#17119: Disallow pari(None)
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Reporter: jdemeyer | Owner:
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-6.4
Component: interfaces | Resolution:
Keywords: | Merged in:
Authors: Jeroen Demeyer | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/jdemeyer/ticket/17119 | 4476266587733622582d32b1b7015c56ae97bf69
Dependencies: | Stopgaps:
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Comment (by pbruin):
PARI has its own equivalent of `None`, namely `gnil`. This is internally
a `t_INT` with value 0, but is treated specially by virtue of its memory
address. It is returned by things like evaluating the empty string,
constructions like `if(0,1)`, etc. In Sage we currently convert `gnil` to
None:
{{{
sage: pari('') is None
True
sage: pari('if(0,1)') is None
True
}}}
I think we should decide on a uniform behaviour for the relationship
between `gnil` and `None`. It seems that we have the following options:
- Always convert `gnil` to Python `None`, and make `pari(None)` return
`None`.
- Allow wrapping `gnil` (and possibly other PARI objects that are not on
the stack) in a Sage `gen` without copying it, so PARI recognises it as
`gnil`. In particular, `pari(None)` (like `pari('')`), will be such a
`gen` wrapping `gnil`. This probably prints as 0 by default, but this
could be changed by the `__repr__()` method.
- Stop treating `gnil` specially, identifying it with `gen_0`. Make this
consistent by defining `pari(None) = pari(0)`. This is what GP does when
assigning to a variable:
{{{
gp > if(0,1) \\ returns gnil
gp > x = if(0,1); x \\ the result of gcopy(gnil) is identical to gen_0
%2 = 0
}}}
--
Ticket URL: <http://trac.sagemath.org/ticket/17119#comment:5>
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