_text="an image")
+        ]
+
+        self.assertDictEqual(
+            BlockAttachment(blocks=blocks).to_dict(), {"blocks": [b.to_dict() for b in blocks]}
+        )
+