Type hints

[MBradbury]

I have seen #755 and wanted to take a look at providing type hints also. I would appreciate a review of this, as there are some choices I made which need to be checked and others where a decision needs to be made.

Summary:

• Increases minimum version to Python 3.9, as this is when hinting list[T] was introduced (instead of using List[T])
• Added Lpt* type aliases to avoid excessive Unions across the code
• Overloads for dict, dicts and matrix
• Most of the examples type check successfully
• Removed LpElement
• Needed to move away from dicts to dataclasses to serialise MPS. Ability to convert to dicts is provided by Python, but a separate package (dacite) has been needed to load from json into the dataclasses.
• Removed the ability for `LpAffineExpression to inherit from an OrderedDict, always inherit from dict.

Choices to make:

1. What type should be used to store coefficients? Currently a mixture of int and float is allowed.
2. dicts and matrix cannot be fully typed with Python's type hints. I have added overloads for 3 dimensional inputs, how many is appropriate?
3. What is LpConstraintVar? Is it a variable or is it a constraint? There are several pieces of code in LpAffineExpression that assume that all the items it contains are either a LpVariable or a LpConstraintVar. Several options:
   • Could LpConstraintVar inherit from LpVariable?
   • Manage dict of LpConstraintVar separately?
   • Does LpConstraintVar support operators in a LpAffineExpression?
4. There seems to be a number of problems with FractionElasticSubProblem as parameters seemed to be in the incorrect order for the __init__.

There is also a test failure to investigate (PULP_CBC_CMDTest.test_dual_variables_reduced_costs), but I am having trouble finding the root cause.

[pchtsp]

Thanks, I will happy contribute to this initiative of adding types.

I have no problem moving to 3.9, although if it's only to avoid writing from typing import List then it's a bit too much, isn't it?
Moving to dataclasses seems a good idea. I'm not well versed on them so I cannot say much. I know pydantic is also used to type objects, not sure if relevant (I don't regularly used these libraries).
On the dacite question, I would try hard not to add new dependencies unless really justified. The package currently has none and I think that's quite nice.

Many of your questions are for @stumitchell to answer, I believe.
On the first two questions:

• What type should be used to store coefficients? Currently a mixture of int and float is allowed.
  • Without checking the "int" cases I would assume coefficients should be float (because that's what LP understands best).
• dicts and matrix cannot be fully typed with Python's type hints. I have added overloads for 3 dimensional inputs, how many is appropriate?
  • It's hard to impose anything... I myself make dicts of tuples or various size. But part of the charm of PuLP is that it's very flexible and people use it very differently

[christiansegercrantz]

Amazing work, thank you! I can also help with reviewing and changes if needed, although currently a bit busy.

Regarding python 3.8, it's already end-of-lifecycle, so I don't see a reason not to move away from it, people shouldn't be using it anyway.

[MBradbury]

For dacite, alternatively, pulp moves to pickle for disk serialisation which would eliminate the need for dacite, but then human-readability of the files would be lost.

The type hints do not prevent dicts / matrix being used with more dimensions, just means that IDEs won't provide accurate information on the returned type.