implement warning in __init_subclass__ instead

spacy/errors.py

@@ -85,9 +85,9 @@ class Warnings:
             "attribute or operator.")
 
     # TODO: fix numbering after merging develop into master
-    W089 = ("The 'begin_training' method has been renamed to 'initialize', "
-            "for calls to 'nlp' as well as for the individual pipeline "
-            "components.")
+    W088 = ("This component implements a 'begin_training' method, "
+            "which should probably be renamed to 'initialize'.")
+    W089 = ("The nlp.begin_training method has been renamed to nlp.initialize.")
     W090 = ("Could not locate any {format} files in path '{path}'.")
     W091 = ("Could not clean/remove the temp directory at {dir}: {msg}.")
     W092 = ("Ignoring annotations for sentence starts, as dependency heads are set.")

spacy/language.py

@@ -1207,11 +1207,7 @@ class Language:
             )
             self.tokenizer.initialize(get_examples, nlp=self, **tok_settings)
         for name, proc in self.pipeline:
-            # backwards compatibility for older components
-            if hasattr(proc, "begin_training"):
-                warnings.warn(Warnings.W089, DeprecationWarning)
-                proc.begin_training(get_examples, pipeline=self.pipeline, sgd=self._optimizer)
-            elif hasattr(proc, "initialize"):
+            if hasattr(proc, "initialize"):
                 p_settings = I["components"].get(name, {})
                 p_settings = validate_init_settings(
                     proc.initialize, p_settings, section="components", name=name

spacy/pipeline/pipe.pyx

@@ -1,4 +1,5 @@
 # cython: infer_types=True, profile=True
+import warnings
 from typing import Optional, Tuple
 import srsly
 from thinc.api import set_dropout_rate, Model
@@ -6,7 +7,7 @@ from thinc.api import set_dropout_rate, Model
 from ..tokens.doc cimport Doc
 
 from ..training import validate_examples
-from ..errors import Errors
+from ..errors import Errors, Warnings
 from .. import util
 
 
@@ -33,6 +34,13 @@ cdef class Pipe:
         self.name = name
         self.cfg = dict(cfg)
 
+    @classmethod
+    def __init_subclass__(cls, **kwargs):
+        """Raise a warning if an inheriting class implements 'begin_training'
+         (from v2) instead of the new 'initialize' method (from v3)"""
+        if hasattr(cls, "begin_training"):
+            warnings.warn(Warnings.W088)
+
     @property
     def labels(self) -> Optional[Tuple[str]]:
         return []