test: compare snapshots by what they draw, not how they were encoded

tests/conftest.py

@@ -1,9 +1,11 @@
 import os
 import tempfile
+from io import BytesIO
 from pathlib import Path
 
 import pytest
 from inline_snapshot import Format, register_format
+from PIL import Image, UnidentifiedImageError
 
 # Pin fontconfig (used by cairosvg) to the repo fonts so SVG rasterization in
 # snapshot tests resolves the same font files as the PIL pipeline, independent
@@ -23,11 +25,46 @@
 os.environ["FONTCONFIG_FILE"] = str(_FONTCONFIG_DIR / "fonts.conf")
 
 
+class RenderedImage:
+    """A stored image compared by what it draws, not by how it was encoded.
+
+    Two encoders can write the same picture as different bytes: PNG carries a
+    deflate stream, and the same pixels compress differently across zlib builds.
+    Comparing the files byte for byte therefore tests the encoder that happened
+    to be installed, and a snapshot suite that does it drifts out of date
+    without a single pixel having moved.
+    """
+
+    def __init__(self, data: bytes) -> None:
+        self._data = data
+
+    @staticmethod
+    def _pixels(data: bytes):
+        with Image.open(BytesIO(data)) as image:
+            return image.size, image.convert("RGBA").tobytes()
+
+    def __eq__(self, other: object) -> bool:
+        if isinstance(other, RenderedImage):
+            return self._pixels(self._data) == self._pixels(other._data)
+        if isinstance(other, bytes):
+            try:
+                return self._pixels(self._data) == self._pixels(other)
+            except UnidentifiedImageError:
+                return self._data == other
+        return NotImplemented
+
+    def __hash__(self) -> int:
+        return hash(self._data)
+
+    def __repr__(self) -> str:
+        return f"RenderedImage({len(self._data)} bytes)"
+
+
 class ImageFormat(Format):
     suffix = ".png"
 
-    def decode(self, path: Path) -> bytes:
-        return path.read_bytes()
+    def decode(self, path: Path) -> RenderedImage:
+        return RenderedImage(path.read_bytes())
 
     def encode(
         self,