Python binding for decode stream

Different API because Python cannot handle lifetimes properly.

bindings/python/py_src/tokenizers/decoders/__init__.py

@@ -12,3 +12,4 @@
 BPEDecoder = decoders.BPEDecoder
 CTC = decoders.CTC
 Sequence = decoders.Sequence
+DecodeStream = decoders.DecodeStream

bindings/python/py_src/tokenizers/decoders/__init__.pyi

@@ -1,4 +1,12 @@
 # Generated content DO NOT EDIT
+class DecodeStream:
+    """
+    Class needed for streaming decode
+
+    """
+    def __init__(self, skip_special_tokens):
+        pass
+
 class Decoder:
     """
     Base class for all decoders

bindings/python/src/decoders.rs

@@ -1,6 +1,7 @@
 use std::sync::{Arc, RwLock};
 
 use crate::pre_tokenizers::from_string;
+use crate::tokenizer::PyTokenizer;
 use crate::utils::PyPattern;
 use pyo3::exceptions;
 use pyo3::prelude::*;
@@ -590,9 +591,71 @@ pub fn decoders(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<PyBPEDecoder>()?;
     m.add_class::<PyCTCDecoder>()?;
     m.add_class::<PySequenceDecoder>()?;
+    m.add_class::<PyDecodeStream>()?;
     Ok(())
 }
 
+/// Class needed for streaming decode
+///
+#[pyclass(module = "tokenizers.decoders", name = "DecodeStream")]
+#[derive(Clone)]
+pub struct PyDecodeStream {
+    /// Regular decode option that is kept throughout.
+    skip_special_tokens: bool,
+    /// A temporary buffer of the necessary token_ids needed
+    /// to produce valid string chunks.
+    /// This typically contains 3 parts:
+    ///  - read
+    ///  - prefix
+    ///  - rest
+    ///
+    /// Read is the bit necessary to surround the prefix
+    /// so decoding the whole ids produces a valid prefix.
+    /// Prefix is the previously produced string, kept around to trim off of
+    /// the next valid chunk
+    ids: Vec<u32>,
+    /// The previously returned chunk that needs to be discarded from the
+    /// decoding of the current ids to produce the next chunk
+    prefix: String,
+    /// The index within the ids corresponding to the prefix so we can drain
+    /// correctly
+    prefix_index: usize,
+    /// We need to keep 2 prefixes.
+    /// Prefix is the second one that was already emitted to discard the part
+    /// of the text of all the ids
+    /// read is the prefix kept only for starting side effects of the prefix
+    read_index: usize,
+}
+
+#[pymethods]
+impl PyDecodeStream {
+    #[new]
+    #[pyo3(signature = (skip_special_tokens), text_signature = "(self, skip_special_tokens)")]
+    fn new(skip_special_tokens: bool) -> Self {
+        PyDecodeStream {
+            skip_special_tokens,
+            ids: vec![],
+            prefix: "".to_string(),
+            prefix_index: 0,
+            read_index: 0,
+        }
+    }
+
+    #[pyo3(signature = (tokenizer, id), text_signature = "(self, tokenizer, id)")]
+    fn step(&mut self, tokenizer: &PyTokenizer, id: u32) -> PyResult<Option<String>> {
+        ToPyResult(tk::tokenizer::step_decode_stream(
+            &tokenizer.tokenizer,
+            id,
+            self.skip_special_tokens,
+            &mut self.ids,
+            &mut self.prefix,
+            &mut self.prefix_index,
+            &mut self.read_index,
+        ))
+        .into()
+    }
+}
+
 #[cfg(test)]
 mod test {
     use std::sync::{Arc, RwLock};

bindings/python/src/tokenizer.rs

@@ -467,7 +467,7 @@ type Tokenizer = TokenizerImpl<PyModel, PyNormalizer, PyPreTokenizer, PyPostProc
 #[derive(Clone, Serialize)]
 #[serde(transparent)]
 pub struct PyTokenizer {
-    tokenizer: Tokenizer,
+    pub(crate) tokenizer: Tokenizer,
 }
 
 impl PyTokenizer {

bindings/python/tests/bindings/test_tokenizer.py

@@ -9,6 +9,7 @@
 from tokenizers.pre_tokenizers import ByteLevel, Metaspace
 from tokenizers.processors import RobertaProcessing, TemplateProcessing
 from tokenizers.normalizers import Strip, Lowercase, Sequence
+from tokenizers.decoders import ByteFallback, DecodeStream, Metaspace as DecoderMetaspace
 
 
 from ..utils import bert_files, data_dir, multiprocessing_with_parallelism, roberta_files
@@ -365,6 +366,37 @@ def test_decode(self):
         output = tokenizer.decode_batch([[0, 1, 2, 3], [4]])
         assert output == ["my name is john", "pair"]
 
+        # Can decode stream
+        stream = DecodeStream(skip_special_tokens=False)
+        assert stream.step(tokenizer, 0) == "my"
+        assert stream.step(tokenizer, 1) == " name"
+        assert stream.step(tokenizer, 2) == " is"
+        assert stream.step(tokenizer, 3) == " john"
+
+    def test_decode_stream(self):
+        vocab = [
+            ("<unk>", 0.0),
+            ("<0x20>", -0.1),
+            ("<0xC3>", -0.2),
+            ("<0xA9>", -0.3),
+        ]
+        tokenizer = Tokenizer(Unigram(vocab, 0, byte_fallback=True))
+        tokenizer.decoder = ByteFallback()
+        stream = DecodeStream(skip_special_tokens=False)
+        assert stream.step(tokenizer, 1) == " "
+        assert stream.step(tokenizer, 2) == None
+        assert stream.step(tokenizer, 3) == "é"
+
+        vocab = [
+            ("<unk>", 0.0),
+            ("▁This", -0.1),
+        ]
+        tokenizer = Tokenizer(Unigram(vocab, 0, byte_fallback=False))
+        tokenizer.decoder = DecoderMetaspace()
+        stream = DecodeStream(skip_special_tokens=False)
+        assert stream.step(tokenizer, 1) == "This"
+        assert stream.step(tokenizer, 1) == " This"
+
     def test_get_vocab(self):
         tokenizer = Tokenizer(BPE())
         tokenizer.add_tokens(["my", "name", "is", "john", "pair"])

tokenizers/src/tokenizer/mod.rs

@@ -1069,24 +1069,50 @@ where
 
     /// See [`DecodeStream`]
     pub fn step(&mut self, id: u32) -> Result<Option<String>> {
-        self.ids.push(id);
-        let string = self
-            .tokenizer
-            .decode(self.ids.as_slice(), self.skip_special_tokens)?;
-        if string.len() > self.prefix.len() && !string.ends_with('�') {
-            if !(string.starts_with(&self.prefix)) {
-                return Err(Box::new(DecodeStreamError::InvalidPrefix));
-            }
-            let new_text = &string[self.prefix.len()..].to_string();
-            let new_prefix_index = self.ids.len() - self.prefix_index;
-            self.ids = self.ids.drain(self.read_index..).collect();
-            self.prefix = self.tokenizer.decode(&self.ids, self.skip_special_tokens)?;
-            self.read_index = self.prefix_index;
-            self.prefix_index = new_prefix_index;
-            Ok(Some(new_text.to_string()))
-        } else {
-            Ok(None)
+        step_decode_stream(
+            &self.tokenizer,
+            id,
+            self.skip_special_tokens,
+            &mut self.ids,
+            &mut self.prefix,
+            &mut self.prefix_index,
+            &mut self.read_index,
+        )
+    }
+}
+
+/// Internal function exposed only to bypass python limitations
+pub fn step_decode_stream<M, N, PT, PP, D>(
+    tokenizer: &TokenizerImpl<M, N, PT, PP, D>,
+    id: u32,
+    skip_special_tokens: bool,
+    ids: &mut Vec<u32>,
+    prefix: &mut String,
+    prefix_index: &mut usize,
+    read_index: &mut usize,
+) -> Result<Option<String>>
+where
+    M: Model,
+    N: Normalizer,
+    PT: PreTokenizer,
+    PP: PostProcessor,
+    D: Decoder,
+{
+    ids.push(id);
+    let string = tokenizer.decode(ids.as_slice(), skip_special_tokens)?;
+    if string.len() > prefix.len() && !string.ends_with('�') {
+        if !(string.starts_with(&*prefix)) {
+            return Err(Box::new(DecodeStreamError::InvalidPrefix));
         }
+        let new_text = &string[prefix.len()..].to_string();
+        let new_prefix_index = ids.len() - *prefix_index;
+        *ids = ids.drain(*read_index..).collect();
+        *prefix = tokenizer.decode(&ids, skip_special_tokens)?;
+        *read_index = *prefix_index;
+        *prefix_index = new_prefix_index;
+        Ok(Some(new_text.to_string()))
+    } else {
+        Ok(None)
     }
 }