Raise if on_match is not callable or None

spacy/errors.py

@@ -476,6 +476,8 @@ class Errors(object):
     E168 = ("Unknown field: {field}")
     E169 = ("Can't find module: {module}")
     E170 = ("Cannot apply transition {name}: invalid for the current state.")
+    E171 = ("Matcher.add received invalid on_match callback argument: expected "
+            "callable or None, but got: {arg_type}")
 
 
 @add_codes

spacy/matcher/matcher.pyx

@@ -103,6 +103,8 @@ cdef class Matcher:
         *patterns (list): List of token descriptions.
         """
         errors = {}
+        if on_match is not None and not hasattr(on_match, "__call__"):
+            raise ValueError(Errors.E171.format(arg_type=type(on_match)))
         for i, pattern in enumerate(patterns):
             if len(pattern) == 0:
                 raise ValueError(Errors.E012.format(key=key))

spacy/tests/matcher/test_matcher_api.py

@@ -410,3 +410,11 @@ def test_matcher_schema_token_attributes(en_vocab, pattern, text):
     assert len(matcher) == 1
     matches = matcher(doc)
     assert len(matches) == 1
+
+
+def test_matcher_valid_callback(en_vocab):
+    """Test that on_match can only be None or callable."""
+    matcher = Matcher(en_vocab)
+    with pytest.raises(ValueError):
+        matcher.add("TEST", [], [{"TEXT": "test"}])
+    matcher(Doc(en_vocab, words=["test"]))