Native DateOnly to Python datetime.date

[buvinghausen]

Add DateOnly ↔ datetime.date interop

Adds bidirectional conversion between C#'s DateOnly and Python's datetime.date.

Conversion approach

The conversion uses the ordinal day representation to avoid any string parsing, locale issues, or intermediate DateTime allocations — making it the cleanest and fastest round-trip.

• C# → Python: date.DayNumber + 1 is passed to Python's date.fromordinal()
• Python → C#: Python's date.toordinal() result minus 1 is passed to DateOnly.FromDayNumber()

The offset of 1 exists because C#'s DayNumber is zero-based (day 0 = January 1, 0001) while Python's ordinal is one-based (ordinal 1 = January 1, 0001).

Changes

Runtime (CSnakes.Runtime)

• PyObjectTypeConverter.Date.cs — new partial class implementing ConvertFromDateOnly and ConvertToDateOnly via the ordinal approach
• PyObject.cs — added PyObject.From(DateOnly) and PyObject.From(DateOnly?) factory overloads; As<DateOnly>() dispatch added to the type-switch
• PyObjectImporters.cs — added [Experimental("PRTEXP001")] public sealed class Date : IPyObjectImporter<DateOnly>
• PublicAPI.Unshipped.txt (net8.0 & net9.0) — new public surface area recorded

Source generation (CSnakes.SourceGeneration)

• PythonTypeSpec.cs — added DateType singleton record (base("date"))
• PythonParser.TypeDef.cs — parser maps "date" and "datetime.date" to DateType
• TypeReflection.cs — DateType maps to the C# type name DateOnly
• ResultConversionCodeGenerator.cs — DateType case wired to the Date scalar generator

Tests

• CSnakes.Runtime.Tests/Converter/DateConverterTest.cs — round-trip unit tests via both obj.As<DateOnly>() and obj.ImportAs<DateOnly, PyObjectImporters.Date>() covering MinValue, MaxValue, Unix epoch, and a known date
• CSnakes.Tests/TypeReflectionTests.cs — added ("date", "DateOnly") and ("datetime.date", "DateOnly") to the type reflection theory data
• CSnakes.Tests/PythonTypeSpecTests.cs — added DateTypeTests and UuidTypeTests inner test classes; both singletons added to Singletons_AreExpectedTypes
• CSnakes.Tests/PythonStaticGeneratorTests/FormatClassFromMethods.test_date.approved.txt — snapshot approval confirming DateOnly and global::CSnakes.Runtime.Python.PyObjectImporters.Date appear correctly in generated code
• Integration.Tests/python/test_date.py — Python test module with test_date_roundtrip and test_create_date
• Integration.Tests/DateTests.cs — integration tests: roundtrip for a known date, DateOnly.MinValue, and construction from year/month/day components

Example

# test_date.py
import datetime

def test_date_roundtrip(value: datetime.date) -> datetime.date:
    return value

def test_create_date(year: int, month: int, day: int) -> datetime.date:
    return datetime.date(year, month, day)

var module = env.TestDate();

DateOnly date = new DateOnly(2026, 3, 8);
DateOnly result = module.TestDateRoundtrip(date);   // round-trips through Python

DateOnly created = module.TestCreateDate(2026, 3, 8); // constructed in Python

[Copilot]

Pull request overview

This PR adds bidirectional conversion between C#'s DateOnly and Python's datetime.date, using ordinal-based conversion (avoiding string parsing or intermediate DateTime allocations). The implementation follows the established patterns for type conversion throughout the CSnakes codebase.

Changes:

• Adds DateOnly ↔ datetime.date conversion via ordinal representation (PyObjectTypeConverter.Date.cs), with PyObject.From(DateOnly) factory overloads and As<DateOnly>() dispatch in the runtime
• Adds source generation support: DateType type spec, parser mapping for "date"/"datetime.date", C# type reflection to DateOnly, and result conversion code generation
• Adds comprehensive tests: unit tests for round-trip conversion, source generation snapshot tests, type reflection tests, and integration tests

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/CSnakes.Runtime/PyObjectTypeConverter.Date.cs	New partial class with ConvertFromDateOnly and ConvertToDateOnly using ordinal-based conversion
src/CSnakes.Runtime/Python/PyObject.cs	Added From(DateOnly), From(DateOnly?) overloads and As<DateOnly>() dispatch
src/CSnakes.Runtime/Python/PyObjectImporters.cs	Added Date importer class implementing IPyObjectImporter<DateOnly>
src/CSnakes.SourceGeneration/Parser/Types/PythonTypeSpec.cs	Added DateType singleton record
src/CSnakes.SourceGeneration/Parser/PythonParser.TypeDef.cs	Maps "date" and "datetime.date" to DateType
src/CSnakes.SourceGeneration/Reflection/TypeReflection.cs	Maps DateType to DateOnly
src/CSnakes.SourceGeneration/ResultConversionCodeGenerator.cs	Wires DateType to the scalar Date code generator
src/CSnakes.Runtime/PublicAPI/net8.0/PublicAPI.Unshipped.txt	Records new public API surface (net8.0)
src/CSnakes.Runtime/PublicAPI/net9.0/PublicAPI.Unshipped.txt	Records new public API surface (net9.0)
src/CSnakes.Runtime.Tests/Converter/DateConverterTest.cs	Round-trip unit tests for DateOnly conversion
src/CSnakes.Tests/TypeReflectionTests.cs	Adds date/datetime.date → DateOnly test cases
src/CSnakes.Tests/PythonTypeSpecTests.cs	Adds DateTypeTests and singleton assertion
src/CSnakes.Tests/PythonStaticGeneratorTests/FormatClassFromMethods.test_date.approved.txt	Snapshot approval for generated code
src/Integration.Tests/python/test_date.py	Python test module for date round-trip and creation
src/Integration.Tests/DateTests.cs	Integration tests for date conversion

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