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[Python-Dev] Issue #26204: compiler now emits a SyntaxWarning on constant statement

Steven D'Aprano steve at pearwood.info
Mon Feb 8 20:02:02 EST 2016 
On Mon, Feb 08, 2016 at 05:43:25PM -0500, Yury Selivanov wrote:
> 
> 
> On 2016-02-08 5:19 PM, Terry Reedy wrote:
> >On 2/8/2016 4:51 PM, Victor Stinner wrote:
> >>2016-02-08 22:28 GMT+01:00 Alexander Walters <tritium-list at sdamon.com>:
> >>>What incantation do you need to do to make that behavior apparent?
> >>
> >>I didn't know. I just checked. It's assert used with a non-empty tuple:
> >>
> >>>>>assert ("tuple",)
> >><stdin>:1: SyntaxWarning: assertion is always true, perhaps remove 
> >>parentheses?
> >
> >I think this should be left to linters also.
> >
> 
> I agree.  I'd remove that warning.


Please don't remove the warning, it is very useful. 

Compare an assertion written correctly:

py> assert 1==2, "Error in arithmetic"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Error in arithmetic


with the simple mistake of wrapping the "tuple" in parens:

py> assert (1==2, "Error in arithmetic")
<stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
py> 


This especially hurts people who think that assert is a function. In 
Python 2.5 and older, you get no warning, and can write wrong code:

py> x = 2
py> assert(x==1, 'expected 1 but got %s' % x)
py>


Removing this warning would be a regression.


-- 
Steve
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