fix default format dimensionless (#2012)

CHANGES

@@ -6,6 +6,7 @@ Pint Changelog
 
 - Fix custom formatter needing the registry object. (PR #2011)
 - Support python 3.9 following difficulties installing with NumPy 2. (PR #2019)
+- Fix default formatting of dimensionless unit issue. (PR #2012)
 
 0.24 (2024-06-07)
 -----------------

pint/delegates/formatter/_compound_unit_helpers.py

@@ -256,6 +256,12 @@ def prepare_compount_unit(
 
     # out: unit_name, unit_exponent
 
+    if len(out) == 0:
+        if "~" in spec:
+            return ([], [])
+        else:
+            return ([("dimensionless", 1)], [])
+
     if "~" in spec:
         if registry is None:
             raise ValueError(

pint/delegates/formatter/_format_helpers.py

@@ -131,6 +131,8 @@ def join_mu(joint_fstring: str, mstr: str, ustr: str) -> str:
 
     This avoids that `3 and `1 / m` becomes `3 1 / m`
     """
+    if ustr == "":
+        return mstr
     if ustr.startswith("1 / "):
         return joint_fstring.format(mstr, ustr[2:])
     return joint_fstring.format(mstr, ustr)

pint/testsuite/test_issues.py

@@ -1255,3 +1255,21 @@ def test_issue1949(registry_empty):
 def test_issue1772(given, expected):
     ureg = UnitRegistry(non_int_type=decimal.Decimal)
     assert f"{ureg(given):Lx}" == expected
+
+
+def test_issue2007():
+    ureg = UnitRegistry()
+    q = ureg.Quantity(1, "")
+    assert f"{q:P}" == "1 dimensionless"
+    assert f"{q:C}" == "1 dimensionless"
+    assert f"{q:D}" == "1 dimensionless"
+    assert f"{q:H}" == "1 dimensionless"
+
+    assert f"{q:L}" == "1\\ \\mathrm{dimensionless}"
+    #  L returned '1\\ dimensionless' in pint 0.23
+
+    assert f"{q:Lx}" == "\\SI[]{1}{}"
+    assert f"{q:~P}" == "1"
+    assert f"{q:~C}" == "1"
+    assert f"{q:~D}" == "1"
+    assert f"{q:~H}" == "1"