Converting your own types

Rather than converting at every call site, hand the Webview a default — the same hook json.dumps(default=...) takes. It is called with any value outside the Bridge contract and must return one inside it.

from dataclasses import asdict, is_dataclass
from datetime import datetime
from decimal import Decimal

from dry import Webview


def default(value):
    if isinstance(value, datetime):
        return value.isoformat()
    if isinstance(value, Decimal):
        return float(value)
    if is_dataclass(value):
        return asdict(value)
    raise TypeError(f'{type(value).__name__} is not JSON serializable')


wv = Webview(app_id='com.example.myapp', html=HTML, api=api, default=default)
wv.run()

What the hook returns is checked in turn, so it may return another value the hook itself handles — a dataclass holding a Decimal converts in two steps.

Raise from it for anything you do not want to convert, and the Call rejects, or the emit raises, exactly as it would have without a hook.

It is consulted last

The hook is the final step, reached only once every other rule has declined the value. It is therefore never asked about:

• a set, frozenset, bytes or bytearray — those are refused before it, because a silent conversion is what the contract exists to prevent;
• an int outside ±2**53, NaN or Infinity;
• a dictionary key. json.dumps does not pass keys to default either. Convert the keys yourself before the dictionary crosses.

One hook, for everything

default= is read while the Webview is being built and applies to every value Dry sends: Call return values, Event values, in every direction. Assigning it after run() raises.

It applies on the way out only. Values arriving from the frontend are already inside the contract by construction — JSON has nothing else in it.