How to Map Values to Date Ranges and RangeKeyDict

Situations where you need to return different values depending on a specific date come up often. As in the example, when the point accrual rate changes based on January 1, February 2, and August 8, 2020, you could handle it with if statements, but the comparison expressions get long and changing the rules becomes inconvenient.

A better approach is to treat dates and values as pairs and match them in order. If you set the applicable dates in descending order, the logic becomes simpler, but the input order still feels unnatural and some burden remains in understanding the code.

This is where RangeKeyDict comes in handy. Using the form RangeKeyDict({(start, end): value}), you can use a range as a key to map values, allowing you to treat date ranges as data rather than code.

In the example, the policy is expressed as follows.

• (date.min, date(2020, 1, 1)) → None
• (date(2020, 1, 1), date(2020, 2, 2)) → Decimal('0.01')
• (date(2020, 2, 2), date(2020, 8, 8)) → Decimal('0.02')
• (date(2020, 8, 8), date.max) → Decimal('0.08')

This way, date_to_point_rate_v3(base_date) is complete just by putting a date into the mapping object. Beyond dates, this is also useful for problems where values differ by range, such as laws, policies, or version-specific rules.