Why are 'unknown' tokens randomly added to my tokenized input?

[tshmak]

Here's a script to reproduce what I've observed:

import json
from transformers import Wav2Vec2CTCTokenizer

phones = 'ɔː,(en),oɜ,inɡɜ,o4,b,f,enɡ1,oi2,aa7,eɪ,eː,au7,aaiɜ,onɡ4,oe6,uiɜ,ɒ,iə,c,aa2,oenɡ1,ei7,oenɡ6,au1,ŋ5,iu5,aɪə,ou4,d,ai7,k,i2,eoi5,aai2,j,oenɡɜ,u1,ŋ4,i,m,oi6,unɡɜ,ou2,au2,p,yu1,a,yu4,onɡ1,ɛ,e5,əʊ,ou6,yu5,aɜ,oi1,onɡ5,ai5,aau5,inɡ5,ai1,eɜ,ei5,uɜ,o2,i5,nɡ6,enɡ4,ɐ,l,o1,iu4,enɡ6,ou5,onɡ7,anɡ1,tʃ,aau2,eo6,aa6,iː,enɡ7,oenɡ5,ŋ,aau1,u5,eo5,yu7,oi7,aaɜ,oiɜ,yu2,aa5,ɑː,oe1,n,eoi2,ui2,oenɡ2,inɡ1,anɡ4,t,au4,ei4,u2,aanɡ2,ui4,dʒ,[PAD],a1,e,oenɡ7,aau4,onɡɜ,eoi6,unɡ5,ɹ,e6,yu6,ɪ,ʃ,ei2,aauɜ,enɡɜ,unɡ1,aɪ,i6,eiɜ,aanɡ1,inɡ6,iu1,o5,ui1,inɡ2,unɡ4,eoi4,eo4,uː,ei1,oenɡ4,aa4,aanɡ7,a2,e4,enɡ2,a5,auɜ,iɜ,əl,ai6,iu2,a4,e2,ouɜ,eoi1,anɡ2,[UNK],h,onɡ6,aau6,nɡ5,nɡ4,enɡ5,oeɜ,inɡ4,a6,eoiɜ,e1,ʊ,i1,o7,z,au6,ai4,anɡ6,aai1,oi5,aʊ,v,iu6,unɡ7,au5,eoɜ,aanɡ6,ou1,aanɡ5,(zhy),anɡɜ,oi4,onɡ2,a7,w,ui5,ui6,oe5,unɡ6,aanɡ4,ɔɪ,inɡ7,ɡ,s,o6,aa1,u6,aai4,ʌ,ou7,yuɜ,ɜː,ei6,aiɜ,ə,anɡ7,ai2,u4,iu7,iuɜ,eo1,aai6,eo2,i4,i7,aai5,unɡ2'.split(',')

phones_dict = {x:i for i, x in enumerate(phones)}
with open('test.json', 'w') as f: 
    json.dump(phones_dict, f, indent=4, ensure_ascii=False)

tokenizer = Wav2Vec2CTCTokenizer('test.json', unk_token='[UNK]', pad_token='[PAD]')
text = 'ɡ ei2 j a4 n ɡ eɜ p anɡ4 j au5 t aaɜ n z o2 j a1 t h au2 h eiɜ'
print(tokenizer(text))
print(tokenizer.decode(tokenizer(text)['input_ids'], spaces_between_special_tokens=True)) 

The output:

{'input_ids': [200, 122, 35, 152, 96, 157, 200, 62, 45, 101, 35, 182, 102, 90, 96, 157, 172, 65, 35, 110, 102, 157, 158, 44, 158, 128], 'attention_mask': [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]}
ɡ ei2 j a4 n [UNK] ɡ eɜ p anɡ4 j au5 t aaɜ n [UNK] z o2 j a1 t [UNK] h au2 h eiɜ

As you can see, the 'unknown' token ([UNK]) is added at random locations even though all the words/phones were defined in phones_dict.

What is happening?

I'm using transformers version 4.29.2.

[tshmak]

Sorry, it should be posted to transformers rather than here. Link: https://github.com/huggingface/transformers/issues/30561

[ArthurZucker]

thanks