[Python-Dev] Re: PEP 285: Adding a bool typen (original) (raw)

Gerald S. Williams gsw@agere.com
Mon, 1 Apr 2002 12:04:38 -0500

I'm not questioning whether it's broken or not, just stating that this is a likely mistake for people to make. Especially since the latter case does work if expr is the result of an operation with a logical result but not for other expressions that are often used in a logical context.

I probably should have used "expr_result" rather than "expr". I really meant for "expr" to be a variable, and was thinking about the case where you would do this:

expr = ...some expression with a "logical" outcome...

if not expr: ....

In a strongly-typed language, this isn't an issue, since you'd have declared "expr" as a boolean type. In that case, "not expr" is the same as "expr == False".

Considering Python's roots as a teaching language, it would be preferable to immediately catch situations where Boolean values are compared to non-Boolean ones and do something about it.

-Jerry

-O Gerald S. Williams, 22Y-103GA : mailto:gsw@agere.com O- -O AGERE SYSTEMS, 555 UNION BLVD : office:610-712-8661 O- -O ALLENTOWN, PA, USA 18109-3286 : mobile:908-672-7592 O-

Paul Svensson wrote:

On Mon, 1 Apr 2002, Gerald S. Williams wrote: >Consider that the main source of confusion won't come from >comparisons to True, but comparisons to False. These two >statements might look identical to the casual observer, but >they really aren't: > > if not expr: > > if expr == False:

Of course they aren't; the first example is good code, the second is broken, and deserves to lose.