[Python-ideas] True and False are singletons

David Mertz mertz at gnosis.cx
Mon Mar 18 17:52:45 EDT 2019

There are few cases where I would approve of 'if x is True'. However, the
names used in the example suggest it could be one of those rare cases.
Settings of True/False/None (i.e. not set) seem like a reasonable pattern.
In fact, in code like that, merely "truthy" values are probably a bug that
should not pass silently. Obviously this depends on the surrounding code to

On Mon, Mar 18, 2019, 5:44 PM Greg Ewing <greg.ewing at canterbury.ac.nz>

> Richard Damon wrote:
> > On 3/18/19 7:27 AM, Greg Ewing wrote:
> >
> >>    if settings[MY_KEY]:
> >>        ...
>  >
> > That means something VERY different.
> Yes, but there needs to be justification for why the difference
> matters and why this particular way is the best way to deal
> with it.
> Whenever you write 'x is True' or 'x == True', you are putting
> a burden on all code that assigns to x to ensure that the
> value is actually an instance of bool rather than just a
> truthy or falsy value. That's an unusual requiremebt that
> can lead to obscure bugs.
> In the tri-state example, the way I would do it is to guard
> uses of it with 'if x is not None' and then treat the other
> values as truthy or falsy.
> --
> Greg
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