[Python-Dev] Can Python implementations reject semantically invalid expressions?

[Glyph Lefkowitz]

On Jul 2, 2010, at 12:28 AM, Steven D'Aprano wrote:

> This question was inspired by something asked on #python today. Consider 
> it a hypothetical, not a serious proposal.
> 
> We know that many semantic errors in Python lead to runtime errors, e.g. 
> 1 + "1". If an implementation rejected them at compile time, would it 
> still be Python? E.g. if the keyhole optimizer raised SyntaxError (or 
> some other exception) on seeing this:
> 
> def f():
>    return 1 + "1"
> 
> instead of compiling something which can't fail to raise an exception, 
> would that still be a legal Python implementation?

I'd say "no".  Python has defined semantics in this situation: a TypeError is raised.

To me, this seems akin to a keyhole optimizer arbitrarily deciding that

	raise TypeError()

should cause the compiler to abort.

If this type of expression were common, it would be within the rights of, for example, a Python JIT to generate a fast path through 'f' that wouldn't bother to actually invoke its 'int' type's '__add__' method, since there is no possible way for a Python program to tell the difference, since int.__add__ is immutable.