modify_exception introduces another (nested) exception

[BagelOrb]

fastai 2.7.14
fastcore 1.5.29

Describe the bug
When producing some exception from a plain assert, the exception is modified by fastai, but the modify_exception call introduces another error, which leads to a very convoluted stack trace.

When in any callback you have an assert without description, the code which handles the exception and reraises it (i.e. e.args[0]) will itself produce another exception, because it assumes that the exception had arguments, while it didn't. Please apply the fix proposed at the bottom of this issue report.

To Reproduce

# Python
import torch
from torch.utils.data import TensorDataset
from fastai.vision.all import *
from fastai.callback.core import Callback

# Create random noise data
x = torch.randn(100, 28)
y = torch.randint(0, 2, (100,1))

# Create a TensorDataset and then a DataLoader
train_dl = DataLoader(TensorDataset(x, y.float()), batch_size=64, shuffle=True)
dls = DataLoaders(train_dl, train_dl).cuda()

# Define a custom callback
class CustomCallback(Callback):
    def after_fit(self):
        assert False

# Create a learner and train
learn = Learner(dls, torch.nn.Linear(28, 1), loss_func=nn.BCEWithLogitsLoss(), cbs=CustomCallback())
learn.fit(1)

Expected behavior

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
Cell In[14], [line 22](vscode-notebook-cell:?execution_count=14&line=22)
     [20](vscode-notebook-cell:?execution_count=14&line=20) # Create a learner and train
     [21](vscode-notebook-cell:?execution_count=14&line=21) learn = Learner(dls, torch.nn.Linear(28, 1), loss_func=nn.BCEWithLogitsLoss(), cbs=CustomCallback())
---> [22](vscode-notebook-cell:?execution_count=14&line=22) learn.fit(1)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:264](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:264), in Learner.fit(self, n_epoch, lr, wd, cbs, reset_opt, start_epoch)
    [262](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:262) self.opt.set_hypers(lr=self.lr if lr is None else lr)
    [263](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:263) self.n_epoch = n_epoch
--> [264](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:264) self._with_events(self._do_fit, 'fit', CancelFitException, self._end_cleanup)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:201](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:201), in Learner._with_events(self, f, event_type, ex, final)
    [199](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:199) try: self(f'before_{event_type}');  f()
    [200](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:200) except ex: self(f'after_cancel_{event_type}')
--> [201](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:201) self(f'after_{event_type}');  final()

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:172](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:172), in Learner.__call__(self, event_name)
--> [172](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:172) def __call__(self, event_name): L(event_name).map(self._call_one)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/foundation.py:156](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/foundation.py:156), in L.map(self, f, *args, **kwargs)
--> [156](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/foundation.py:156) def map(self, f, *args, **kwargs): return self._new(map_ex(self, f, *args, gen=False, **kwargs))

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:840](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:840), in map_ex(iterable, f, gen, *args, **kwargs)
    [838](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:838) res = map(g, iterable)
    [839](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:839) if gen: return res
--> [840](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:840) return list(res)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:825](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:825), in bind.__call__(self, *args, **kwargs)
    [823](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:823)     if isinstance(v,_Arg): kwargs[k] = args.pop(v.i)
    [824](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:824) fargs = [args[x.i] if isinstance(x, _Arg) else x for x in self.pargs] + args[self.maxi+1:]
--> [825](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:825) return self.func(*fargs, **kwargs)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:176](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:176), in Learner._call_one(self, event_name)
    [174](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:174) def _call_one(self, event_name):
    [175](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:175)     if not hasattr(event, event_name): raise Exception(f'missing {event_name}')
--> [176](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:176)     for cb in self.cbs.sorted('order'): cb(event_name)

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:62](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:62), in Callback.__call__(self, event_name)
     [60](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:60)     try: res = getcallable(self, event_name)()
     [61](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:61)     except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): raise
---> [62](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:62)     except Exception as e: raise modify_exception(e, f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\n\t{e.args[0]}', replace=True)
     [63](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:63) if event_name=='after_fit': self.run=True #Reset self.run to True at each end of fit
     [64](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:64) return res

File [~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:60](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:60), in Callback.__call__(self, event_name)
     [58](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:58) res = None
     [59](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:59) if self.run and _run: 
---> [60](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:60)     try: res = getcallable(self, event_name)()
     [61](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:61)     except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): raise
     [62](https://file+.vscode-resource.vscode-cdn.net/home/tim.kuipers/dev/deeplearning/projects/iris_drone_classification/pipeline_2023_fastai2/~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:62)     except Exception as e: raise modify_exception(e, f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\n\t{e.args[0]}', replace=True)

Cell In[14], [line 18](vscode-notebook-cell:?execution_count=14&line=18)
     [17](vscode-notebook-cell:?execution_count=14&line=17) def after_fit(self):
---> [18](vscode-notebook-cell:?execution_count=14&line=18)     assert False, "SDg"

AssertionError: Exception occured in `CustomCallback` when calling event `after_fit`

Actual behavior

{
	"name": "IndexError",
	"message": "tuple index out of range",
	"stack": "---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:60, in Callback.__call__(self, event_name)
     59 if self.run and _run: 
---> 60     try: res = getcallable(self, event_name)()
     61     except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): raise

Cell In[15], line 18, in CustomCallback.after_fit(self)
     17 def after_fit(self):
---> 18     assert False

AssertionError: 

During handling of the above exception, another exception occurred:

IndexError                                Traceback (most recent call last)
Cell In[15], line 22
     20 # Create a learner and train
     21 learn = Learner(dls, torch.nn.Linear(28, 1), loss_func=nn.BCEWithLogitsLoss(), cbs=CustomCallback())
---> 22 learn.fit(1)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:264, in Learner.fit(self, n_epoch, lr, wd, cbs, reset_opt, start_epoch)
    262 self.opt.set_hypers(lr=self.lr if lr is None else lr)
    263 self.n_epoch = n_epoch
--> 264 self._with_events(self._do_fit, 'fit', CancelFitException, self._end_cleanup)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:201, in Learner._with_events(self, f, event_type, ex, final)
    199 try: self(f'before_{event_type}');  f()
    200 except ex: self(f'after_cancel_{event_type}')
--> 201 self(f'after_{event_type}');  final()

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:172, in Learner.__call__(self, event_name)
--> 172 def __call__(self, event_name): L(event_name).map(self._call_one)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/foundation.py:156, in L.map(self, f, *args, **kwargs)
--> 156 def map(self, f, *args, **kwargs): return self._new(map_ex(self, f, *args, gen=False, **kwargs))

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:840, in map_ex(iterable, f, gen, *args, **kwargs)
    838 res = map(g, iterable)
    839 if gen: return res
--> 840 return list(res)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastcore/basics.py:825, in bind.__call__(self, *args, **kwargs)
    823     if isinstance(v,_Arg): kwargs[k] = args.pop(v.i)
    824 fargs = [args[x.i] if isinstance(x, _Arg) else x for x in self.pargs] + args[self.maxi+1:]
--> 825 return self.func(*fargs, **kwargs)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/learner.py:176, in Learner._call_one(self, event_name)
    174 def _call_one(self, event_name):
    175     if not hasattr(event, event_name): raise Exception(f'missing {event_name}')
--> 176     for cb in self.cbs.sorted('order'): cb(event_name)

File ~/miniforge3/envs/dl/lib/python3.11/site-packages/fastai/callback/core.py:62, in Callback.__call__(self, event_name)
     60     try: res = getcallable(self, event_name)()
     61     except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): raise
---> 62     except Exception as e: raise modify_exception(e, f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\
\\t{e.args[0]}', replace=True)
     63 if event_name=='after_fit': self.run=True #Reset self.run to True at each end of fit
     64 return res

IndexError: tuple index out of range"
}

Additional context
Fix the following code in core.py:

# %% ../../nbs/13_callback.core.ipynb 16
@funcs_kwargs(as_method=True)
class Callback(Stateful,GetAttr):
[...]
    def __call__(self, event_name):
        "Call `self.{event_name}` if it's defined"
        _run = (event_name not in _inner_loop or (self.run_train and getattr(self, 'training', True)) or
               (self.run_valid and not getattr(self, 'training', False)))
        res = None
        if self.run and _run: 
            try: res = getcallable(self, event_name)()
            except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): raise
            except Exception as e: raise modify_exception(e, f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\n\t{e.args[0]}', replace=True)

Code fix suggested by GPT:

try: 
    res = getcallable(self, event_name)()
except (CancelBatchException, CancelBackwardException, CancelEpochException, CancelFitException, CancelStepException, CancelTrainException, CancelValidException): 
    raise
except Exception as e: 
    error_message = e.args[0] if e.args else str(e)
    raise modify_exception(e, f'Exception occured in `{self.__class__.__name__}` when calling event `{event_name}`:\n\t{error_message}', replace=True)

[BagelOrb]

In my current project I am hitting an assert failure from fastai itself: assert self.monitor in self.recorder.metric_names[1:]. However, my debugger is not able to follow the exception, because of the issue described in this ticket. This means I cannot debug the issue.

Besides fixing this ticket, you might also want to check all your asserts to see if they have a message associated with them.