Testing with doubles

Bind production types in one config and in-memory doubles in another. Tests should not mock time, the filesystem, or sys.stdout.

from pyiv import Config, get_injector
from pyiv.clock import Clock, RealClock, SyntheticClock
from pyiv.console import Console, MemoryConsole, RealConsole
from pyiv.datetime_service import DateTimeService, MockDateTimeService, PythonDateTimeService
from pyiv.filesystem import Filesystem, MemoryFilesystem, RealFilesystem

class ProdConfig(Config):
    def configure(self):
        self.register(Clock, RealClock)
        self.register(Filesystem, RealFilesystem)
        self.register(Console, RealConsole)
        self.register(DateTimeService, PythonDateTimeService)

class TestConfig(Config):
    def configure(self):
        self.register(
            Clock,
            lambda: SyntheticClock(start_time=100.0),
            singleton=True,
        )
        self.register(Filesystem, MemoryFilesystem)
        self.register(Console, MemoryConsole)
        self.register(DateTimeService, MockDateTimeService)

injector = get_injector(TestConfig)
clock = injector.inject(Clock)
clock.advance(5.0)
clock.time()  # 105.0

fs = injector.inject(Filesystem)
fs.write_text("test.txt", "content")  # in memory, not the repo

console = injector.inject(Console)
print("hello", file=console)
console.getvalue()  # contains "hello"

What to swap

Clock / SyntheticClock — freeze and advance time; no real sleeps.
Filesystem / MemoryFilesystem — read and write paths without touching disk.
Console / MemoryConsole — capture print(..., file=console).
DateTimeService / MockDateTimeService — fixed UTC timestamps.

Keep the rest of the object graph on real classes. Only replace the edges that talk to the world.

Override overlays

When the production graph is large, overlay only the doubles with override(ProdConfig).with_(TestConfig) instead of rewriting every binding:

from pyiv import get_injector, override
from pyiv.clock import Clock, SyntheticClock

class TestOverrides(Config):
    def configure(self):
        self.register(
            Clock,
            lambda: SyntheticClock(start_time=100.0),
            singleton=True,
        )

injector = get_injector(override(ProdConfig).with_(TestOverrides))