FreeGroup(0)([]) broken

[darijgr]

This is a border case, but I'd still prefer it to work:

sage: F = FreeGroup(0)
sage: F([])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-47-49d58f580ab1> in <module>()
----> 1 F([])

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/structure/parent.so in sage.structure.parent.Parent.__call__ (build/cythonized/sage/structure/parent.c:9603)()

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/structure/coerce_maps.so in sage.structure.coerce_maps.DefaultConvertMap_unique._call_ (build/cythonized/sage/structure/coerce_maps.c:4256)()

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/structure/coerce_maps.so in sage.structure.coerce_maps.DefaultConvertMap_unique._call_ (build/cythonized/sage/structure/coerce_maps.c:4163)()

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/groups/free_group.pyc in _element_constructor_(self, *args, **kwds)
    808             P = x.parent()
    809         except AttributeError:
--> 810             return self.element_class(self, x, **kwds)
    811         if isinstance(P, FreeGroup_class):
    812             names = set(P._names[abs(i)-1] for i in x.Tietze())

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/groups/free_group.pyc in __init__(self, parent, x)
    225                     i=i+1
    226             AbstractWordTietzeWord = libgap.eval('AbstractWordTietzeWord')
--> 227             x = AbstractWordTietzeWord(l, parent._gap_gens())
    228         ElementLibGAP.__init__(self, parent, x)
    229 

/scratch/sage-6.4.beta2/local/lib/python2.7/site-packages/sage/libs/gap/element.so in sage.libs.gap.element.GapElement_Function.__call__ (build/cythonized/sage/libs/gap/element.c:15598)()

ValueError: libGAP: Error, List Element: <list>[1] must have an assigned value

CC: @tscrim

Component: group theory

Keywords: gap, group, free group, border case

Author: Travis Scrimshaw

Branch/Commit: 40c165c

Reviewer: Darij Grinberg

Issue created by migration from https://trac.sagemath.org/ticket/17246

[tscrim]

comment:1

I will get a fix in a day or two.

[tscrim]

comment:2

I made the input to element construct just return the identity anytime a bool(x) is False (which includes empty lists and tuples), so it fixes the issue (and possibly slightly faster than before).

---

New commits:

9121d13

Fix for trivial free group.

[tscrim]

Commit: 9121d13

[tscrim]

Author: Travis Scrimshaw

[tscrim]

Branch: public/groups/trivial_free_group-17246

[darijgr]

comment:3

Looks good to me, though a shade of doubt remains about whether not x doesn't encompass a few cases too many. Can you run this by someone who knows Sage's free groups well?

[sagetrac-git]

Changed commit from 9121d13 to c65cb5e

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

c65cb5e

Prevent 0 from being used to construct the identity.

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

40c165c

Changed test to explicitly check x == [] or x == ().

[sagetrac-git]

Changed commit from c65cb5e to 40c165c

[tscrim]

comment:6

It did overextend to include 0 going to the identity. However the input is suppose to be a list or tuple whose entries correspond to (inverse) generators, so I've changed it to explicitly check x == [] or x == ().

[darijgr]

comment:7

Thanks!

[tscrim]

comment:8

NP. Thanks for doing the review.

[tscrim]

Reviewer: Darij Grinberg

[vbraun]

Changed branch from public/groups/trivial_free_group-17246 to 40c165c