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from __future__ import barry_as_FLUFL doesn't work #125331
Description
Bug report
Bug description:
% ./python.exe -c 'from __future__ import barry_as_FLUFL; print(1 <> 2)'
File "<string>", line 1
from __future__ import barry_as_FLUFL; print(1 <> 2)
^^
SyntaxError: invalid syntax
But with Barry as the FLUFL, <> is the correct way to test for inequality.
The future works only if you pass the right flag to compile() (as test_flufl.py does), but there's no way to do that in a file.
Obviously this future is a joke so this doesn't matter greatly, but I think it indicates that future imports that affect the parser aren't handled properly. That's something we should fix in case we get other futures in the ... future that also affect the parser. The joke future is also useful as a way to demonstrate how future statements work in general without having to rely on a specific future that will go away in a few releases.
CPython versions tested on:
3.12, CPython main branch
Operating systems tested on:
No response
Activity
nineteendo commentedon Oct 11, 2024
We would have to be careful to make sure that this easter egg can't be used in production...
Wulian233 commentedon Oct 11, 2024
This issue was discussed recently, and IDLE is not a bug
#124999 (comment)
#124999 (comment)
ZeroIntensity commentedon Oct 12, 2024
Clearly this should be a blocker for all upcoming releases 😄
pablogsal commentedon Oct 14, 2024
I'm investigating and working on a possible fix. I am not fully convinced that is a but but I agree it looks like one.
pablogsal commentedon Oct 14, 2024
Not really: the parser receives all its configuration when executed, including the flags. The parser cannot change the configuration mid execution (among other things because it doesn't "know" what an import is supposed to do).
The way most futures work is by changing the compiler, not the parser. This is the exception and in reality is not even an exception because the way this was added was to only work in interactive mode. Indeed if you try this in a file:
it won't work:
Is only prepared to work on sequential-execution where the previous statement modifies the flags. That's why it works in the REPL:
this is because when executing the first statement, the second call to parse changed flags. But this only happened after it was executed (not parsed).
pablogsal commentedon Oct 14, 2024
The toll to fix this is not trivial: every call to parse/compile would need to be actually 2 calls to parse/compile OR we need to teach the parser to change flags mid run (which is not trivial because future imports have certain restrictions):
gives:
and these restrictions are not checked by the parser but the compiler (in the symbol table):
cpython/Python/symtable.c
Line 1777 in 843d28f
pablogsal commentedon Oct 14, 2024
I think that given that this will introduce some slowness to fix or maintenance burden in the parser and this was the way this worked since it was introduced I suggest to close as not a bug.
JelleZijlstra commentedon Oct 14, 2024
The Python 2-era future imports that changed the parser did work this way:
But you're right that more recent future imports have been implemented in the compiler, not the parser, and I believe you that it's difficult to implement this as desired in the parser. Still would be nice to make this work, but probably not worth it until we have a non-joke future import that needs it.
pablogsal commentedon Oct 14, 2024
Yeah it was basically in a function called
future_hack, that tells you everything you need to know to know about the maintenance effect :)pythongh-125331: Allow the parser to activate future imports on the fly
pablogsal commentedon Oct 14, 2024
Ok I have a
future_hackPEG edition:#125482
We could go with this now if @lysnikolaou doesn't find it too gross. This is relying on the fact that if one of these nodes gets successfully parsed the backtrack will not change its meaning (which is sort of gross).
thatbirdguythatuknownot commentedon Oct 15, 2024
I wanted to make this issue along with the other
barry_as_FLUFLissue I found a few months ago but I thought it would get shut down immediately :pwimglenn commentedon Dec 9, 2024
Coincidentally, I asked similar on stack overflow back in Sept- https://stackoverflow.com/q/79004026/674039
Nobody was able to answer it there (yet), but Pablo's comments here effectively answer the question.
pythongh-125331: Allow the parser to activate future imports on the fly
gh-125331: Allow the parser to activate future imports on the fly (#1…
pythongh-125331: Allow the parser to activate future imports on the f…
pythongh-125331: Allow the parser to activate future imports on the f…
[3.12] gh-125331: Allow the parser to activate future imports on the …
[3.13] gh-125331: Allow the parser to activate future imports on the …