Difference between fraction and as_numer_denom

[Paul Royik]

I used as_numer_denom earlier and now found fraction.

Is there any difference between them?

[Aaron Meurer]

It looks like fraction takes into account assumptions, but on the
other hand, it doesn't do any rewriting of the expression to combine
sums of fractions (e.g., fraction(1/x + 1/y) gives (1/x + 1/y, 1),
whereas (1/x + 1/y).as_numer_denom() gives (x + y, x*y)).

Aaron Meurer

On Thu, Mar 26, 2015 at 12:09 PM, Paul Royik <distan...@gmail.com> wrote:
> I used as_numer_denom earlier and now found fraction.
> Is there any difference between them?
>

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[Paul Royik]

Both fraction and as_numer_denom return (2x+2, sin(x)+1) for expression 2(x+1)/(sin(x)+1), i.e. they rewrite numerator and denominator.

I suggest simple fix for fraction: substitute last line return Mul(*numer), Mul(*denom) with return Mul(*numer, evaluate=False), Mul(*denom, evaluate=False)

For as_numer_denom fix seems harder to implement.

Is it possible to change them?

[Chris Smith]

That seems reasonable since fraction is allowing the expr to rewrite:

>>> 2*e

2*(x + 1)/(x - 1)

>>> fraction(_)

(2*x + 2, x - 1)

[Aaron Meurer]

Isn't this just because the Mul evaluates itself. I don't think it's
actually fraction() that is rewriting things.

Aaron Meurer

> https://groups.google.com/d/msgid/sympy/12fcfa1f-5701-400f-ace6-31ee0aa00daf%40googlegroups.com.

[Paul Royik]

This dosn't seem reasonable.

Expanding goes on the line Mul(*numer). Mul(*numer, evaluate=False) fixes things.

[Aaron Meurer]

We need to just remove this automatic distribution of numbers. Until
we do that, we will continue to have confusing behavior like this. See
https://github.com/sympy/sympy/issues/4596 for more information.

Aaron Meurer

> https://groups.google.com/d/msgid/sympy/c592330a-4b9c-4812-a342-4e44573aac2b%40googlegroups.com.

[Paul Royik]

I see now.