What a coincidence -- the patch I am working on *will* (have the capability to) return both a boolean and an isomorphism for is_isomorphic() on elliptic curves. I was wondering how to do that and assumed that it would be a tuple. Rather than make the tuple's length (1 or 2) depend on the input I would always return a tuple of length 2 but the second item would be None iff the first has false. Isn't that a reasonable way of doing it?
I don't like requiring the trapping of errors, though I realise that it is very Pythonesque to do so. But if we allow the input line sage: phi = E1.isomorphism(E2) then we *must* raise an error when the things are not isomorphic, just as sage: QQ(2).nth_root(2) raises as ValueError (though sqrt(2) does not as it creates a symbolic sqrt(2)). This is going beyond the original thread, I know. On 16/01/2008, David Kohel <[EMAIL PROTECTED]> wrote: > > There was a thread on multiple return values coming from Magma, > since renamed to "Integer points on conics" > > William pointed out that one can access the multiple return values > using nvals: > > x = magma.XGCD(15, 10) > x,y,z = magma.XGCD(15, 10, nvals = 3) > > The first returns an integer, while the second returns a tuple. > > Q1: Is this an acceptable general construction for Python and/or SAGE, > namely to return a different type depending on the values passed into > it? Is it acceptable when it is controlled by nvals in exactly this > way? > > As William pointed out, very different algorithms can be called > depending on the > number of arguments requested in Magma. The following is an example: > > sage: E = magma.EllipticCurve([1,3]) > sage: magma.IsIsomorphic(E,E) > true > sage: bool, phi = magma.IsIsomorphic(E,E,nvals=2) > sage: phi > Elliptic curve isomorphism from: Elliptic Curve defined by y^2 = x^3 + > x + 3 over Rational Field to Elliptic Curve defined by y^2 = x^3 + x + > 3 over Rational Field > Taking (x : y : 1) to (x : y : 1) > > Currently in SAGE we have: > > sage: E = EllipticCurve([1,3]) > sage: E.is_isomorphic(E) > True > > Q2: Would the following be acceptable way of implementing equivalent > functionality in SAGE?: > > sage: bool, phi = E.is_isomorphic(E, nvals=2) > > Note that often (in many concexts) it more expensive to return an > isomorphism, but only slightly more so compared with recomputing the > isomorphism test. If the answer to my question is "no, it not > acceptable SAGE/Python syntax", then how should we implement this > functionality? One possibility is: > > sage: phi = E1.isomorphism(E2) > > and to trap an error if is_isomorphic returns false. Is this a > better? > > > > -- John Cremona --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-devel@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sage-devel URLs: http://sage.scipy.org/sage/ and http://modular.math.washington.edu/sage/ -~----------~----~----~----~------~----~------~--~---
