Wrapper pattern as per Complete Python Testing Guide

create_user.py

from dataclasses import dataclass

from time_provider import TimeProvider
from user import User


class CreateUser:

    def __init__(self, time_provider: TimeProvider) -> None:
        self._time_provider = time_provider

    def execute(self, email: str) -> User:
        return User(
            email=email,
            created_at=self._time_provider.now(),
        )

test_create_user.py

from datetime import datetime, timezone

from create_user import CreateUser
from time_provider import FixedTimeProvider


def test_user_is_created() -> None:
    created_at = datetime(2025, 1, 15, 12, 30, 0, tzinfo=timezone.utc)
    use_case = CreateUser(FixedTimeProvider(fixed))

    user = use_case.execute(email="a@example.com")

    assert user.email == "a@example.com"
    assert user.created_at == created_at

time_provider.py

from datetime import datetime, timezone
from typing import Protocol


class TimeProvider(Protocol):
    """Protocol for getting current time."""

    def now(self) -> datetime: ...


class UTCTimeProvider:
    """Production implementation returning current UTC time."""

    def now(self) -> datetime:
        return datetime.now(timezone.utc)


class FixedTimeProvider:
    """Test implementation with controllable time."""

    def __init__(self, fixed_time: datetime) -> None:
        self._time = fixed_time

    def now(self) -> datetime:
        return self._time

user.py

from dataclasses import dataclass
from datetime import datetime


@dataclass(frozen=True)
class User:
    email: str
    created_at: datetime