bpo-43002: Fix description of behaviour of an exception class in 'from' clause

[mdickinson]

In a raise <expr> from <expr2> statement, <expr2> is permitted to be an exception instance, an exception class, or None. The reference manual suggests that in the first two cases, the value of <expr2> is attached directly to the raise exception as the __cause__. But in the implementation, an exception class is first instantiated before being attached to the raised exception:

cpython/Python/ceval.c

Lines 4758 to 4763 in b745a61

	if (PyExceptionClass_Check(cause)) {
	fixed_cause = _PyObject_CallNoArg(cause);
	if (fixed_cause == NULL)
	goto raise_error;
	Py_DECREF(cause);
	}

This PR fixes the documentation to match the implementation.

https://bugs.python.org/issue43002

[cool-RR]

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[mdickinson]

This PR is stale because it has been open for 30 days with no activity.

Thanks, but I'm not quite sure what to do with this information.

Reviews are welcome. I'll plan to merge within the next couple of weeks.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[miss-islington]

Thanks @mdickinson for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-25341 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

GH-25342 is a backport of this pull request to the 3.8 branch.