#17378: Preparser gets lost with iterated ellipsis_range
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Reporter: tmonteil | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.5
Component: user interface | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nbruin):
I think we're really out of our depth here. `ellipsis_range` can actually
take multiple `..` in its arguments, so we need to distinguish between
{{{
[1,2..10,12..20]
}}}
and
{{{
[1,2..len([10,12..20])]
}}}
The code in question is presently (see sage.misc.preparser line 520)
{{{
ix = code.find('..')
while ix != -1:
if ix == 0:
raise SyntaxError("Cannot start line with ellipsis.")
elif code[ix-1]=='.':
# '...' be valid Python in index slices
code = code[:ix-1] + "Ellipsis" + code[ix+2:]
elif len(code) >= ix+3 and code[ix+2]=='.':
# '...' be valid Python in index slices
code = code[:ix] + "Ellipsis" + code[ix+3:]
else:
start_list, end_list = containing_block(code, ix, ['()','[]'])
(*) arguments = code[start_list+1:end_list-1].replace('...',
',Ellipsis,').replace('..', ',Ellipsis,')
arguments = re.sub(r',\s*,', ',', arguments)
if preparse_step:
arguments = arguments.replace(';', ', step=')
range_or_iter = 'range' if code[start_list]=='[' else 'iter'
code = "%s(ellipsis_%s(%s))%s" % (code[:start_list],
range_or_iter,
arguments,
code[end_list:])
ix = code.find('..')
}}}
The problem is in the line marked (*). It really does need to be prepared
to make multiple substitutions, but it should leave ".." that are enclosed
in some kind of bracket for later substitutions.
This is really one of those cases where string-substitutions aren't really
powerful enough. It we'd start with a '..' that has a containing block
that isn't properly contained in a containing block of any other '..',
rather than the first '..' we find, then we'd be ok. (because inside that
containing block all the '..' do belong to the same `ellipsis_range`).
I'm a little worried about cost with that approach: what happens if a
whole ".sage" file gets preparsed that has a whole lot of '..' in it? does
this routine then execute on that huge string? execution time quadratic
(or worse) in the number of '..' might be problematic.
--
Ticket URL: <http://trac.sagemath.org/ticket/17378#comment:1>
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