Hi all,
On Sat, 18 Jan 2020 14:27:23 +0200
Ram Rachum <r...@rachum.com> wrote:
> [...] In any case, the
> way Python chains exceptions when showing them is orthogonal to this
> proposed change. Python already displays the exceptions chained even
> if we don't use "raise from", the only thing that "raise from"
> changes is the text that Python puts between the 2 chained exceptions.
>
Indeed. Well, almost: there is one more thing that addnig the `from`
changes, and that is the attributes on the exception (actually, it's
these attributes which really control the display): Without `from`, the
original exception is placed in a `__context__` attribute; with `from`,
it is placed in a `__cause__` attribute.
At first I thought this was a reason to object to this change -- I
thought code which catches exceptions and looks into their `__context__`
might break because of it. But as it turns out, `from` puts the
original exception on the `__cause__` in *addition* to `__context__`:
In [8]: try:
...: try:
...: 1/0
...: except ZeroDivisionError as e:
...: raise Exception("hi") from e
...: except Exception as ex:
...: exc = ex
...:
In [10]: exc
Out[10]: Exception('hi')
In [13]: exc.__cause__ is exc.__context__
Out[13]: True
So that is not a concern.
> Regarding automatically enforcing this format going forward: I looked
> at the list of Flake8 rules <https://www.flake8rules.com/> and
> couldn't find anything about it.
>
Indeed, I don't think there can be -- it cannot be a style violation to
run into an error while handling another error...
Shai.
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