oggopus: provide a bitrate property (#704)

Estimate the bitrate based on the file size when seeking after the tag pages.
This makes sure the bitrate doesn't change when changing the tags.

Fixes #475

Author: lazka

mutagen/oggopus.py

@@ -41,10 +41,12 @@ class OggOpusInfo(StreamInfo):
     Attributes:
         length (`float`): File length in seconds, as a float
         channels (`int`): Number of channels
+        bitrate (`int`): Bitrate in bits per second, as an int
     """
 
     length = 0
     channels = 0
+    bitrate = 0
 
     def __init__(self, fileobj):
         page = OggPage(fileobj)
@@ -68,13 +70,20 @@ def __init__(self, fileobj):
             raise OggOpusHeaderError("version %r unsupported" % major)
 
     def _post_tags(self, fileobj):
+        audio_size = get_size(fileobj) - fileobj.tell()
+
         page = OggPage.find_last(fileobj, self.serial, finishing=True)
         if page is None:
             raise OggOpusHeaderError
         self.length = (page.position - self.__pre_skip) / float(48000)
 
+        if self.length:
+            self.bitrate = round(audio_size * 8 / self.length)
+        else:
+            self.bitrate = 0
+
     def pprint(self):
-        return u"Ogg Opus, %.2f seconds" % (self.length)
+        return u"Ogg Opus, %.2f seconds, %d bps" % (self.length, self.bitrate)
 
 
 class OggOpusVComment(VCommentDict):

tests/test_oggopus.py

@@ -79,3 +79,45 @@ def test_preserve_non_padding(self):
 
     def test_init_padding(self):
         self.assertEqual(self.audio.tags._padding, 196)
+
+    def test_pprint(self):
+        assert self.audio.info.pprint() == "Ogg Opus, 11.35 seconds, 45243 bps"
+
+    def test_bitrate(self):
+        assert self.audio.info.bitrate == 45243
+
+    def test_bitrate_stable_after_tag_change(self):
+        bitrate_before = self.audio.info.bitrate
+        assert bitrate_before != 0
+
+        self.audio["ARTIST"] = ["Test Artist"]
+        self.audio["ALBUM"] = ["Test Album"]
+        self.audio["TITLE"] = ["Test Title"]
+        self.audio.save(padding=lambda x: 0)
+
+        audio_after = self.Kind(self.filename)
+        bitrate_after = audio_after.info.bitrate
+        assert bitrate_before == bitrate_after
+
+    def test_bitrate_stable_with_large_metadata(self):
+        bitrate_initial = self.audio.info.bitrate
+        assert bitrate_initial != 0
+
+        large_comment = "x" * 50000
+        self.audio["COMMENT"] = [large_comment]
+        self.audio["DESCRIPTION"] = [large_comment]
+        self.audio["CUSTOM"] = [large_comment]
+        self.audio.save(padding=lambda x: 0)
+
+        audio_large = self.Kind(self.filename)
+        bitrate_after_large = audio_large.info.bitrate
+        assert bitrate_initial == bitrate_after_large
+
+        audio_large["COMMENT"] = ["small"]
+        del audio_large["DESCRIPTION"]
+        del audio_large["CUSTOM"]
+        audio_large.save(padding=lambda x: 0)
+
+        audio_small = self.Kind(self.filename)
+        bitrate_after_small = audio_small.info.bitrate
+        assert bitrate_initial == bitrate_after_small