Update error handling and docstring

spacy/errors.py

@@ -475,6 +475,17 @@ class Errors:
             "issue tracker: http://github.com/explosion/spaCy/issues")
 
     # TODO: fix numbering after merging develop into master
+    E886 = ("Can't replace {name} -> {tok2vec} listeners: path '{path}' not "
+            "found in config for component '{name}'.")
+    E887 = ("Can't replace {name} -> {tok2vec} listeners: the paths to replace "
+            "({paths}) don't match the available listeners in the model ({n_listeners}).")
+    E888 = ("Can't replace listeners for '{name}' ({pipe}): invalid upstream "
+            "component that doesn't seem to support listeners. Expected Tok2Vec "
+            "or Transformer component. If you didn't call nlp.replace_listeners "
+            "manually, this is likely a bug in spaCy.")
+    E889 = ("Can't replace listeners of component '{name}' because it's not "
+            "in the pipeline. Available components: {opts}. If you didn't call "
+            "nlp.replace_listeners manually, this is likely a bug in spaCy.")
     E890 = ("Cannot add the alias '{alias}' to the Knowledge base. "
             "Each alias should be a meaningful string.")
     E891 = ("Alias '{alias}' could not be added to the Knowledge base. "

spacy/language.py

@@ -676,11 +676,36 @@ class Language:
         tok2vec_name: str,
         pipe_name: str,
         listeners: Iterable[str] = SimpleFrozenList(),
-    ):
+    ) -> None:
+        """Find listener layers (connecting to a token-to-vector embedding
+        component) of a given pipeline component model and replace
+        them with a standalone copy of the token-to-vector layer. This can be
+        useful when training a pipeline with components sourced from an existing
+        pipeline: if multiple components (e.g. tagger, parser, NER) listen to
+        the same tok2vec component, but some of them are frozen and not updated,
+        their performance may degrade significally as the tok2vec component is
+        updated with new data. To prevent this, listeners can be replaced with
+        a standalone tok2vec layer that is owned by the component and doesn't
+        change if the component isn't updated.
+
+        tok2vec_name (str): Name of the token-to-vector component, typically
+            "tok2vec" or "transformer".
+        pipe_name (str): Name of pipeline component to replace listeners for.
+        listeners (Iterable[str]): The paths to the listeners, relative to the
+            component config, e.g. ["model.tok2vec"]. Typically, implementations
+            will only connect to one tok2vec component, [model.tok2vec], but in
+            theory, custom models can use multiple listeners. The value here can
+            either be an empty list to not replace any listeners, or a complete
+            (!) list of the paths to all listener layers used by the model.
+
+        DOCS: https://nightly.spacy.io/api/language#replace_listeners
+        """
         if tok2vec_name not in self.pipe_names:
-            raise ValueError  # TODO:
+            err = Errors.E889.format(name=tok2vec_name, opts=", ".join(self.pipe_names))
+            raise ValueError(err)
         if pipe_name not in self.pipe_names:
-            raise ValueError  # TODO:
+            err = Errors.E889.format(name=pipe_name, opts=", ".join(self.pipe_names))
+            raise ValueError(err)
         tok2vec = self.get_pipe(tok2vec_name)
         tok2vec_cfg = self.get_pipe_config(tok2vec_name)
         if (
@@ -688,7 +713,7 @@ class Language:
             or not hasattr(tok2vec, "listener_map")
             or "model" not in tok2vec_cfg
         ):
-            raise ValueError  # TODO: likely bug in spaCy if this happens
+            raise ValueError(Errors.E888.format(name=tok2vec_name, pipe=type(tok2vec)))
         pipe_listeners = tok2vec.listener_map.get(pipe_name, [])
         pipe_cfg = self._pipe_configs[pipe_name]
         if listeners:
@@ -697,7 +722,13 @@ class Language:
                 # The number of listeners defined in the component model doesn't
                 # match the listeners to replace, so we won't be able to update
                 # the nodes and generate a matching config
-                raise ValueError(f"{listeners}, {pipe_listeners}")  # TODO:
+                err = Errors.E887.format(
+                    name=pipe_name,
+                    tok2vec=tok2vec_name,
+                    paths=listeners,
+                    n_listeners=len(pipe_listeners),
+                )
+                raise ValueError(err)
             pipe = self.get_pipe(pipe_name)
             # Go over the listener layers and replace them
             for listener in pipe_listeners:
@@ -709,7 +740,10 @@ class Language:
                 try:
                     util.dot_to_object(pipe_cfg, listener_path)
                 except KeyError:
-                    raise ValueError  # TODO:
+                    err = Errors.E886.format(
+                        name=pipe_name, tok2vec=tok2vec_name, path=listener_path
+                    )
+                    raise ValueError(err)
                 util.set_dot_to_object(pipe_cfg, listener_path, tok2vec_cfg["model"])
 
     def create_pipe_from_source(