Added bit_depth attribute to PngImageFile

Tests/test_file_png.py

@@ -222,6 +222,20 @@ def test_interlace(self) -> None:
 
             im.load()
 
+    @pytest.mark.parametrize(
+        "test_file, expected_bit_depth",
+        (
+            ("Tests/images/hopper.png", 8),
+            ("Tests/images/hopper_bw_500.png", 1),
+            ("Tests/images/tiny.png", 4),
+            ("Tests/images/i_trns.png", 16),
+        ),
+    )
+    def test_bit_depth(self, test_file: str, expected_bit_depth: int) -> None:
+        with Image.open(test_file) as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
+            assert im.bit_depth == expected_bit_depth
+
     def test_load_transparent_p(self) -> None:
         test_file = "Tests/images/pil123p.png"
         with Image.open(test_file) as im:

docs/handbook/image-file-formats.rst

@@ -895,6 +895,14 @@ decompression bombs. Additionally, the total size of all of the text
 chunks is limited to :data:`.PngImagePlugin.MAX_TEXT_MEMORY`, defaulting to
 64MB.
 
+The :py:class:`~PIL.PngImagePlugin.PngImageFile` class also exposes the
+following attribute:
+
+**bit_depth**
+    The per-sample bit depth read from the PNG ``IHDR`` chunk. Valid values
+    depend on the PNG color type and are one of ``1``, ``2``, ``4``, ``8`` or
+    ``16``.
+
 .. _png-saving:
 
 Saving

src/PIL/PngImagePlugin.py

@@ -391,6 +391,7 @@ def __init__(self, fp: IO[bytes]) -> None:
         self.im_text: dict[str, str | iTXt] = {}
         self.im_size = (0, 0)
         self.im_mode = ""
+        self.im_bit_depth = 0
         self.im_tile: list[ImageFile._Tile] = []
         self.im_palette: tuple[str, bytes] | None = None
         self.im_custom_mimetype: str | None = None
@@ -459,6 +460,7 @@ def chunk_IHDR(self, pos: int, length: int) -> bytes:
             msg = "Truncated IHDR chunk"
             raise ValueError(msg)
         self.im_size = i32(s, 0), i32(s, 4)
+        self.im_bit_depth = s[8]
         try:
             self.im_mode, self.im_rawmode = _MODES[(s[8], s[9])]
         except Exception:
@@ -801,6 +803,7 @@ def _open(self) -> None:
         self._mode = self.png.im_mode
         self._size = self.png.im_size
         self.info = self.png.im_info
+        self.bit_depth = self.png.im_bit_depth
         self._text: dict[str, str | iTXt] | None = None
         self.tile = self.png.im_tile
         self.custom_mimetype = self.png.im_custom_mimetype