227. Cpu bus ppu default factory

Add a default PPU to CpuBus.

Lesson 227 of 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_227_cpu_bus_ppu_default_factory.py

File to update

emulator/bus/cpu_bus.py

What to add

CpuBus should own a PPU instance.

Why this step exists

The NES CPU communicates with the PPU through memory-mapped registers. Before CpuBus can route reads/writes in $2000-$3FFF, the bus needs a PPU object to forward those accesses to.

Important design choice

Use default_factory instead of PPU() directly in the dataclass field.

Why

Each CpuBus should get its own PPU instance. Sharing one mutable PPU between bus instances would create hidden coupling between tests and emulator instances.

Suggested implementation pseudocode

from dataclasses import dataclass, field
from emulator.ppu.ppu import PPU


@dataclass
class CpuBus:
    ...
    ppu: PPU = field(default_factory=PPU)

Run this lesson

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_227_cpu_bus_ppu_default_factory.py -v

Example implementation

Suggested implementation pseudocode

from dataclasses import dataclass, field
from emulator.ppu.ppu import PPU


@dataclass
class CpuBus:
    ...
    ppu: PPU = field(default_factory=PPU)

Show test codetest_227_cpu_bus_ppu_default_factory.py

from emulator.bus.cpu_bus import CpuBus
from emulator.ppu.ppu import PPU


def test_cpu_bus_has_default_ppu_instance():
    """
    Objective:
    CpuBus should have a PPU available by default.
    """
    bus = CpuBus()

    assert isinstance(bus.ppu, PPU)


def test_each_cpu_bus_gets_its_own_ppu_instance():
    """
    Objective:
    Avoid shared mutable PPU state between CpuBus instances.

    Why:
    If two buses accidentally share the same PPU, one test or emulator instance
    could change PPU state observed by another. That kind of hidden coupling is
    difficult to debug.
    """
    first_bus = CpuBus()
    second_bus = CpuBus()

    assert first_bus.ppu is not second_bus.ppu


def test_cpu_bus_allows_injecting_specific_ppu_instance():
    """
    Objective:
    Tests and future emulator orchestration should be able to provide a specific
    PPU instance.

    Why:
    This makes integration tests easier because they can inspect the exact PPU
    object connected to the bus.
    """
    ppu = PPU()

    bus = CpuBus(ppu=ppu)

    assert bus.ppu is ppu

Test functions (3)

test_cpu_bus_has_default_ppu_instance
test_each_cpu_bus_gets_its_own_ppu_instance
test_cpu_bus_allows_injecting_specific_ppu_instance