Upload comic/backends.py with huggingface_hub

comic/backends.py

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+"""Swappable model backends behind two tiny interfaces.
+
+  WriterBackend.chat(messages) -> str        # Gemma: returns the full reply text
+  ArtistBackend.render(prompt, seed) -> bytes # FLUX: returns JPEG/PNG image bytes
+
+Selected by COMIC_BACKEND (or an explicit arg):
+  "mock"  -> offline stand-ins (no GPU, no cost) for building/testing the whole flow.
+  "modal" -> the real Gemma vLLM OpenAI endpoint + the resident FLUX endpoint on Modal.
+
+The app, engine and tests only ever see these two interfaces, so deploying real
+models is a config flip, not a code change.
+"""
+
+from __future__ import annotations
+
+import os
+from abc import ABC, abstractmethod
+
+
+class WriterBackend(ABC):
+    @abstractmethod
+    def chat(self, messages: list) -> str:
+        """Return the model's full text reply for an OpenAI-style message list."""
+        raise NotImplementedError
+
+    def warm(self) -> bool:
+        """Best-effort: nudge the backend so a later call is warm. Never raises."""
+        return True
+
+
+class ArtistBackend(ABC):
+    @abstractmethod
+    def render(self, prompt: str, seed: int = 0) -> bytes:
+        """Return image bytes (JPEG/PNG) for a text-free image prompt."""
+        raise NotImplementedError
+
+    def render_batch(self, prompts: list, seeds: list) -> list:
+        """Render several prompts. Default: one at a time. Real backends override to
+        batch through the GPU in a single pass for much higher throughput."""
+        seeds = list(seeds) + [0] * (len(prompts) - len(seeds))
+        return [self.render(p, s) for p, s in zip(prompts, seeds)]
+
+    def warm(self) -> bool:
+        return True
+
+
+# ── factory ──────────────────────────────────────────────────────────────────
+
+def make_backends(kind: str | None = None):
+    """(writer, artist) pair. kind defaults to $COMIC_BACKEND or 'mock'."""
+    kind = (kind or os.environ.get("COMIC_BACKEND", "mock")).lower()
+    if kind == "modal":
+        from .modal_backend import ModalWriter, ModalArtist
+        return ModalWriter(), ModalArtist()
+    if kind == "mock":
+        from .mock_backend import MockWriter, MockArtist
+        return MockWriter(), MockArtist()
+    raise ValueError(f"Unknown COMIC_BACKEND: {kind!r} (expected 'mock' or 'modal')")