Merge pull request #1629 from Yoshanuikabundi/patch-1

Fix cryptic error when re-registering a formatter

2 files changed, 16 insertions, 1 deletions

diff --git a/pint/formatting.py b/pint/formatting.py
index 8d59756..554b381 100644
--- a/pint/formatting.py
+++ b/pint/formatting.py
@@ -157,7 +157,7 @@ def register_unit_format(name):
 
     def wrapper(func):
         if name in _FORMATTERS:
-            raise ValueError(f"format {name:!r} already exists")  # or warn instead
+            raise ValueError(f"format {name!r} already exists")  # or warn instead
         _FORMATTERS[name] = func
 
     return wrapper
diff --git a/pint/testsuite/test_formatting.py b/pint/testsuite/test_formatting.py
index d287948..48e770b 100644
--- a/pint/testsuite/test_formatting.py
+++ b/pint/testsuite/test_formatting.py
@@ -52,3 +52,18 @@ def test_split_format(format, default, flag, expected):
     result = fmt.split_format(format, default, flag)
 
     assert result == expected
+
+
+def test_register_unit_format(func_registry):
+    @fmt.register_unit_format("custom")
+    def format_custom(unit, registry, **options):
+        return "<formatted unit>"
+
+    quantity = 1.0 * func_registry.meter
+    assert f"{quantity:custom}" == "1.0 <formatted unit>"
+
+    with pytest.raises(ValueError, match="format 'custom' already exists"):
+
+        @fmt.register_unit_format("custom")
+        def format_custom_redefined(unit, registry, **options):
+            return "<overwritten>"