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Swallow potential exceptions from showtraceback() (#13934)...
Swallow potential exceptions from showtraceback() (#13934) The nbgrader project is aware of a form of cheating where students disrupt `InteractiveShell.showtraceback` in hopes of hiding exceptions to avoid losing points. They have implemented a solution to prevent this cheating from working on the client side, and have some tests to demonstrate this technique: https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt.ipynb https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt-alternative.ipynb In essence, these attacks import the interactive shell and erase the traceback handler so that their failing tests won't report failures. ```python import IPython.core.interactiveshell IPython.core.interactiveshell.InteractiveShell.showtraceback = None ``` The problem is that this causes an exception inside the kernel, leading to a stalled execution. The kernel has stopped working, but the client continues to wait for messages. So far, nbgrader's solution to this is to require a timeout value so the client can eventually decide it is done. This prevents allowing a value of `None` for `Execute.timeout` because this would cause a test case to infinitely hang. This commit addresses the problem by making `InteractiveShell._run_cell` a little more protective around it's call to `showtraceback()`. There is already a try/except block around running the cell. This commit adds a finally clause so that the method will _always_ return an `ExecutionResult`, even if a new exception is thrown within the except clause. For the record, the exception thrown is: TypeError: 'NoneType' object is not callable Accepting this change will allow nbgrader to update `nbgrader.preprocessors.Execute` to support a type of `Integer(allow_none=True)` as the parent `NotebookClient` intended. Discussion about this is ongoing in jupyter/nbgrader#1690.
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Test Output Callbacks.ipynb
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Basic Output

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from IPython.display import display
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print('hi')
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display('hi')
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1
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%matplotlib inline
import matplotlib.pyplot as plt
plt.plot([1,3,2])
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%%javascript
console.log("I ran!");
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%%html
<b>bold</b>
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%%latex
$$
a = 5
$$

input_request

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raw_input("prompt > ")

set_next_input

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%%writefile tst.py
def foo():
    pass
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%load tst.py

Pager in execute_reply

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plt?

object_info

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# press shift-tab after parentheses
int(

complete

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# press tab after f
f

clear_output

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import sys, time
from IPython.display import clear_output
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for i in range(10):
    clear_output()
    time.sleep(0.25)
    print(i)
    sys.stdout.flush()
    time.sleep(0.25)
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for i in range(10):
    clear_output(wait=True)
    time.sleep(0.25)
    print(i)
    sys.stdout.flush()
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