Feat add rate from bracket indice method to rate tax scale like

[PiGo86]

As suggested by @benjello, this PR is intended to allow, for a given tax scale, the determination of the tax rate according to the tax bracket indice.

This feature would be useful for a fix in openfisca-france : https://github.com/openfisca/openfisca-france/pull/1831#discussion_r835448750

New features

• Introduce rate_from_bracket_indice method on RateTaxScaleLike class
  • Allows for the determination of the tax rate based on the tax bracket indice

[benjello]

Thank you very much @PiGo86 for this PR

May I ask you to add to new useful methods very similar to what you already did ?:

• rate_from_tax_base
• threshold_from_tax_base

Thanks !

[PiGo86]

Thank you very much @PiGo86 for this PR

May I ask you to add to new useful methods very similar to what you already did ?:

* `rate_from_tax_base`

* `threshold_from_tax_base`

Thanks !

Sure!

For threshold_from_tax_base, which threshold should be returned? The lower one or the upper one? If it is the upper one, which value has to be returned for the last bracket?

[benjello]

The lower one !

[PiGo86]

Ok, I have just added the two new methods (rate_from_tax_base and threshold_from_tax_base).

However, I wonder if it is appropriate to add the methods rate_from_bracket_indice and rate_from_tax_base to RateTaxScaleLike class.

Indeed, I understand that these methods are not really accurate for the LinearAverageRateTaxScale class, as the returned rate seems to be false in this context.

It should be OK for threshold_from_tax_base method.

In addition, I have moved all tests for these three methods in a new test_rate_tax_scale_like test file (it seemed more logical to me). Let me know if this is not convenient.

[benjello]

@PiGo86 : could you move rate_from_bracket_indice and rate_from_tax_base to MarginalRateTaxScale ?

Thanks again.

[PiGo86]

Ok, done! :wink:

[coveralls]

Coverage increased (+0.01%) to 78.922% when pulling ff61a9f56a1f93f7815889a78ace3c4c8f6c2c44 on PiGo86:feat-add-rate_from_bracket_indice-method-to-RateTaxScaleLike into 30fc3db38f5ba0483d92ff39d41c5682732eb261 on openfisca:master.

[benjello]

Thanks @PiGo86 !