Thanks Tom! Absolutely fascinating! I was trying to grok this and thinking,
"but what if we want 'or' to return a value that will later be used as a
conditional, surely it should return bool?" But of course whatever it
returns will be correctly interpreted as a bool in a conditional context!
Delayed/lazy bool casting, in a sense. Very clever indeed.
There's quite a few places where this would make my code quite a bit
cleaner! =)
Thanks again!
Juan.
On Thu, May 14, 2015 at 12:21 PM, Thomas Caswell <tcasw...@gmail.com> wrote:
> The `a or b` syntax evaluates if a is 'trueish' and if so returns a if not
> returns b so `c = None or {}` -> c == {} but `c = {'a': 1} or {}` -> c ==
> {'a': 1}
>
> See
> https://docs.python.org/3.5/reference/expressions.html#grammar-token-or_test
> for the docs on or. and works almost the same, but returns a if a is False
> and b in a is True.
>
> In the grammar for calls it should be looking for thing like "'**'
> expression" which means in the parsing anything that is part of the
> expression gets evaluated before the unpacking of the mapping. If you
> chase far enough back in the grammar an 'or_test' is an 'expression' (I may
> be butchering the terminology here, only just learned how lexing/parsing
> works a few weeks ago) so it should be fully evaluated before trying to
> unpack.
>
> See https://docs.python.org/3.5/reference/expressions.html#calls for the
> official docs.
>
> I suspect the source of this bug is that the grammar is getting rearranged
> a bit to allow for things like d = {**other_dict, 'x':6} and b = (*a, *c)
> to work as expected and something did not get changed quite right.
>
> Tom
>
> On Wed, May 13, 2015 at 8:33 PM Juan Nunez-Iglesias <jni.s...@gmail.com>
> wrote:
>
>> Fascinating! Can you "unpack" (heh) that error for us mere mortals? In
>> particular:
>>
>> - never seen that "or" syntax before... Is it coercing both expressions
>> as bool, or is it evaluating to left if bool(left) evaluates to True, else
>> to right?
>> - Why do you expect the second expression to work? Is ** supposed to have
>> lower preference than "or"? (Which seems weird to me.)
>>
>> Thanks!
>>
>> Juan.
>>
>> On Thu, May 14, 2015 at 5:08 AM, Thomas Caswell <tcasw...@gmail.com>
>> wrote:
>>
>>>
>>> The failures on python nightly are currently due to a bug in python (
>>> http://bugs.python.org/issue24176)
>>>
>>> Tom
>>>
>>>
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