Add copy/__copy__ methods to CVXOPT, PPL, InteractiveLP backends, and __deepcopy__ to MixedIntegerLinearProgram and backends

[mkoeppe]

The COIN, CPLEX, GLPK, and Gurobi backends supply a copy method. But it's not documented as a backend interface method in GenericBackend, and the CVXOPT, PPL, InteractiveLP backends do not implement it. It is used by MixedIntegerLinearProgram.__copy__.

The backend method should actually probably be called __copy__ as well -- see ​https://docs.python.org/2/library/copy.html

The branch on the ticket does this.

We also add __deepcopy__ methods to MixedIntegerLinearProgram and the backends. This fixes #15159, though the semantics of copy and deepcopy of a MixedIntegerLinearProgram remains questionable due to its interaction with MIPVariable; see #15159 and #19523.

See also #20323 (in which copying a backend could be the basis for more diverse tests).

[git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

​0bd7a87	GenericBackend._test_add_variables(): Add tests from CVXOPTBackend
​6b55e16	Add a classmethod _test_solve
​e14afd3	GenericBackend._test_solve: Finish
​48b9fe5	Change from mutating instance _test methods to class methods
​5394729	New method _test_ncols_nonnegative
​7138fa0	GenericBackend: Remove failing _test methods from this ticket to make the patchbot and its friends happy
​452b406	Merge branch 'u/mkoeppe/common_testsuite_for_mip_backends' into t/20414/add_copy___copy___methods_to_cvxopt__ppl__interactivelp_backends
​8f968ed	_test_copy: New
​3e26c4d	_test_copy_does_not_share_data: New
​cdd159d	test_copy_some_mips: New

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​4b9fd05

InteractiveLPBackend.__copy__: New

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​fe1a8b9	PPLBackend: Implement __copy__
​d9f1ec8	CVXOPTBackend: Implement __copy__
​751d621	InteractiveLPBackend.__copy__: Fix doctest
​9d39e0c	CoinBackend: In add_col, don't forget to append to col_names
​7c97097	GenericBackend.__copy__: Fix doctest
​ce10c70	GenericBackend._test_copy_some_mips: Change test so it does not use 0 coefficients
​580e913	GenericBackend._do_test_problem_data: Compare types as well

[mkoeppe]

The branch is on top of #20323 (Common TestSuite for MIP backends). The _test_copy_some_mips method exposed a bug in CoinBackend, which I have fixed on the way.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​38c6be9

MixedIntegerLinearProgram, GenericBackend: Add __deepcopy__ methods

[mkoeppe]

Now also contains a fix for #15159 (see description). Needs review.

[dimpase]

How reliable is __deepcopy__ here? Is it just working now?

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​3117b01

MixedIntegerLinearProgram.__deepcopy__: Add another doctest

[mkoeppe]

Replying to dimpase:

How reliable is __deepcopy__ here? Is it just working now?

It's working. I've added another test to illustrate deepcopy semantics.

[dimpase]

Replying to mkoeppe:

Replying to dimpase:

How reliable is __deepcopy__ here? Is it just working now?

It's working. I've added another test to illustrate deepcopy semantics.

was there something about variables that are (not) copied?

[mkoeppe]

Replying to dimpase:

was there something about variables that are (not) copied?

Before this patch, deepcopy didn't even copy the backend of a MIP. Now it does, simply by doing the same as copy.

Both copy and deepcopy have weird semantics in relation to the MIPVariables in a problems -- because a MIP does not have an API to gain access to its variables; and trying to use the old MIPVariables with the copy is questionable and definitely broken if one creates new components of this variable, see #15159, #19523. Fixing this would require a redesign; a possible stopgap would be to stop all MIPVariables from creating new components when they have been subjected to copy().

[dimpase]

ok, fine.

[mkoeppe]

Replying to dimpase:

ok, fine.

Thanks for reviewing.

I've created a new ticket, #20461, for solving the issue with the variables of a copied MIP. Please take a look when you have a chance.