Seperate start/end token check for source and target tokenizer (#308)

* Seperate start/end token check for  source/target

* Update changelog

* Dont repeat check if source == target tokenizers

* Add brackets to enforce order of logical operators

Co-authored-by: Pete <petew@allenai.org>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Changed
+
+- Seperate start/end token check in `Seq2SeqDatasetReader` for source and target tokenizers.
+
 ## [v2.7.0](https://github.com/allenai/allennlp-models/releases/tag/v2.7.0) - 2021-09-01
 
 ### Added

allennlp_models/generation/dataset_readers/seq2seq.py

@@ -103,22 +103,14 @@ def __init__(
             or target_add_start_token
             or target_add_end_token
         ):
-            # Check that the tokenizer correctly appends the start and end tokens to
-            # the sequence without splitting them.
-            tokens = self._source_tokenizer.tokenize(start_symbol + " " + end_symbol)
-            err_msg = (
-                f"Bad start or end symbol ('{start_symbol}', '{end_symbol}') "
-                f"for tokenizer {self._source_tokenizer}"
-            )
-            try:
-                start_token, end_token = tokens[0], tokens[-1]
-            except IndexError:
-                raise ValueError(err_msg)
-            if start_token.text != start_symbol or end_token.text != end_symbol:
-                raise ValueError(err_msg)
-
-            self._start_token = start_token
-            self._end_token = end_token
+            if source_add_start_token or source_add_end_token:
+                self._check_start_end_tokens(start_symbol, end_symbol, self._source_tokenizer)
+            if (
+                target_add_start_token or target_add_end_token
+            ) and self._target_tokenizer != self._source_tokenizer:
+                self._check_start_end_tokens(start_symbol, end_symbol, self._target_tokenizer)
+        self._start_token = Token(start_symbol)
+        self._end_token = Token(end_symbol)
 
         self._delimiter = delimiter
         self._source_max_tokens = source_max_tokens
@@ -190,3 +182,22 @@ def apply_token_indexers(self, instance: Instance) -> None:
         instance.fields["source_tokens"]._token_indexers = self._source_token_indexers  # type: ignore
         if "target_tokens" in instance.fields:
             instance.fields["target_tokens"]._token_indexers = self._target_token_indexers  # type: ignore
+
+    def _check_start_end_tokens(
+        self, start_symbol: str, end_symbol: str, tokenizer: Tokenizer
+    ) -> None:
+        """Check that `tokenizer` correctly appends `start_symbol` and `end_symbol` to the
+        sequence without splitting them. Raises a `ValueError` if this is not the case.
+        """
+
+        tokens = tokenizer.tokenize(start_symbol + " " + end_symbol)
+        err_msg = (
+            f"Bad start or end symbol ('{start_symbol}', '{end_symbol}') "
+            f"for tokenizer {self._source_tokenizer}"
+        )
+        try:
+            start_token, end_token = tokens[0], tokens[-1]
+        except IndexError:
+            raise ValueError(err_msg)
+        if start_token.text != start_symbol or end_token.text != end_symbol:
+            raise ValueError(err_msg)