#14535: Mutability of Graphs
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Reporter: SimonKing | Owner: jason, ncohen, rlm
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.10
Component: graph theory | Resolution:
Keywords: mutability graph | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Simon King | Merged in:
Dependencies: #14524 | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:16 nbruin]:
> I hope that last line remains fantasy: Changing the mutability of an
object is a mutation, so should be forbidden on an immutable object (the
only case where `g.mutable(True)` should succeed is when `g` is already
mutable).
Sure it is a mutation. However, I was told a couple of times that the
"pythonic way" is to pretend that programmers are adult people who know
what they do and who are prepared to take responsibility for their
actions. Hence, while there should be some protection against accidental
changes to an immutable object, it should still be possible to work around
the protection.
I conclude that it would be the anti-pythonic way to
- allow to make a mutable object immutable, but not the other way (this is
by making `_is_immutable` a cdef attribute, and restrict the interface to
three methods `is_immutable`, `is_mutable` and `set_immutable`)
- make it virtually impossible that a sub-class overrides a mutation-
protected method by a method that does not check for mutation (this is
possible with metaclasses)
- turn an object into an immutable object as soon as its hash is computed,
so that its hash bucket will henceforth never change.
We need to find a reasonable middle way between the overly permissive
approach ("all graphs are mutable ''and'' hashable, the programmer really
has to be careful") and the overly protective approach (as described in
the bulleted list).
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14535#comment:17>
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