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

[Andrew Barnert]

> On Feb 8, 2016, at 11:13, Guido van Rossum <guido at python.org> wrote:
> 
>> On Mon, Feb 8, 2016 at 9:44 AM, Victor Stinner <victor.stinner at gmail.com> wrote:
>> 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.
> 
> Hum. I'm not excited by this idea. It is not bad syntax. Have you
> actually seen newbies who were confused by such things?

This does overlap to some extent with a problem that newbies *do* get confused by (and that transplants from Ruby don't find confusing, but do keep forgetting): writing an expression as the last statement in a function and then getting a TypeError or AttributeError about NoneType from the caller. Victor's example of a function that was presumably meant to return False, but instead just evaluates False and returns None, does happen.

But often, that last expression isn't a constant, but something like self.y - self.x. So I'm not sure how much this warning would help that case. In fact, it might add to the confusion if sometimes you get a warning and sometimes you don't. (And you wouldn't want a warning about any function with no return whose last statement is an expression, because often that's perfectly reasonable code, where the expression is a mutating method call, like self.spam.append(arg).)

Also, there are plenty of other common newbie/transplant problems that are similar to this one but can't be caught with a warning, like just referencing a function or method instead of calling it because you left the parens off. That's *usually* a bug, but not always--it could be a LBYL check for an attribute's presence, for example.