Fix parsing of string-based dimensions in OpenFOAM files (closes #332)

foamlib/_files/_files.py

@@ -31,6 +31,7 @@
     MutableSubDict,
     StandaloneData,
     StandaloneDataLike,
+    StringDimensionSet,
     SubDict,
     SubDictLike,
 )
@@ -118,6 +119,7 @@ class FoamFile(
 
     Dimensioned = Dimensioned
     DimensionSet = DimensionSet
+    StringDimensionSet = StringDimensionSet
 
     class SubDict(
         MutableMapping[str, Union[Data, MutableSubDict]],

foamlib/_files/_parsing.py

@@ -339,9 +339,20 @@ def _keyword_entry_of(
     | Keyword("false", _IDENTBODYCHARS)
     | Keyword("off", _IDENTBODYCHARS)
 ).set_parse_action(lambda: False)
+
+# Parser for string-based dimension units (e.g., Pa, mm^2, s^-2)
+_UNIT_CHARS = printables.replace("[", "").replace("]", "").replace(";", "")
+_UNIT_STRING = (
+    Word(_UNIT_CHARS)[1, ...].set_parse_action(lambda tks: " ".join(tks))
+)
+
 _DIMENSIONS = (
-    Literal("[").suppress() + common.number[0, 7] + Literal("]").suppress()
-).set_parse_action(lambda tks: DimensionSet(*tks))
+    # Numeric dimensions: [1 -1 -2 0 0 0 0]
+    (Literal("[").suppress() + common.number[0, 7] + Literal("]").suppress()).set_parse_action(lambda tks: DimensionSet(*tks))
+    |
+    # String-based units: [ Pa ] or [Pa mm^2 s^-2]
+    (Literal("[").suppress() + _UNIT_STRING + Literal("]").suppress()).set_parse_action(lambda tks: tks[0])
+)
 _TENSOR = common.ieee_float | (
     Literal("(").suppress()
     + Group(common.ieee_float[3] | common.ieee_float[6] | common.ieee_float[9])

foamlib/_files/_types.py

@@ -81,11 +81,29 @@
 ]
 
 
+class StringDimensionSet:
+    """Represents string-based dimensions like 'Pa', 'mm^2 s^-2', etc."""
+
+    def __init__(self, units: str) -> None:
+        self.units = units
+
+    def __repr__(self) -> str:
+        return f"StringDimensionSet('{self.units}')"
+
+    def __eq__(self, other: object) -> bool:
+        if isinstance(other, StringDimensionSet):
+            return self.units == other.units
+        return False
+
+    def __str__(self) -> str:
+        return self.units
+
+
 class Dimensioned:
     def __init__(
         self,
         value: TensorLike,
-        dimensions: DimensionSet | Sequence[float],
+        dimensions: DimensionSet | StringDimensionSet | Sequence[float] | str,
         name: str | None = None,
     ) -> None:
         if is_sequence(value):
@@ -94,10 +112,12 @@
             assert isinstance(value, (int, float, np.ndarray))
             self.value = float(value)
 
-        if not isinstance(dimensions, DimensionSet):
-            self.dimensions = DimensionSet(*dimensions)
-        else:
+        if isinstance(dimensions, str):
+            self.dimensions = StringDimensionSet(dimensions)
+        elif isinstance(dimensions, (DimensionSet, StringDimensionSet)):
             self.dimensions = dimensions
+        else:
+            self.dimensions = DimensionSet(*dimensions)
 
         self.name = name
 

tests/test_files/test_parsing/test_loads.py

@@ -100,3 +100,40 @@
     assert np.array_equal(
         faces[1][faces[0][1] : faces[0][2]], [3516631, 3516634, 3516633, 3516632]
     )
+
+
+def test_string_dimensions() -> None:
+    """Test parsing of string-based dimensions like [ Pa ], addressing issue #332."""
+    # Test simple string dimensions
+    assert FoamFile.loads("[ Pa ]") == "Pa"
+    assert FoamFile.loads("[Pa]") == "Pa"
+
+    # Test complex string dimensions
+    assert FoamFile.loads("[Pa mm^2 s^-2]") == "Pa mm^2 s^-2"
+    assert FoamFile.loads("[ kg m s^-2 ]") == "kg m s^-2"
+
+    # Test string dimensions in dimensioned values
+    dimensioned = FoamFile.loads("g [ Pa ] 9.81")
+    assert isinstance(dimensioned, FoamFile.Dimensioned)
+    assert isinstance(dimensioned.dimensions, FoamFile.StringDimensionSet)
+    assert dimensioned.dimensions.units == "Pa"
+    assert dimensioned.value == 9.81
+    assert dimensioned.name == "g"
+
+    # Test that numeric dimensions still work as before
+    assert FoamFile.loads("[1 1 -2 0 0 0 0]") == FoamFile.DimensionSet(
+        mass=1, length=1, time=-2
+    )
+
+    # Test in a dictionary context (the original issue scenario)
+    result = FoamFile.loads("""
+    functions
+    {
+        test
+        {
+            type        exprField;
+            dimensions  [ Pa ];
+        }
+    }
+    """)
+    assert result["functions"]["test"]["dimensions"] == "Pa"