Fix recursion error in Formula.__init__ (#1292)

* fix init and add tests

* add RNs

* fix formatting

release-notes/next-release.md

@@ -15,6 +15,7 @@
 * Correct reading and writing of subsystem in mat.
 * General cleanup of code in mat.py
 * fix the pandas deprecation warning in `find_external_compartment`
+* fix an issue where creating a Formula object would give a recursion error
 
 ## Other
 

src/cobra/core/formula.py

@@ -30,7 +30,7 @@ def __init__(self, formula: Optional[str] = None, **kwargs) -> None:
         formula: str, optional
             An string that will be parsed as a formula.
         """
-        super().__init__(self, formula, **kwargs)
+        super().__init__(id=formula, **kwargs)
         self.formula = formula
         self.elements = {}
         if self.formula is not None:

tests/test_core/test_formula.py

@@ -0,0 +1,28 @@
+"""Test functions of formula.py ."""
+
+import pytest
+
+from cobra.core.formula import Formula
+
+
+def test_formula_init() -> None:
+    """Test initialization."""
+    f = Formula("H2O")
+    assert "Formula H2O" in repr(f)
+    f = Formula("H2O", name="Water")
+    assert f.name == "Water"
+
+
+@pytest.mark.parametrize(
+    ["formula", "weight"], [["H2O", 18.01528], ["C6H12O6", 180.15588], ["NO3", 62.0049]]
+)
+def test_formula_weight(formula, weight) -> None:
+    """Test molecular weight calculation."""
+    assert Formula(formula).weight == pytest.approx(weight)
+
+
+def test_formula_wrong() -> None:
+    """Test incorrect formula elements."""
+    with pytest.warns(UserWarning):
+        w = Formula("NOX").weight
+        assert w is None