From e2b61c003e9be2cc4fe35140d6a607e63677aa6f Mon Sep 17 00:00:00 2001
From: Pascal Bourgault <bourgault.pascal@ouranos.ca>
Date: Tue, 11 Jun 2024 14:32:21 -0400
Subject: [PATCH] Fix custom formatters needing registry

---
 CHANGES                                  |  2 +-
 pint/delegates/formatter/_to_register.py |  2 ++
 pint/delegates/formatter/full.py         | 12 +++++++++---
 pint/testsuite/test_formatting.py        |  2 ++
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/CHANGES b/CHANGES
index e3a1b7ad0..8e59b4803 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,7 +4,7 @@ Pint Changelog
 0.25 (unreleased)
 -----------------
 
-Nothing added yet.
+- Fix custom formatter needing the registry object. (PR #2011)
 
 
 0.24 (2024-06-07)
diff --git a/pint/delegates/formatter/_to_register.py b/pint/delegates/formatter/_to_register.py
index 0f8f46788..697973716 100644
--- a/pint/delegates/formatter/_to_register.py
+++ b/pint/delegates/formatter/_to_register.py
@@ -61,6 +61,8 @@ def wrapper(func: Callable[[PlainUnit, UnitRegistry], str]):
             raise ValueError(f"format {name!r} already exists")  # or warn instead
 
         class NewFormatter(BaseFormatter):
+            spec = name
+
             def format_magnitude(
                 self,
                 magnitude: Magnitude,
diff --git a/pint/delegates/formatter/full.py b/pint/delegates/formatter/full.py
index e6d0eee47..adc6f6c83 100644
--- a/pint/delegates/formatter/full.py
+++ b/pint/delegates/formatter/full.py
@@ -103,11 +103,17 @@ def get_formatter(self, spec: str):
                 return v
 
         try:
-            return REGISTERED_FORMATTERS[spec]
+            orphan_fmt = REGISTERED_FORMATTERS[spec]
         except KeyError:
-            pass
+            return self._formatters["D"]
 
-        return self._formatters["D"]
+        try:
+            fmt = orphan_fmt.__class__(self._registry)
+            spec = getattr(fmt, "spec", spec)
+            self._formatters[spec] = fmt
+            return fmt
+        except Exception:
+            return orphan_fmt
 
     def format_magnitude(
         self, magnitude: Magnitude, mspec: str = "", **babel_kwds: Unpack[BabelKwds]
diff --git a/pint/testsuite/test_formatting.py b/pint/testsuite/test_formatting.py
index e74c09c50..d8f10715b 100644
--- a/pint/testsuite/test_formatting.py
+++ b/pint/testsuite/test_formatting.py
@@ -59,6 +59,8 @@ def test_split_format(format, default, flag, expected):
 def test_register_unit_format(func_registry):
     @fmt.register_unit_format("custom")
     def format_custom(unit, registry, **options):
+        # Ensure the registry is correct..
+        registry.Unit(unit)
         return "<formatted unit>"
 
     quantity = 1.0 * func_registry.meter