gh-102542 Refactor the mime audio module

Lib/email/mime/audio.py

@@ -6,7 +6,6 @@
 
 __all__ = ['MIMEAudio']
 
-from io import BytesIO
 from email import encoders
 from email.mime.nonmultipart import MIMENonMultipart
 
@@ -36,65 +35,20 @@ def __init__(self, _audiodata, _subtype=None,
         constructor, which turns them into parameters on the Content-Type
         header.
         """
-        if _subtype is None:
-            _subtype = _what(_audiodata)
-        if _subtype is None:
-            raise TypeError('Could not find audio MIME subtype')
+        _subtype = _subtype or _infer_subtype(_audiodata)
         MIMENonMultipart.__init__(self, 'audio', _subtype, policy=policy,
                                   **_params)
         self.set_payload(_audiodata)
         _encoder(self)
 
 
-_rules = []
-
-
-# Originally from the sndhdr module.
-#
-# There are others in sndhdr that don't have MIME types. :(
-# Additional ones to be added to sndhdr? midi, mp3, realaudio, wma??
-def _what(data):
-    # Try to identify a sound file type.
-    #
-    # sndhdr.what() had a pretty cruddy interface, unfortunately.  This is why
-    # we re-do it here.  It would be easier to reverse engineer the Unix 'file'
-    # command and use the standard 'magic' file, as shipped with a modern Unix.
-    hdr = data[:512]
-    fakefile = BytesIO(hdr)
-    for testfn in _rules:
-        if res := testfn(hdr, fakefile):
-            return res
-    else:
-        return None
-
-
-def rule(rulefunc):
-    _rules.append(rulefunc)
-    return rulefunc
-
-
-@rule
-def _aiff(h, f):
-    if not h.startswith(b'FORM'):
-        return None
-    if h[8:12] in {b'AIFC', b'AIFF'}:
+def _infer_subtype(h: bytes) -> str:
+    """Infer the audio format based on the data's header"""
+    if   h.startswith(b'FORM') and h[8:12] in (b'AIFC', b'AIFF'):
         return 'x-aiff'
-    else:
-        return None
-
-
-@rule
-def _au(h, f):
-    if h.startswith(b'.snd'):
+    elif h.startswith(b'.snd'):
         return 'basic'
-    else:
-        return None
-
+    elif h.startswith(b'RIFF') and h[8:16] == b'WAVEfmt ':
+        return 'x-wav'
 
-@rule
-def _wav(h, f):
-    # 'RIFF' <len> 'WAVE' 'fmt ' <len>
-    if not h.startswith(b'RIFF') or h[8:12] != b'WAVE' or h[12:16] != b'fmt ':
-        return None
-    else:
-        return "x-wav"
+    raise TypeError('Could not find audio MIME subtype')