Strange behavior in relabeling a graph in Sage 9.2

[Nikos Apostolakis]

Dear list,

Relabeling a graph in Sage 9.2 exhibits some strange behavior.  If the argument is a dictionary constructed by dictionary comprehension Sage seems to just ignore it.  If the dictionary is explicitly given then everything works.  Here is an example

sage: bar = DiGraph([((2, 3), (1, 2), 2), ((3, 4), (3, 5), 3), ((3, 5), (2, 3), 3), ((5, 6), (5, 7), 5), ((5, 7), (3, 5), 5)])
sage: foo = [(5,6), (3,4), (5,7), (3,5), (2,3), (1,2)]
sage: bar.relabel({foo[i]: i for i in range(6)})
sage: bar.vertices()
[(1, 2), (2, 3), (3, 4), (3, 5), (5, 6), (5, 7)]
sage: ## However if I explicitly give the dictionary
sage: {foo[i]: i for i in range(6)}
{(5,6): 0, (3,4): 1, (5,7): 2, (3,5): 3, (2,3): 4, (1,2): 5}
sage: bar.relabel({(5,6): 0, (3,4): 1, (5,7): 2, (3,5): 3, (2,3): 4, (1,2): 5})
sage: bar.vertices()
[0, 1, 2, 3, 4, 5]
sage: sage0_version()
'SageMath version 9.2, Release Date: 2020-10-24'


This problem doesn't seem to happen in older versions of Sage. I have Sage 8.8 in another box and the problem doesn't occur:

sage: bar = DiGraph([((2, 3), (1, 2), 2), ((3, 4), (3, 5), 3), ((3, 5), (2, 3), 3), ((5, 6), (5, 7), 5), ((5, 7), (3, 5), 5)])
sage: foo = [(5,6), (3,4), (5,7), (3,5), (2,3), (1,2)]
sage: bar.relabel({foo[i]: i for i in range(6)})
sage: bar.vertices()
[0, 1, 2, 3, 4, 5]
sage: sage0_version()
'SageMath version 8.8, Release Date: 2019-06-26'

I wonder if this is caused by the transition to Python3. Some kind of lazy evaluation?

Thanks,
Nikos Apostolakis

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