Fix: Apply clean_up_tokenization_spaces in TokenizersBackend._decode

[Aznix07]

What does this PR do?

This PR fixes a regression in v5.0.0rc1 where the _decode method in TokenizerBackend was not respecting the clean_up_tokenization_spaces paramter, causing unwanted spaces to appear before punctuation in decoded output.

Reproduction:

from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained('mlx-community/Llama-3.2-1B-Instruct-4bit')
text = tokenizer.decode([128000, 64, 1174, 65])
print(text)

Behavior:

• v4.57.3: <|begin_of_text|>a,b ✅
• v5.0.0rc1 (before fix): <|begin_of_text|>a ,b ❌ (extra space before comma)
• v5.0.0rc1 (after fix): <|begin_of_text|>a,b ✅

Solution

Added the missing clean_up_tokenization_spaces logic to TokenizersBackend._decode() method. When enabled (default behavior), it removes extra spaces before punctuation using regex pattern matching.

Fixes #42913

Who can review?

@ArthurZucker @itazap

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=42916&sha=99c932

[ArthurZucker]

Hey! is there a motivation to have that? We removed it because its unintuitive, and can be done by the user itself outside. do you have a specific usecase in mind?

[Aznix07]

Hi @ArthurZucker!

Thanks for the response! I realize I should have clarified this before submitting the PR - apologize for that.

What I observed:

• The clean_up_tokenization_spaces param exists in decode() and defaults to True
• But in TokenizersBackend._decode(), it's not being applied
• This causes: decode([128000, 64, 1174, 65]) → 'a ,b' instead of 'a,b'

My question:
Was removing this cleanup intentional for v5.0.0?

If yes, I can close this PR. If it was unintended, Im happy to adjust the implementation based on your guidance.

Thanks!

[apaniukov]

Hey! is there a motivation to have that? We removed it because its unintuitive, and can be done by the user itself outside. do you have a specific usecase in mind?

@ArthurZucker the issue is that in v4 clean_up_tokenization_spaces flag worked for all tokenizers, and in v5 it works only when the particular tokenizer implementation defined clean_up_tokenization method.

1. It is inconsistent with PythonBackend tokenizer, where there is default implementation in _decode method.
2. clean_up_tokenization_spaces flag in decode method now works for some tokenizers, and stop working for other.
3. (Some) Tokenizers that has self.clean_up_tokenization_spaces=True change behavior in v5, like Llama form the issue.

Removing the behavior and keeping the clean_up_tokenization_spaces flag in the decode is unintuitive, because we need to check does it really work for each tokenizer we use.

I created the issue about it earlier: #42898