gh-91217: deprecate imghdr (#91461)

* Deprecate imghdr

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* Update Doc/whatsnew/3.11.rst

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* Inline `imghdr` into `email.mime.image`

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: Barry Warsaw <barry@python.org>

Lib/email/mime/image.py

@@ -6,13 +6,113 @@
 
 __all__ = ['MIMEImage']
 
-import imghdr
-
 from email import encoders
 from email.mime.nonmultipart import MIMENonMultipart
 
 
-
+# Originally from the imghdr module.
+def _what(h):
+    for tf in tests:
+        if res := tf(h):
+            return res
+    else:
+        return None
+
+tests = []
+
+def _test_jpeg(h):
+    """JPEG data with JFIF or Exif markers; and raw JPEG"""
+    if h[6:10] in (b'JFIF', b'Exif'):
+        return 'jpeg'
+    elif h[:4] == b'\xff\xd8\xff\xdb':
+        return 'jpeg'
+
+tests.append(_test_jpeg)
+
+def _test_png(h):
+    if h.startswith(b'\211PNG\r\n\032\n'):
+        return 'png'
+
+tests.append(_test_png)
+
+def _test_gif(h):
+    """GIF ('87 and '89 variants)"""
+    if h[:6] in (b'GIF87a', b'GIF89a'):
+        return 'gif'
+
+tests.append(_test_gif)
+
+def _test_tiff(h):
+    """TIFF (can be in Motorola or Intel byte order)"""
+    if h[:2] in (b'MM', b'II'):
+        return 'tiff'
+
+tests.append(_test_tiff)
+
+def _test_rgb(h):
+    """SGI image library"""
+    if h.startswith(b'\001\332'):
+        return 'rgb'
+
+tests.append(_test_rgb)
+
+def _test_pbm(h):
+    """PBM (portable bitmap)"""
+    if len(h) >= 3 and \
+        h[0] == ord(b'P') and h[1] in b'14' and h[2] in b' \t\n\r':
+        return 'pbm'
+
+tests.append(_test_pbm)
+
+def _test_pgm(h):
+    """PGM (portable graymap)"""
+    if len(h) >= 3 and \
+        h[0] == ord(b'P') and h[1] in b'25' and h[2] in b' \t\n\r':
+        return 'pgm'
+
+tests.append(_test_pgm)
+
+def _test_ppm(h):
+    """PPM (portable pixmap)"""
+    if len(h) >= 3 and \
+        h[0] == ord(b'P') and h[1] in b'36' and h[2] in b' \t\n\r':
+        return 'ppm'
+
+tests.append(_test_ppm)
+
+def _test_rast(h):
+    """Sun raster file"""
+    if h.startswith(b'\x59\xA6\x6A\x95'):
+        return 'rast'
+
+tests.append(_test_rast)
+
+def _test_xbm(h):
+    """X bitmap (X10 or X11)"""
+    if h.startswith(b'#define '):
+        return 'xbm'
+
+tests.append(_test_xbm)
+
+def _test_bmp(h):
+    if h.startswith(b'BM'):
+        return 'bmp'
+
+tests.append(_test_bmp)
+
+def _test_webp(h):
+    if h.startswith(b'RIFF') and h[8:12] == b'WEBP':
+        return 'webp'
+
+tests.append(_test_webp)
+
+def _test_exr(h):
+    if h.startswith(b'\x76\x2f\x31\x01'):
+        return 'exr'
+
+tests.append(_test_exr)
+
+
 class MIMEImage(MIMENonMultipart):
     """Class for generating image/* type MIME documents."""
 
@@ -20,11 +120,11 @@ class MIMEImage(MIMENonMultipart):
                  _encoder=encoders.encode_base64, *, policy=None, **_params):
         """Create an image/* type MIME document.
 
-        _imagedata is a string containing the raw image data.  If this data
-        can be decoded by the standard Python `imghdr' module, then the
-        subtype will be automatically included in the Content-Type header.
-        Otherwise, you can specify the specific image subtype via the _subtype
-        parameter.
+        _imagedata is a string containing the raw image data.  If the data
+        type can be detected (jpeg, png, gif, tiff, rgb, pbm, pgm, ppm,
+        rast, xbm, bmp, webp, and exr attempted), then the subtype will be
+        automatically included in the Content-Type header. Otherwise, you can
+        specify the specific image subtype via the _subtype parameter.
 
         _encoder is a function which will perform the actual encoding for
         transport of the image data.  It takes one argument, which is this
@@ -38,9 +138,8 @@ class MIMEImage(MIMENonMultipart):
         header.
         """
         if _subtype is None:
-            _subtype = imghdr.what(None, _imagedata)
-        if _subtype is None:
-            raise TypeError('Could not guess image MIME subtype')
+            if (_subtype := _what(_imagedata)) is None:
+                raise TypeError('Could not guess image MIME subtype')
         MIMENonMultipart.__init__(self, 'image', _subtype, policy=policy,
                                   **_params)
         self.set_payload(_imagedata)