[Python-Dev] Constancy of None (original) (raw)

Raymond Hettinger python at rcn.com
Fri Jul 16 11:47:05 CEST 2004

[Raymond]

> That raises the question. Since there was a SyntaxWarning in Py2.3, > should it have been escalated to an error for Py2.4?

[AMK]

> +1. > > (And is it time to make assignments to True and False trigger a > SyntaxWarning, or should that wait until 2.5?)

[Guido]

We should be much more conservative with that. There's tons of code out there that assigns these (conditionally, if it's 2.3-aware) to truth values and we don't want to break that.

We may never (prior to Py3.0) be able to block assignments to True/False; however, some lesser restriction could be feasible. For example, it may be possible to require that the assignment have the same boolean value so that "True=x" rejects when bool(x)!=True. The would allow "True=1" and "True=(1==1)" statements to work while still enabling the compiler to optimize the test out of "while True".

While I'm not sure this is a great idea, I would have no remorse for wrecking a piece of code that assigned "True=0" ;-)

[Guido]

The situation with None is much better, since it was always there; AFAIK only a few speed freaks in Zope-land have ever assigned to it (and always using the form "None = None").

Guido's reasoning here is confirmed by Py2.3's SyntaxWarning not causing the slightest stir. With respect to the constancy of None, we are on firm ground.

If there are no objections, I would like to leave in yesterday's patch and upgrade the Py2.4 None assignment SyntaxWarning to a SyntaxError.

Raymond

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