Solution

solution/main.py

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+import hmac
+import hashlib
+
+from pprint import pprint
+
+import requests
+
+from benedict import benedict
+
+from dotenv import dotenv_values
+
+from fastapi import Body, FastAPI, Request, Response, status
+
+config = dotenv_values(".env")
+
+app = FastAPI()
+
+
+# This token can be obtained from the `OAuth Tokens for Your Workspace` section
+# In `OAuth & Permissions` in the bot settings
+BOT_TOKEN = config["BOT_TOKEN"]
+SIGNING_SECRET = config["SIGNING_SECRET"]
+
+
+def slack_validate_request(timestamp, body, slack_signature):
+    sig_basestring = 'v0:' + timestamp + ':' + body
+
+    signature = 'v0=' + hmac.new(
+        SIGNING_SECRET.encode(),
+        sig_basestring.encode(),
+        hashlib.sha256
+    ).hexdigest()
+
+    return signature == slack_signature
+
+
+@app.post("/echo")
+async def echo(request: Request, response: Response, data=Body(...)):
+    raw_body = await request.body()
+    body = raw_body.decode("utf-8")
+
+    # We are following the guide from here:
+    # https://api.slack.com/authentication/verifying-requests-from-slack
+    slack_timestamp = request.headers['x-slack-request-timestamp']
+    slack_signature = request.headers['x-slack-signature']
+
+    is_valid = slack_validate_request(slack_timestamp, body, slack_signature)
+
+    if not is_valid:
+        response.status_code = status.HTTP_403_FORBIDDEN
+        return
+
+    # We need to add the `channels:read` scope
+    # From the `OAuth & Permissions` section in the bot settings
+    # This will give us `data.event.channel`, which is the channe id,
+    # So we can post back a message
+    pprint(data)
+
+    payload = benedict(data)
+
+    event_type = payload.get("event.type")
+    channel_id = payload.get("event.channel")
+    thread_ts = payload.get("event.thread_ts")
+
+    if event_type == "app_mention":
+        send_message_payload = {
+            "channel": channel_id,
+            "text": "Pinging back in the channel"
+        }
+
+        if thread_ts is not None:
+            send_message_payload["thread_ts"] = thread_ts
+            send_message_payload["text"] = "Replying back in a thread"
+
+        headers = {
+            "Authorization": f"Bearer {BOT_TOKEN}"
+        }
+
+        response = requests.post(
+            "https://slack.com/api/chat.postMessage",
+            headers=headers,
+            json=send_message_payload
+        )
+        # We do that to debug any incoming errors from Slack
+        pprint(response.text)
+
+    return {
+        "data": data,
+        "raw_body": body,
+        "headers": request.headers
+    }

solution/requirements.txt

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+fastapi==0.67.0
+uvicorn==0.14.0
+requests==2.26.0
+python-benedict==0.24.0
+python-dotenv==0.18.0