[Python-Dev] Expert floats (original) (raw)

Tim Peters tim.one at comcast.net
Tue Mar 30 13:30:14 EST 2004

[Aahz]

>>> 1.1 1.1000000000000001

Just a fact of life. :-/

[Ping]

I regret that this "feature" was ever introduced or "fixed" or what have you. Things were much better when repr(1.1) was "1.1" a few versions ago.

Naturally, I disagree. The immediate motivation at the time was that marshal uses repr(float) to store floats in code objects, so people who use floats seriously found that results differed between running a module directly and importing the same module via a .pyc/.pyo file. That's flatly intolerable for serious work.

That could have been repaired by changing the marshal format, at some cost in compatibility headaches. But since we made the change anyway, it had a wonderful consequence: fp newbies gripe about an example very much like the above right away, and we have a tutorial appendix now that gives them crucial education about the issues involved early in their Python career. Before, they were bit by a large variety of subtler fp surprises much later in their Python life, harder to explain, each requiring a different detailed explanation. Since I'm the guy who traditionally tried to help newbies with stuff like that over the last decade, my testimony that life is 10x better after the change shouldn't be dismissed lightly.

A display hook was added to sys so that people who give a rip (not naming Ping specifically ) could write and share code to format interactive responses following whatever rules they can tolerate. It's still a surprise to me that virtually nobody seems to have cared enough to bother.

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