[Python-Dev] Adding a Rational Type to Python

[Michael Hudson]

"Tim Peters" <tim.one@home.com> writes:

> [Michael Hudson]
> > ...
> > Having ddd.ddd be a rational bothers me.  *No* langauge does that at
> > present, do they?
> 
> ABC (Python's closest predecessor) did.  6.02e23 and 1.073242e-301
> were also exact rationals.  *All* numeric literals were.  This
> explains why they aren't in Python, but doesn't explain exactly why:
> i.e., it didn't work well in ABC, but it's unclear whether that's
> because rationals suck, or because you got rationals even when
> 10,000 years of computer history <wink> told you that "." would get
> you something else.

Well, it seems likely that it wouldn't work in Python too, doesn't it?
Especially with 10010 years of computer history.

> > Also, writing rational numbers as decimal floats strikes me as a
> > bit loopy.  Is
> >
> >   0.33333333
> >
> > 1/3 or 3333333/10000000?
> 
> Neither, it's 33333333/100000000 (which is what I expect you intended for
> your 2nd choice).

Err, yes.  I was feeling too lazy to count 0's.

[snip]
> OTOH, it's possible to do rational<->string conversion with an
> extended notation for "repeating decimals", e.g.
> 
>    str(1/3) == "0.(3)"
>    eval("0.(3)") == 1/3
> 
> would be possible (indeed, I've implemented it in my own rational
> classes, but not by default since identifying "the repeating part"
> in rat->string can take space proportional to the magnitude of the
> denominator).

Hmm, I wonder what the repr of rational(1,3) is...

> but-"."-is-mnemonic-for-the-"point"-in-"floating-point"-ly y'rs  - tim

Quite.

Cheers,
M.

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