Support repeated calls to Table.convert()

* Test repeated calls to Table.convert()
* Register Table.convert() functions under their own `lambda_hash` name
* Raise exception on registering identical function names

Refs #525

sqlite_utils/db.py

@@ -219,6 +219,11 @@ class AlterError(Exception):
     pass
 
 
+class FunctionAlreadyRegistered(Exception):
+    "A function with this name and arity was already registered"
+    pass
+
+
 class NoObviousTable(Exception):
     "Could not tell which table this operation refers to"
     pass
@@ -409,7 +414,7 @@ class Database:
             fn_name = name or fn.__name__
             arity = len(inspect.signature(fn).parameters)
             if not replace and (fn_name, arity) in self._registered_functions:
-                return fn
+                raise FunctionAlreadyRegistered(f'Already registered function with name "{fn_name}" and identical arity')
             kwargs = {}
             registered = False
             if deterministic:
@@ -434,7 +439,7 @@ class Database:
 
     def register_fts4_bm25(self):
         "Register the ``rank_bm25(match_info)`` function used for calculating relevance with SQLite FTS4."
-        self.register_function(rank_bm25, deterministic=True)
+        self.register_function(rank_bm25, deterministic=True, replace=True)
 
     def attach(self, alias: str, filepath: Union[str, pathlib.Path]):
         """
@@ -2687,13 +2692,16 @@ class Table(Queryable):
                     return v
                 return jsonify_if_needed(fn(v))
 
-            self.db.register_function(convert_value)
+            fn_name = fn.__name__
+            if fn_name == '<lambda>':
+                fn_name = f'lambda_{hash(fn)}'
+            self.db.register_function(convert_value, name=fn_name)
             sql = "update [{table}] set {sets}{where};".format(
                 table=self.name,
                 sets=", ".join(
                     [
-                        "[{output_column}] = convert_value([{column}])".format(
-                            output_column=output or column, column=column
+                        "[{output_column}] = {fn_name}([{column}])".format(
+                            output_column=output or column, column=column, fn_name=fn_name
                         )
                         for column in columns
                     ]