This post is a result of many attempts at trying to find an existing solution, deciding that nothing did what I needed, and writing the code myself. Specifically, I wanted to be able to verify whether or not that a file is a valid MP3 file from Python. I did not want any dependency on non-Python code (for cross-platform reasons), nor did I need to encode, decode, play, record, or any other such operations to the file. I just needed to know if it was an MP3 or not, and that is all. Oh yeah, and the file will probably have a random file name without the .mp3 extension.
At first, I downloaded several python libraries. The documentation was poor on most of them so I had to experiment to figure out if they did what I needed. All were failures or required something external like ffmpeg. I found library that seemed to check if an   mp3 file was valid, but discovered it only worked if the file was named with the mp3 extension. A closer look at its code revealed that it was just checking the file’s mime-type based on the file extension. That was useless for me.
So I decided that this was something I needed to do myself. With this mp3 file format specification as a reference, I sat down and wrote the code that follows, which seems to work very well. Basically the code searches for the first valid audio frame, makes sure that the frame’s header values are sane, and then checks that the second frame seems to start where it should. This code does not decode any audio in those frames.
Here is the code:
def isMp3Valid(file_path):
is_valid = False
f = open(file_path, 'r')
block = f.read(1024)
frame_start = block.find(chr(255))
block_count = 0 #abort after 64k
while len(block)>0 and frame_start == -1 and block_count<64:
block = f.read(1024)
frame_start = block.find(chr(255))
block_count+=1
if frame_start > -1:
frame_hdr = block[frame_start:frame_start+4]
is_valid = frame_hdr[0] == chr(255)
mpeg_version = ''
layer_desc = ''
uses_crc = False
bitrate = 0
sample_rate = 0
padding = False
frame_length = 0
if is_valid:
is_valid = ord(frame_hdr[1]) & 0xe0 == 0xe0 #validate the rest of the frame_sync bits exist
if is_valid:
if ord(frame_hdr[1]) & 0x18 == 0:
mpeg_version = '2.5'
elif ord(frame_hdr[1]) & 0x18 == 0x10:
mpeg_version = '2'
elif ord(frame_hdr[1]) & 0x18 == 0x18:
mpeg_version = '1'
else:
is_valid = False
if is_valid:
if ord(frame_hdr[1]) & 6 == 2:
layer_desc = 'Layer III'
elif ord(frame_hdr[1]) & 6 == 4:
layer_desc = 'Layer II'
elif ord(frame_hdr[1]) & 6 == 6:
layer_desc = 'Layer I'
else:
is_valid = False
if is_valid:
uses_crc = ord(frame_hdr[1]) & 1 == 0
bitrate_chart = [
[0,0,0,0,0],
[32,32,32,32,8],
[64,48,40,48,16],
[96,56,48,56,24],
[128,64,56,64,32],
[160,80,64,80,40],
[192,96,80,96,40],
[224,112,96,112,56],
[256,128,112,128,64],
[288,160,128,144,80],
[320,192,160,160,96],
[352,224,192,176,112],
[384,256,224,192,128],
[416,320,256,224,144],
[448,384,320,256,160]]
bitrate_index = ord(frame_hdr[2]) >> 4
if bitrate_index==15:
is_valid=False
else:
bitrate_col = 0
if mpeg_version == '1':
if layer_desc == 'Layer I':
bitrate_col = 0
elif layer_desc == 'Layer II':
bitrate_col = 1
else:
bitrate_col = 2
else:
if layer_desc == 'Layer I':
bitrate_col = 3
else:
bitrate_col = 4
bitrate = bitrate_chart[bitrate_index][bitrate_col]
is_valid = bitrate > 0
if is_valid:
sample_rate_chart = [
[44100, 22050, 11025],
[48000, 24000, 12000],
[32000, 16000, 8000]]
sample_rate_index = (ord(frame_hdr[2]) & 0xc) >> 2
if sample_rate_index != 3:
sample_rate_col = 0
if mpeg_version == '1':
sample_rate_col = 0
elif mpeg_version == '2':
sample_rate_col = 1
else:
sample_rate_col = 2
sample_rate = sample_rate_chart[sample_rate_index][sample_rate_col]
else:
is_valid = False
if is_valid:
padding = ord(frame_hdr[2]) & 1 == 1
padding_length = 0
if layer_desc == 'Layer I':
if padding:
padding_length = 4
frame_length = (12 * bitrate * 1000 / sample_rate + padding_length) * 4
else:
if padding:
padding_length = 1
frame_length = 144 * bitrate * 1000 / sample_rate + padding_length
is_valid = frame_length > 0
# Verify the next frame
if(frame_start + frame_length < len(block)):
is_valid = block[frame_start + frame_length] == chr(255)
else:
offset = (frame_start + frame_length) - len(block)
block = f.read(1024)
if len(block) > offset:
is_valid = block[offset] == chr(255)
else:
is_valid = False
f.close()
return is_valid