[Python-ideas] Make "raise" an expression

[Michael Selik]

On Sat, Aug 6, 2016 at 10:13 PM Terry Reedy <tjreedy at udel.edu> wrote:

> On 8/6/2016 8:57 PM, Michael Selik wrote:
> > On Sat, Aug 6, 2016, 7:24 PM אלעזר
> > <elazarg at gmail.com
> > <mailto:elazarg at gmail.com>> wrote:
> >     The raise statement cannot be used as an expression, even though
> >     there is no theoretical reason for it not to. I don't see any
> >     practical reason either - except the fact that it requires changing
> >     the syntax.
> >
> >     I have found this idea mentioned by Anthony Lee in a thread from
> >     2014, regarding "or
> >     raise",
> https://mail.python.org/pipermail/python-ideas/2014-November/029921.html.
> But there
> >     is nothing against the idea there.
> >
> >     Yes, it could easily be implemented using a function, but this seems
> >     to be an argument against *augmenting* the language, not against
> >     *changing* it. Moreover, this function will have uglier name, since
> >     `raise` is taken. (It could have been the other way around -
> >     omitting the statement entirely, leaving only a built in function
> >     `raise()`. I'm not suggesting that, of course)
> >
> >
> > Luckily, "throw" is available for you.
> >
> > def throw(exception):
> >     raise exception
>
> Nice.  This makes the 'or raise' construct available to any who like it
> in all versions.  There is precedent for this name in generator.throw.
>

I suppose it should really be more like
In [1]: def throw(exception=None, *, cause=None):
    ...:     if exception is None:
    ...:         raise
    ...:     if cause is None:
    ...:         raise exception
    ...:     raise exception from cause


I'm not sure if I like the use in a shortcut expression. Too easy to
accidentally use ``or`` when you mean ``and`` and vice versa.

    False or throw(RuntimeError('you shoulda been True to me'))
    True and throw(RuntimeError('I knew it was you, Fredo.'))
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