From 9adf24ce539038c380452e5123e2066fa1b0d01a Mon Sep 17 00:00:00 2001
From: Josh Hope-Collins <joshua.hope-collins13@imperial.ac.uk>
Date: Thu, 20 Nov 2025 16:45:28 +0000
Subject: [PATCH 1/2] OptionsManager: do not turn an empty prefix into an
 underscore

---
 petsctools/options.py | 2 +-
 tests/test_options.py | 8 ++++++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/petsctools/options.py b/petsctools/options.py
index 413c2bd..ab7527c 100644
--- a/petsctools/options.py
+++ b/petsctools/options.py
@@ -134,7 +134,7 @@ def _validate_prefix(prefix):
     """Valid prefixes are strings ending with an underscore.
     """
     prefix = str(prefix)
-    if not prefix.endswith("_"):
+    if prefix and not prefix.endswith("_"):
         prefix += "_"
     return prefix
 
diff --git a/tests/test_options.py b/tests/test_options.py
index 67527fc..a4d5130 100644
--- a/tests/test_options.py
+++ b/tests/test_options.py
@@ -84,18 +84,22 @@ def test_options_prefix():
     options = petsctools.OptionsManager({})
     assert options.options_prefix.startswith("petsctools_")
 
+    # User defined empty prefix
+    options = petsctools.OptionsManager({}, options_prefix='')
+    assert options.options_prefix == ""
+
     # User defined default prefix
     options = petsctools.OptionsManager({}, default_prefix="firedrake")
     assert options.options_prefix.startswith("firedrake_")
 
     # Explicit prefix overrides default prefix
     options = petsctools.OptionsManager({}, options_prefix="myobj")
-    assert options.options_prefix.startswith("myobj_")
+    assert options.options_prefix == "myobj_"
 
     # Explicit prefix overrides default prefix
     options = petsctools.OptionsManager({}, options_prefix="myobj",
                                         default_prefix="firedrake")
-    assert options.options_prefix.startswith("myobj_")
+    assert options.options_prefix == "myobj_"
 
 
 @pytest.mark.skipnopetsc4py

From c11327be83d3318d5a1be267e3b8fd85f5749b76 Mon Sep 17 00:00:00 2001
From: Josh Hope-Collins <joshua.hope-collins13@imperial.ac.uk>
Date: Thu, 20 Nov 2025 16:56:41 +0000
Subject: [PATCH 2/2] cannot turn None into a valid prefix

---
 petsctools/options.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/petsctools/options.py b/petsctools/options.py
index ab7527c..1c6b078 100644
--- a/petsctools/options.py
+++ b/petsctools/options.py
@@ -133,6 +133,8 @@ def _warn_unused_options(all_options: Iterable, used_options: Iterable,
 def _validate_prefix(prefix):
     """Valid prefixes are strings ending with an underscore.
     """
+    if prefix is None:
+        raise ValueError("Cannot validate None as a prefix")
     prefix = str(prefix)
     if prefix and not prefix.endswith("_"):
         prefix += "_"