bug: Seq2Seq teacher forcing disabled by unconditional overwrite — intentional?

Description

In Seq2Seq.forward() (transliterate/thai2rom.py), the teacher forcing if-else block is immediately overwritten by an unconditional assignment:

# line 421-426
if teacher_force and target_seq is not None:
    decoder_input = target_seq[:, di].reshape(batch_size, 1)
else:
    decoder_input = topi.detach()

decoder_input = topi.detach()  # ← this always runs, overwriting the above

Line 426 makes the if-else block dead code — teacher_forcing_ratio has no effect.

History

Looking at git history, this appears to be an incomplete revert:

1. fd4b6676 (2019-08-02) — Original: teacher forcing with ternary expression. Correct.
2. dd6410d4 (2022-10-24) — Removed teacher forcing, left decoder_input = topi.detach().
3. d48dc503 (2022-10-25) — "fix: bringback original thai2rom code" — added back the if-else but did not remove the old line from step 2.

Question

Was removing teacher forcing in step 2 intentional? If the intent of step 3 was to restore it, then line 426 should be removed. The pre-trained model shipped with PyThaiNLP was trained before this change, so romanize(engine="thai2rom") still works — only retraining/fine-tuning is affected.

If teacher forcing is no longer needed (e.g. the model is only used for inference), the if-else block at lines 421-424 could be removed instead.

Steps to reproduce

import inspect
from pythainlp.transliterate.thai2rom import Seq2Seq

source = inspect.getsource(Seq2Seq.forward)
# Lines 421-426 show the if-else followed by unconditional overwrite

PyThaiNLP version

5.3.3

Files

• pythainlp/transliterate/thai2rom.py (lines 421-426)

Start in pythainlp/transliterate/thai2rom.py at Seq2Seq.forward() and inspect the teacher-forcing block around lines 421-426. Compare commits fd4b6676, dd6410d4, and d48dc503, then use the inspect-based reproduction to confirm the current behavior. Done means the intended training behavior is established and the unconditional overwrite or dead block is resolved accordingly.