Interpret True, False, and None as python literals

formulae/expr.py

@@ -19,7 +19,7 @@ def __repr__(self):  # pragma: no cover
 
     def __str__(self):  # pragma: no cover
         right = "  ".join(str(self.value).splitlines(True))
-        return f"Assign(name={self.name}, value={right}\n)"
+        return f"Assign(name={self.name}, value={right})"
 
     def accept(self, visitor):
         return visitor.visitAssignExpr(self)

formulae/parser.py

@@ -195,6 +195,8 @@ def primary(self):  # pylint: disable=too-many-return-statements
             return Literal(token.literal, lexeme=token.lexeme)
         elif self.match("BQNAME"):
             return QuotedName(self.previous())
+        elif self.match("PYTHON_LITERAL"):
+            return Literal(self.previous().literal)
         elif self.match("LEFT_PAREN"):
             expr = self.expression()
             self.consume("RIGHT_PAREN", "Expect ')' after expression.")

formulae/scanner.py

@@ -188,11 +188,17 @@ def number(self):
 
         self.add_token("NUMBER", token)
 
+    # pylint: disable=eval-used
     def identifier(self):
         # 'mod.function' is also an identifier
         while self.peek().isalnum() or self.peek() in [".", "_"]:
             self.advance()
-        self.add_token("IDENTIFIER")
+
+        token = self.code[self.start : self.current]
+        if token in ("True", "False", "None"):  # These are actually literals, not variable names
+            self.add_token("PYTHON_LITERAL", eval(token))  # Pass literals, not strings
+        else:
+            self.add_token("IDENTIFIER")
 
     def char(self):
         while self.peek() not in ["'", '"'] and not self.at_end():

formulae/terms/call_resolver.py

@@ -93,8 +93,6 @@ class LazyVariable:
         The name of the variable it represents.
     """
 
-    BUILTINS = {"True": True, "False": False, "None": None}
-
     def __init__(self, name):
         self.name = name
 
@@ -129,16 +127,13 @@ def eval(self, data_mask, env):
         result:
             The value represented by this name in either the data mask or the environment.
         """
-        if self.name in self.BUILTINS:
-            result = self.BUILTINS[self.name]
-        else:
+        try:
+            result = data_mask[self.name]
+        except KeyError:
             try:
-                result = data_mask[self.name]
-            except KeyError:
-                try:
-                    result = env.namespace[self.name]
-                except KeyError as e:
-                    raise e
+                result = env.namespace[self.name]
+            except KeyError as e:
+                raise e
         return result
 
 

tests/test_parser.py

@@ -264,3 +264,20 @@ def test_unclosed_function_call():
     data = pd.DataFrame({"y": [1, 2], "x": [1, 2]})
     with pytest.raises(ParseError, match="after arguments"):
         design_matrices("y ~ f(x", data)
+
+
+def test_parse_python_literals():
+    result = Parser(Scanner("f(x, True, y=False, z=None)").scan(False)).parse()
+
+    assert isinstance(result.args[0], Variable)
+
+    assert isinstance(result.args[1], Literal)
+    assert result.args[1].value is True
+
+    assert isinstance(result.args[2], Assign)
+    assert isinstance(result.args[2].value, Literal)
+    assert result.args[2].value.value is False
+
+    assert isinstance(result.args[3], Assign)
+    assert isinstance(result.args[3].value, Literal)
+    assert result.args[3].value.value is None