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

[Alexander Walters]

I am not keen on a SyntaxWarning.  Either something is python syntax, or 
it is not.  This warning catches something linters have been catching 
for ages.  I really don't see the value in adding this, and can see it 
causing more confusion than it solves.  In the #python irc channel, we 
see quite a few newbie mistakes, but declaring a constant that isn't 
used is rarely if ever one of them.

On 2/8/2016 12:44, Victor Stinner wrote:
> Hi,
>
> I changed the Python compiler to ignore any kind "constant
> expressions", whereas it only ignored strings and integers before:
> http://bugs.python.org/issue26204
>
> The compiler now also emits a SyntaxWarning on such case. IMHO the
> warning can help to detect bugs for developers who just learnt Python.
>
> The warning is *not* emited for strings, since triple quoted strings
> are a common syntax for multiline comments.
>
> The warning is *not* emited neither for ellispis (...) since "f():
> ..." is a legit syntax for abstract function.
>
> Are you ok with the new warning?
>
>
> New behaviour:
>
> haypo at smithers$ ./python
> Python 3.6.0a0 (default:759a975e1230, Feb  8 2016, 18:21:23)
>>>> def f():
> ...  False
> ...
> <stdin>:2: SyntaxWarning: ignore constant statement
>
>>>> import dis; dis.dis(f)
>    2           0 LOAD_CONST               0 (None)
>                3 RETURN_VALUE
>
>
> Old behaviour:
>
> haypo at smithers$ python3
> Python 3.4.3 (default, Jun 29 2015, 12:16:01)
>>>> def f():
> ...  False
> ...
>>>> import dis; dis.dis(f)
>    2           0 LOAD_CONST               1 (False)
>                3 POP_TOP
>                4 LOAD_CONST               0 (None)
>                7 RETURN_VALUE
>
>
>
> Before strings and numbers were already ignored. Example:
>
> haypo at smithers$ python3
> Python 3.4.3 (default, Jun 29 2015, 12:16:01)
>
>>>> def f():
> ...  123
> ...
>>>> import dis; dis.dis(f)
>    2           0 LOAD_CONST               0 (None)
>                3 RETURN_VALUE
>
>
> Victor
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