UpdateNotCalled as a singleton may not be a good idea

[jamadden]

Because on Python 3, exceptions are stateful. They store their traceback indefinitely. So there are two potential problems: (1) thread safety and (2) leaks.

In [1]: from zope.contentprovider.interfaces import UpdateNotCalled

In [2]: raise UpdateNotCalled
---------------------------------------------------------------------------
UpdateNotCalled                           Traceback (most recent call last)
<ipython-input-2-ddcbc1dc41ad> in <module>()
----> 1 raise UpdateNotCalled

UpdateNotCalled: ``update()`` was not called yet.

In [3]: UpdateNotCalled.__traceback__
Out[3]: <traceback at 0x10a90af88>

In [4]: UpdateNotCalled.__traceback__.tb_frame.f_locals
Out[4]:
{'code_obj': <code object <module> at 0x10a5a6c00, file "<ipython-input-2-ddcbc1dc41ad>", line 1>,
 'old_excepthook': <bound method BaseIPythonApplication.excepthook of <IPython.terminal.ipapp.TerminalIPythonApp object at 0x10809ac88>>,
 'outflag': True,
 'result': <ExecutionResult object at 10a531a58, execution_count=2 error_before_exec=None error_in_exec=``update()`` was not called yet. result=None>,
 'self': <IPython.terminal.interactiveshell.TerminalInteractiveShell at 0x10a347ef0>}

[mgedmin]

Whoa. Does Python even allow you to write except: ExceptionInstance:???

[mgedmin]

Yeah, this doesn't work (exception is never caught) on Python 2 and is not allowed on Python 3:

TypeError: catching classes that do not inherit from BaseException is not allowed

[jamadden]

Yeah, you'd have to except RuntimeError or except Exception to catch it.

To be fair, no code in zopefoundation seems to raise it. It's documented as optional for implementations of content providers.