Do not count literal factors toward degree (fixing sorting of e.g. gl…

…obal intercept).

formulaic/formula.py

@@ -177,7 +177,7 @@ def _prepare_item(self, key: str, item: FormulaSpec) -> Union[List[Term], Formul
         # Order terms appropriately
         orderer = None
         if self._ordering is OrderingMethod.DEGREE:
-            orderer = lambda terms: sorted(terms, key=lambda term: len(term.factors))
+            orderer = lambda terms: sorted(terms, key=lambda term: term.degree)
         elif self._ordering is OrderingMethod.SORT:
             orderer = lambda terms: sorted(
                 [Term(factors=sorted(term.factors)) for term in terms]

formulaic/parser/types/term.py

@@ -14,14 +14,23 @@ class Term:
     a formula is made up of a sum of terms.
 
     Attributes:
-        factors: The set of factors to be multipled to form the term.
+        factors: The set of factors to be multiplied to form the term.
     """
 
     def __init__(self, factors: Iterable["Factor"]):
         self.factors = tuple(dict.fromkeys(factors))
         self._factor_key = tuple(factor.expr for factor in sorted(self.factors))
         self._hash = hash(":".join(self._factor_key))
 
+    @property
+    def degree(self) -> int:
+        """
+        The degree of the `Term`. Literal factors do not contribute to the degree.
+        """
+        return len(
+            tuple(f for f in self.factors if f.eval_method != f.eval_method.LITERAL)
+        )
+
     # Transforms and comparisons
 
     def __mul__(self, other: Any) -> Term:
@@ -41,9 +50,9 @@ def __eq__(self, other: Any) -> bool:
 
     def __lt__(self, other: Any) -> bool:
         if isinstance(other, Term):
-            if len(self.factors) == len(other.factors):
+            if self.degree == other.degree:
                 return sorted(self.factors) < sorted(other.factors)
-            if len(self.factors) < len(other.factors):
+            if self.degree < other.degree:
                 return True
             return False
         return NotImplemented

tests/parser/types/test_term.py

@@ -42,3 +42,9 @@ def test_sort(self, term1, term2, term3):
 
     def test_repr(self, term1):
         assert repr(term1) == "c:b"
+
+    def test_degree(self, term1, term3):
+        assert term1.degree == 2
+        assert term3.degree == 3
+        assert Term([Factor("1", eval_method="literal")]).degree == 0
+        assert Term([Factor("1", eval_method="literal"), Factor("x")]).degree == 1

tests/test_formula.py

@@ -31,10 +31,10 @@ def data(self):
     def test_constructor(self):
         assert [str(t) for t in Formula(["a", "b", "c"])] == ["a", "b", "c"]
         assert [str(t) for t in Formula(["a", "c", "b", "1"])] == [
+            "1",
             "a",
             "c",
             "b",
-            "1",
         ]
 
         f = Formula((["a", "b"], ["c", "d"]))