Asserts fail with no message

[cab938]

I was trying to programmatically interact with the nbclient package and kept getting blank assertion errors being thrown. It was a bit of a mess on my end to track down since nbclient was being hidden behind numerous other libraries I was working with, here's one of the ones which would regularly throw, indicating that there is no kernel available:

nbclient/nbclient/client.py

Line 960 in 2650b21

assert self.kc is not None

Any reason to use asserts here instead of something more friendly to calling libraries, e.g. a new KernelNotConfiguredException?

[davidbrochart]

It seems that this was used for static type checking, meaning that it should never happen at runtime either. Could you share how you use nbclient?

[cab938]

Yes, I'm building a wrapper for nbclient to make available some of the notebook functions to an LLM agent. More specifically, I'm making a few high level functions like execute_cell() available as tools to langchain agents so that one can ask an LLM with tool support to do things like load, change, execute, and evaluate notebooks.

Here's a bit of pseudocode showing what the workflow looks like:

notebook = nbformat.reads(notebook_content, as_version=4)
client = NotebookClient(notebook)

index=0
for cell in notebook.cells:
    if cell.id == id:
        result = client.execute_cell(cell, index)
   else:
        index += 1

[cab938]

That code likely won't run nicely fwiw, it's sort of ripped out of the stuff I'm building now, and it's possible that code does actually run and initialization of a NotebookClient without a kernel or kernel manager isn't an issue normally and my langchain code is getting in the way.

[cab938]

Here is a better MWE:

from nbclient import NotebookClient
import nbformat

notebook = nbformat.reads(open('test.ipynb','r').read(), as_version=4)
client = NotebookClient(notebook, kernel_name="python3")

for index, cell in enumerate(notebook.cells):
    result = client.execute_cell(cell, index)

Which gives unexpected error:

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
Cell In[14], [line 8](vscode-notebook-cell:?execution_count=14&line=8)
      [5](vscode-notebook-cell:?execution_count=14&line=5) client = NotebookClient(notebook, kernel_name="python3")
      [7](vscode-notebook-cell:?execution_count=14&line=7) for index, cell in enumerate(notebook.cells):
----> [8](vscode-notebook-cell:?execution_count=14&line=8)     result = client.execute_cell(cell, index)

File ~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:159, in run_sync.<locals>.wrapped(*args, **kwargs)
    [157](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:157)     if name not in _runner_map:
    [158](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:158)         _runner_map[name] = _TaskRunner()
--> [159](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:159)     return _runner_map[name].run(inner)
    [160](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:160) except RuntimeError:
    [161](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:161)     pass

File ~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:126, in _TaskRunner.run(self, coro)
    [124](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:124)         self.__runner_thread.start()
    [125](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:125) fut = asyncio.run_coroutine_threadsafe(coro, self.__io_loop)
--> [126](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/jupyter_core/utils/__init__.py:126) return fut.result(None)

File /usr/lib/python3.12/concurrent/futures/_base.py:456, in Future.result(self, timeout)
    [454](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:454)     raise CancelledError()
    [455](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:455) elif self._state == FINISHED:
--> [456](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:456)     return self.__get_result()
    [457](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:457) else:
    [458](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:458)     raise TimeoutError()

File /usr/lib/python3.12/concurrent/futures/_base.py:401, in Future.__get_result(self)
    [399](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:399) if self._exception:
    [400](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:400)     try:
--> [401](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:401)         raise self._exception
    [402](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:402)     finally:
    [403](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:403)         # Break a reference cycle with the exception in self._exception
    [404](https://file+.vscode-resource.vscode-cdn.net/usr/lib/python3.12/concurrent/futures/_base.py:404)         self = None

File ~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:960, in NotebookClient.async_execute_cell(self, cell, cell_index, execution_count, store_history)
    [920](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:920) async def async_execute_cell(
    [921](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:921)     self,
    [922](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:922)     cell: NotebookNode,
   (...)
    [925](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:925)     store_history: bool = True,
    [926](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:926) ) -> NotebookNode:
    [927](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:927)     """
    [928](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:928)     Executes a single code cell.
    [929](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:929) 
   (...)
    [958](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:958)         The cell which was just processed.
    [959](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:959)     """
--> [960](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:960)     assert self.kc is not None
    [962](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:962)     await run_hook(self.on_cell_start, cell=cell, cell_index=cell_index)
    [964](https://file+.vscode-resource.vscode-cdn.net/home/brooksch/sandboxes/jupyter_tool/~/sandboxes/jupyter_tool/.venv/lib/python3.12/site-packages/nbclient/client.py:964)     if cell.cell_type != "code" or not cell.source.strip():

AssertionError:

[cab938]

Odd to me is that I can actually execute the whole notebook it appears, so maybe I am misusing the cell execution api? This code seems to work (and at the least does not throw the assertion error):

from nbclient import NotebookClient
import nbformat

notebook = nbformat.reads(open('test.ipynb','r').read(), as_version=4)
client = NotebookClient(notebook, kernel_name="python3")
client.execute()

[cab938]

Just an update for those who might be running into a similar problem or are trying to use the nbclient package in a similar way, this is how I create and init a new NotebookClient such that I can execute individual cells.

from nbclient import NotebookClient
import nbformat

notebook = nbformat.reads(open('test.ipynb','r').read(), as_version=4)
client = NotebookClient(notebook, kernel_name="python3")
client.km = client.create_kernel_manager()
client.start_new_kernel()
client.kc=client.start_new_kernel_client()

for index, cell in enumerate(notebook.cells):
    print(f"Executing cell {index}")
    result = client.execute_cell(cell, index)