Prototype of deregister and with db.register_functions, refs #589

sqlite_utils/db.py

@@ -413,6 +413,7 @@ class Database:
         deterministic: bool = False,
         replace: bool = False,
         name: Optional[str] = None,
+        deregister: bool = False,
     ):
         """
         ``fn`` will be made available as a function within SQL, with the same name and number
@@ -434,28 +435,42 @@ class Database:
         :param deterministic: set ``True`` for functions that always returns the same output for a given input
         :param replace: set ``True`` to replace an existing function with the same name - otherwise throw an error
         :param name: name of the SQLite function - if not specified, the Python function name will be used
+        :param deregister: set ``True`` to deregister the function
         """
 
         def register(fn):
             fn_name = name or fn.__name__
+            fn_to_register = fn
             arity = len(inspect.signature(fn).parameters)
-            if not replace and (fn_name, arity) in self._registered_functions:
+            if (
+                not replace
+                and not deregister
+                and (fn_name, arity) in self._registered_functions
+            ):
                 return fn
             kwargs = {}
+            if deregister:
+                fn_to_register = None
             registered = False
             if deterministic:
                 # Try this, but fall back if sqlite3.NotSupportedError
                 try:
                     self.conn.create_function(
-                        fn_name, arity, fn, **dict(kwargs, deterministic=True)
+                        fn_name,
+                        arity,
+                        fn_to_register,
+                        **dict(kwargs, deterministic=True),
                     )
                     registered = True
                 except (sqlite3.NotSupportedError, TypeError):
                     # TypeError is Python 3.7 "function takes at most 3 arguments"
                     pass
             if not registered:
-                self.conn.create_function(fn_name, arity, fn, **kwargs)
+                self.conn.create_function(fn_name, arity, fn_to_register, **kwargs)
-            self._registered_functions.add((fn_name, arity))
+            if deregister:
+                self._registered_functions.remove((fn_name, arity))
+            else:
+                self._registered_functions.add((fn_name, arity))
             return fn
 
         if fn is None:
@@ -463,6 +478,18 @@ class Database:
         else:
             register(fn)
 
+    @contextlib.contextmanager
+    def register_functions(self, *functions: Callable):
+        "Register functions for the duration of a with block, then unregister them"
+        for func in functions:
+            self.register_function(func)
+        try:
+            yield
+        finally:
+            # Unregister the functions
+            for func in functions:
+                self.register_function(func, deregister=True)
+
     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, replace=True)