IndexError when re-evaluating snekbox code
Sentry Issue: [BOT-2C](https://sentry.io/organizations/python-discord/issues/1567544008/?referrer=github_integration)

```
IndexError: list index out of range
  File "discord/ext/commands/core.py", line 83, in wrapped
    ret = await coro(*args, **kwargs)
  File "bot/cogs/snekbox.py", line 281, in eval_command
    code = await self.continue_eval(ctx, response)
  File "bot/cogs/snekbox.py", line 235, in continue_eval
    code = new_message.content.split(' ', maxsplit=1)[1]
```

Using `get_context` seems to be a more robust option than parsing the message ourselves. Either way, if it can't parse it, it should just assume the entire content is the code.
diff --git a/bot/cogs/snekbox.py b/bot/cogs/snekbox.py
index cff7c578..45483692 100644
--- a/bot/cogs/snekbox.py
+++ b/bot/cogs/snekbox.py
@@ -232,7 +232,7 @@ class Snekbox(Cog):
                     timeout=10
                 )
 
-                code = new_message.content.split(' ', maxsplit=1)[1]
+                code = await self.get_code(new_message)
                 await ctx.message.clear_reactions()
                 with contextlib.suppress(HTTPException):
                     await response.delete()
@@ -243,6 +243,26 @@ class Snekbox(Cog):
 
             return code
 
+    async def get_code(self, message: Message) -> Optional[str]:
+        """
+        Return the code from `message` to be evaluated.
+
+        If the message is an invocation of the eval command, return the first argument or None if it
+        doesn't exist. Otherwise, return the full content of the message.
+        """
+        log.trace(f"Getting context for message {message.id}.")
+        new_ctx = await self.bot.get_context(message)
+
+        if new_ctx.command is self.eval_command:
+            log.trace(f"Message {message.id} invokes eval command.")
+            split = message.content.split(maxsplit=1)
+            code = split[1] if len(split) > 1 else None
+        else:
+            log.trace(f"Message {message.id} does not invoke eval command.")
+            code = message.content
+
+        return code
+
     @command(name="eval", aliases=("e",))
     @guild_only()
     @in_channel(Channels.bot_commands, hidden_channels=(Channels.esoteric,), bypass_roles=EVAL_ROLES)
diff --git a/tests/bot/cogs/test_snekbox.py b/tests/bot/cogs/test_snekbox.py
index fd946882..1dec0cca 100644
--- a/tests/bot/cogs/test_snekbox.py
+++ b/tests/bot/cogs/test_snekbox.py
@@ -1,11 +1,13 @@
 import asyncio
 import logging
 import unittest
-from unittest.mock import AsyncMock, MagicMock, Mock, call, patch
+from unittest.mock import AsyncMock, MagicMock, Mock, call, create_autospec, patch
 
+from discord.ext import commands
+
+from bot import constants
 from bot.cogs import snekbox
 from bot.cogs.snekbox import Snekbox
-from bot.constants import URLs
 from tests.helpers import MockBot, MockContext, MockMessage, MockReaction, MockUser
 
 
@@ -23,7 +25,7 @@ class SnekboxTests(unittest.IsolatedAsyncioTestCase):
 
         self.assertEqual(await self.cog.post_eval("import random"), "return")
         self.bot.http_session.post.assert_called_with(
-            URLs.snekbox_eval_api,
+            constants.URLs.snekbox_eval_api,
             json={"input": "import random"},
             raise_for_status=True
         )
@@ -43,10 +45,10 @@ class SnekboxTests(unittest.IsolatedAsyncioTestCase):
 
         self.assertEqual(
             await self.cog.upload_output("My awesome output"),
-            URLs.paste_service.format(key=key)
+            constants.URLs.paste_service.format(key=key)
         )
         self.bot.http_session.post.assert_called_with(
-            URLs.paste_service.format(key="documents"),
+            constants.URLs.paste_service.format(key="documents"),
             data="My awesome output",
             raise_for_status=True
         )
@@ -279,11 +281,14 @@ class SnekboxTests(unittest.IsolatedAsyncioTestCase):
         """Test that the continue_eval function does continue if required conditions are met."""
         ctx = MockContext(message=MockMessage(add_reaction=AsyncMock(), clear_reactions=AsyncMock()))
         response = MockMessage(delete=AsyncMock())
-        new_msg = MockMessage(content='!e NewCode')
+        new_msg = MockMessage()
         self.bot.wait_for.side_effect = ((None, new_msg), None)
+        expected = "NewCode"
+        self.cog.get_code = create_autospec(self.cog.get_code, spec_set=True, return_value=expected)
 
         actual = await self.cog.continue_eval(ctx, response)
-        self.assertEqual(actual, 'NewCode')
+        self.cog.get_code.assert_awaited_once_with(new_msg)
+        self.assertEqual(actual, expected)
         self.bot.wait_for.assert_has_awaits(
             (
                 call('message_edit', check=partial_mock(snekbox.predicate_eval_message_edit, ctx), timeout=10),
@@ -302,6 +307,32 @@ class SnekboxTests(unittest.IsolatedAsyncioTestCase):
         self.assertEqual(actual, None)
         ctx.message.clear_reactions.assert_called_once()
 
+    async def test_get_code(self):
+        """Should return 1st arg (or None) if eval cmd in message, otherwise return full content."""
+        prefix = constants.Bot.prefix
+        subtests = (
+            (self.cog.eval_command, f"{prefix}{self.cog.eval_command.name} print(1)", "print(1)"),
+            (self.cog.eval_command, f"{prefix}{self.cog.eval_command.name}", None),
+            (MagicMock(spec=commands.Command), f"{prefix}tags get foo"),
+            (None, "print(123)")
+        )
+
+        for command, content, *expected_code in subtests:
+            if not expected_code:
+                expected_code = content
+            else:
+                [expected_code] = expected_code
+
+            with self.subTest(content=content, expected_code=expected_code):
+                self.bot.get_context.reset_mock()
+                self.bot.get_context.return_value = MockContext(command=command)
+                message = MockMessage(content=content)
+
+                actual_code = await self.cog.get_code(message)
+
+                self.bot.get_context.assert_awaited_once_with(message)
+                self.assertEqual(actual_code, expected_code)
+
     def test_predicate_eval_message_edit(self):
         """Test the predicate_eval_message_edit function."""
         msg0 = MockMessage(id=1, content='abc')
['bot/cogs/snekbox.py', 'tests/bot/cogs/test_snekbox.py']