Ports and adapters

Your domain needs exchange rates, and a provider serves them over HTTP. Define a port: the interface the domain wants, in its own words. Then write adapters that plug into it, one that calls the real API and a fake for tests.

from decimal import Decimal
from typing import Protocol

class Rates(Protocol):
    def rate(self, base: str,
             quote: str) -> Decimal: ...

class FakeRates:
    def __init__(self, table):
        self.table = table

    def rate(self, base, quote):
        return self.table[(base, quote)]

def convert(amount, base, quote, rates):
    return amount * rates.rate(base, quote)

table = {("EUR", "USD"): Decimal("1.1")}
print(convert(Decimal(10), "EUR", "USD",
              FakeRates(table)))

The real adapter takes an httpx.Client instead of creating one. A test can give it a client with an in-memory transport and exercise the real parsing without a network:

import httpx

def api(request):
    return httpx.Response(
        200, json={"rate": "1.1"})

client = httpx.Client(
    base_url="https://rates.test",
    transport=httpx.MockTransport(api))
print(client.get("/rates/EUR/USD").json())

Ports and adapters

Your task