sage.numerical.linear_functions.LinearConstraint is not a hashable type

[sagetrac-cbowen]

I'd like to have sage.numerical.linear_functions.LinearConstraint to be a hashable type.
Currently it is not:

sage: p = MixedIntegerLinearProgram()
sage: b = p.new_variable()
sage: b[0] <= b[1] <= 2
x_0 <= x_1 <= 2
sage: hash(b[0] <= b[1] <= 2)
...
TypeError: unhashable type: 'sage.numerical.linear_functions.LinearConstraint'

This is for the following reasons:

1. When adding constraints to an instance of a MixedIntegerLinearProgram, it is not necessary to add the same or equivalent constraint multiple times. A hash could be used to determine distinct linear constraints that are added into a MILP instance.
2. By having hashable linear constraints, one could allow for linear constraints to be added into sets (http://docs.python.org/2/library/sets.html) or used as keys in dictionaries.

Thanks,
Clinton

CC: @nathanncohen @vbraun @dimpase

Component: linear programming

Keywords: linear constraint, mixed integer linear program

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/15583

[mkoeppe]

comment:5

Added example to the description.

[sagetrac-brown]

comment:7

sage.numerical.linear_functions.LinearConstraint is now a hashable type.

sage: p = MixedIntegerLinearProgram()
sage: b = p.new_variable()
sage: c = (b[0] <= b[1] <= 2)
sage: c.__hash__()
8748240900236
sage: c.__hash__
<method-wrapper '__hash__' of sage.numerical.linear_functions.LinearConstraint object at 0x7f4dbca908c0>

I think we have https://github.com/sagemath/sagetrac-mirror/commits/682ba3e7b8aaf84ba181ac16b8630aefb695c940 to thank for the update.

[dimpase]

Reviewer: Dima Pasechnik

[dimpase]

comment:9

IMHO, typically solvers are able to deal with multiple constraints, etc.

Also, constraints are really held in the backend, and in fact they can get modified once they are processed.

[embray]

comment:10

Closing tickets in the sage-duplicate/invalid/wontfix module with positive_review (i.e. someone has confirmed they should be closed).