Allow bypassing built-in type conversion in msgspec.convert via dec_hook #849

from datetime import date, datetime
from typing import Any, Type
import msgspec

# Required wrapper classes to bypass built-in handling
class CustomDate(date):
    """Workaround to handle date strings like 'DD/MM/YYYY'"""
    pass

class CustomDatetime(datetime):
    """Workaround to handle ISO datetime strings"""
    pass

def dec_hook(type_: Type, obj: Any) -> Any:
    # Custom parsing only works through wrapper classes
    if type_ is CustomDate:
        if isinstance(obj, str):
            day, month, year = map(int, obj.split('/'))
            return CustomDate(year, month, day)
    
    if type_ is CustomDatetime:
        if isinstance(obj, str):
            return CustomDatetime.fromisoformat(obj)
    
    raise NotImplementedError(f"Unsupported type: {type_}")

class Event(msgspec.Struct):
    # Forced to use non-standard types
    start: CustomDatetime
    end: CustomDatetime | None
    registration_deadline: CustomDate

# Usage requires wrapper types
event = msgspec.convert(
    {"start": "2024-01-01T12:00:00", ...},
    Event,
    dec_hook=dec_hook
)

msgspec.convert(
    data,
    cls,
    dec_hook=custom_handler,
    disable_builtin_types={datetime, date}  # New parameter
)

def dec_hook(type_: Type, obj: Any) -> Any:
    # Direct native type handling
    if type_ is date:
        return parse_custom_date(obj)  # Implement your format
    
    if type_ is datetime:
        return parse_custom_datetime(obj)

class Event(msgspec.Struct):
    # Uses proper standard types
    start: datetime
    end: datetime | None
    deadline: date