Add overwrite_existing flag (#1119)

* Add 'overwrite_existing' flag to allow graph rewrites
and include appropriate testing

* Encapsulate test rewriters and use user-facing API

---------

Co-authored-by: Ricardo Vieira <ricardo.vieira1994@gmail.com>

Author: AdvH039

pytensor/graph/rewriting/db.py

@@ -35,6 +35,7 @@ def register(
         rewriter: Union["RewriteDatabase", RewritesType],
         *tags: str,
         use_db_name_as_tag=True,
+        overwrite_existing=False,
     ):
         """Register a new rewriter to the database.
 
@@ -56,7 +57,8 @@ def register(
             ``local_remove_all_assert``. Setting `use_db_name_as_tag` to
             ``False`` removes that behavior. This means that only the rewrite's name
             and/or its tags will enable it.
-
+        overwrite_existing:
+            Overwrite the existing rewriter with a new one having the same name
         """
         if not isinstance(
             rewriter,
@@ -66,22 +68,27 @@ def register(
         ):
             raise TypeError(f"{rewriter} is not a valid rewrite type.")
 
-        if name in self.__db__:
-            raise ValueError(f"The tag '{name}' is already present in the database.")
-
         if use_db_name_as_tag:
             if self.name is not None:
                 tags = (*tags, self.name)
 
         rewriter.name = name
-        # This restriction is there because in many place we suppose that
-        # something in the RewriteDatabase is there only once.
-        if rewriter.name in self.__db__:
-            raise ValueError(
-                f"Tried to register {rewriter.name} again under the new name {name}. "
-                "The same rewrite cannot be registered multiple times in"
-                " an `RewriteDatabase`; use `ProxyDB` instead."
-            )
+
+        # if tag collides with name
+        if name in self.__db__ and name not in self._names:
+            raise ValueError(f"The tag '{name}' is already present in the database.")
+
+        if name in self.__db__ or rewriter.name in self.__db__:
+            if overwrite_existing:
+                self.remove_tags(name, *tags)
+                old_rewriter = self.__db__[name].pop()
+                self._names.remove(name)
+                self.__db__[old_rewriter.__class__.__name__].remove(old_rewriter)
+            else:
+                raise ValueError(
+                    f"The tag '{name}' is already present in the database."
+                )
+
         self.__db__[name] = OrderedSet([rewriter])
         self._names.add(name)
         self.__db__[rewriter.__class__.__name__].add(rewriter)

tests/graph/rewriting/test_db.py

@@ -1,5 +1,6 @@
 import pytest
 
+from pytensor.graph.fg import FunctionGraph
 from pytensor.graph.rewriting.basic import GraphRewriter, SequentialGraphRewriter
 from pytensor.graph.rewriting.db import (
     EquilibriumDB,
@@ -17,6 +18,13 @@ def apply(self, fgraph):
         pass
 
 
+class NewTestRewriter(GraphRewriter):
+    name = "bleh"
+
+    def apply(self, fgraph):
+        pass
+
+
 class TestDB:
     def test_register(self):
         db = RewriteDatabase()
@@ -31,7 +39,7 @@ def test_register(self):
         assert "c" in db
 
         with pytest.raises(ValueError, match=r"The tag.*"):
-            db.register("c", TestRewriter())  # name taken
+            db.register("c", NewTestRewriter())  # name taken
 
         with pytest.raises(ValueError, match=r"The tag.*"):
             db.register("z", TestRewriter())  # name collides with tag
@@ -42,6 +50,40 @@ def test_register(self):
         with pytest.raises(TypeError, match=r".* is not a valid.*"):
             db.register("d", 1)
 
+    def test_overwrite_existing(self):
+        class TestOverwrite1(GraphRewriter):
+            def apply(self, fgraph):
+                fgraph.counter[0] += 1
+
+        class TestOverwrite2(GraphRewriter):
+            def apply(self, fgraph):
+                fgraph.counter[1] += 1
+
+        db = SequenceDB()
+        fg = FunctionGraph([], [])
+        fg.counter = [0, 0]
+
+        db.register("a", TestRewriter(), "basic")
+        rewriter = db.query("+basic")
+        rewriter.rewrite(fg)
+        assert fg.counter == [0, 0]
+
+        with pytest.raises(ValueError, match=r"The tag.*"):
+            db.register("a", TestOverwrite1(), "basic")
+        rewriter = db.query("+basic")
+        rewriter.rewrite(fg)
+        assert fg.counter == [0, 0]
+
+        db.register("a", TestOverwrite1(), "basic", overwrite_existing=True)
+        rewriter = db.query("+basic")
+        rewriter.rewrite(fg)
+        assert fg.counter == [1, 0]
+
+        db.register("a", TestOverwrite2(), "basic", overwrite_existing=True)
+        rewriter = db.query("+basic")
+        rewriter.rewrite(fg)
+        assert fg.counter == [1, 1]
+
     def test_EquilibriumDB(self):
         eq_db = EquilibriumDB()