fasti-dates

Python bindings for fasti — dates, calendars, business-day conventions and day-count fractions for financial code, in native Rust with no floating-point arithmetic anywhere.

The distribution is fasti-dates; the import name is fasti, matching the crate. fasti on PyPI is an unrelated FastAPI project manager, and fasti-py is refused as too similar to it — PyPI strips a py affix before comparing names.

That project also installs its own top-level fasti package, so having both in one environment would leave whichever was installed second. It has published two 0.0.0.dev releases and nothing since, and matching the crate name is worth more here than insuring against a collision with a placeholder — but if you need both, install them in separate environments.

$ pip install fasti-dates

Wheels are abi3 from CPython 3.10 up, so one wheel per platform covers every version, and a new CPython needs no new wheel.

The boundary

This is a translation layer, not a second library. Every name maps to one in the crate, with the same method names, the same semantics and the same argument order.

• Dates in and out are datetime.date, and nothing else. A str is refused, naming datetime.date.fromisoformat. A datetime.datetime is refused too: which day a moment falls on is a time-zone decision, and this library has no time-zone concept — call .date() yourself.
• Year fractions come back as fractions.Fraction. Never a float. The crate is float-free, and that has to survive the boundary.
• Errors are FastiError, a ValueError. Type mistakes are TypeError.

>>> import datetime
>>> from fasti.calendars import us
>>> us.SETTLEMENT.is_holiday("2024-07-04")
Traceback (most recent call last):
TypeError: fasti takes a datetime.date, not a str: ...
>>> us.SETTLEMENT.is_holiday(datetime.datetime(2024, 7, 4, 23, 30))
Traceback (most recent call last):
TypeError: fasti takes a datetime.date, not a datetime.datetime: ...

• Values are immutable. Every mutator returns a new value; every value compares, hashes and pickles.

The operators the crate defines are spelled the Python way, and only those: a date steps by a period, and the two enums the crate orders — Weekday and Frequency — are the two that sort.

>>> import datetime
>>> from fasti import Frequency, Period, Weekday
>>> datetime.date(2026, 1, 15) + Period.months(6)
datetime.date(2026, 7, 15)
>>> datetime.date(2026, 1, 31) + Period.months(1)   # Add clamps; it never snaps
datetime.date(2026, 2, 28)
>>> sorted([Weekday.SUN, Weekday.MON]), Frequency.ANNUAL < Frequency.MONTHLY
([Weekday.MON, Weekday.SUN], True)

End-of-month preservation is Calendar.advance's business, not +'s — the same split the crate makes between Add and Date::advance. A BusinessDayConvention refuses < because the crate gives it no order.

Quickstart

>>> import datetime
>>> from fasti import DayCount, Period, Rule, Schedule
>>> from fasti.calendars import us

>>> us.SETTLEMENT.is_business_day(datetime.date(2024, 7, 4))
False
>>> us.SETTLEMENT.adjust(datetime.date(2024, 7, 4), "following")
datetime.date(2024, 7, 5)
>>> us.SETTLEMENT.advance(datetime.date(2025, 1, 31), Period.months(1),
...                       "modified following", True)
datetime.date(2025, 2, 28)

>>> schedule = Schedule(datetime.date(2025, 1, 15), datetime.date(2026, 1, 15),
...                     "semiannual", us.GOVERNMENT_BOND)
>>> schedule.dates()
[datetime.date(2025, 1, 15), datetime.date(2025, 7, 15), datetime.date(2026, 1, 15)]

>>> DayCount.act_act_icma("semiannual").bind(schedule).year_fraction(
...     datetime.date(2025, 1, 15), datetime.date(2025, 4, 15))
Fraction(45, 181)

A Schedule is its coupon dates: it has a length, indexes, slices, iterates and reverses, because the crate's schedule derefs to [Date]. Slicing gives you the dates; after and until are what give you back a schedule, since a bare run of dates names no lattice.

>>> schedule[1:], schedule[-1], len(schedule)
([datetime.date(2025, 7, 15), datetime.date(2026, 1, 15)], datetime.date(2026, 1, 15), 3)

Calendars compose:

>>> from fasti.calendars import france, us
>>> joint = us.SETTLEMENT.union(france.SETTLEMENT)
>>> joint.is_holiday(datetime.date(2026, 7, 14)), joint.is_holiday(datetime.date(2026, 11, 26))
(True, True)
>>> quiet = joint.with_rule(Rule.one_off(datetime.date(2026, 8, 3))).with_name("Acme")
>>> quiet.name
'Acme'

Reading the API

One rule settles what is a property and what is a method: a field in the crate is a property here, a method there is a method here. So calendar.name and period.unit are properties, while day_count.name(), period.length() and frequency.per_year() are calls. It is not the usual Python instinct — the usual instinct is that anything cheap is a property — but it means you can read the Rust docs and know what to type without a second table.

Equality and pickling

Calendar and Rule compare structurally — two values are equal when they were built the same way. That is not "these two agree on every date": settling that means walking 1901 through 2199. (Rule cannot derive equality in the crate at all, since its escape hatch holds a function pointer.)

Everything pickles. A built-in calendar travels as its registry name, because it holds those function-pointer rules; a derived one travels as the operations applied to it; a generated Schedule replays its generation arguments, because its stub reference grid is not recoverable from the dates alone.

Typing

The package ships type stubs and py.typed. Date positions are spelled datetime.date.

A type checker cannot reject a datetime.datetime where a datetime.date is wanted — datetime subclasses date, so it type-checks. The runtime rejects it.

What is not here

• Rule.Custom. The crate's escape hatch is a bare fn(Date) -> bool pointer, which no Python callable can be.
• Date, Year, Month, Ordinal, Weekend, Fraction. Python already has datetime.date and fractions.Fraction; months, years and ordinals are ints; a weekend is a sequence of Weekday.

Development

See CONTRIBUTING.md. In short:

$ uv sync            # or: pip install maturin pytest mypy
$ maturin develop
$ pytest && mypy

License

Apache-2.0 OR MIT, the same as the crate.