Improvements on scenario.py get_survey_scenario.py

[bonjourmauko]

Changes

• Split unit and integration tests
• Add doc to scenario.py and get_survey_scenario.py
• Add tests to get_survey_scenario.py
• Use multipledispatch to reduce code complexity

Split unit and integration tests

The idea of this separation is to have, in one hand, integration tests like calculating income tax.

And, on the other, tests that only test certain functions in isolation.

This is practical as:

• It helps catching bugs
• Unit tests are faster
• We can leave integration tests to test the important things from a metier point of view

Use multipledispatch to reduce code complexity

This module contains a lot of conditional branching, and that's bug prone and makes difficult to encapsulate functionality.

For example, if we want to add conditions by millésime, we end up with a lot of ifs that render the code extremely hard to maintain.

With dynamic dispatch, we can do so in a cleaner way.

This is closely similar to OpenFisca's formulas, although simpler, generic, and more performant.

[benjello]

Id do not know this decorator and it is not straightforward to understand what it does. Could you provide some ref.

[bonjourmauko]

There is a lot of complexity checking whether a value is present or None, or if it has a certain type, and that implies a lot of if and is bug prone.

What this does is to validate data : depending on the value of the argument used, it'll dispatch to a version of the function that handles that specific case.

What we do is to be sure to dispatch to the expected behaviour by type checking the arguments.

Usually you don't need this, but I think for this repo it'll help us catch several bugs and reduce code complexity with little change.

We'll be able to factor out several branch conditions without risk.

[bonjourmauko]

I can add a comment in the code for other readers as well if that helps.