Add support for the . operator.

docsite/docs/guides/grammar.md

@@ -26,10 +26,12 @@ unless otherwise indicated.
 | `{...}`[^1] | 1 | Quotes python operations, as a more convenient way to do Python operations than `I(...)`, e.g. `` {`my|col`**2} `` | ✓ | ✗ | ✗ |
 | `<function>(...)`[^1] | 1 | Python transform on column, e.g. `my_func(x)` which is equivalent to `{my_func(x)}` | ✓[^2] | ✓ | ✗ |
 |-----|
-| `(...)` | 1 | Groups operations, overriding normal precedence rules. All operations with the parentheses are performed before the result of these operations is permitted to be operated upon by its peers. | ✓ | ✓ | ✓ |
+| `(...)` | 1 | Groups operations, overriding normal precedence rules. All operations with the parentheses are performed before the result of these operations is permitted to be operated upon by its peers. | ✓ | ✗ | ✓ |
 |-----|
-| ** | 2 | Includes all n-th order interactions of the terms in the left operand, where n is the (integral) value of the right operand, e.g. `(a+b+c)**2` is equivalent to `a + b + c + a:b + a:c + b:c`. | ✓ | ✓ | ✓ |
-| ^ | 2 | Alias for `**`. | ✓ | ✗[^3] | ✓ |
+| `.`[^9] | 0 | Stands in as a wild-card for the sum of variables in the data not used on the left-hand side of a formula. | ✓ | ✗ | ✓ |
+|-----|
+| `**` | 2 | Includes all n-th order interactions of the terms in the left operand, where n is the (integral) value of the right operand, e.g. `(a+b+c)**2` is equivalent to `a + b + c + a:b + a:c + b:c`. | ✓ | ✓ | ✓ |
+| `^` | 2 | Alias for `**`. | ✓ | ✗[^3] | ✓ |
 |-----|
 | `:` | 2 | Adds a new term that corresponds to the interaction of its operands (i.e. their elementwise product). | ✓[^4] | ✓ | ✓ |
 |-----|
@@ -123,4 +125,5 @@ and conventions of which you should be aware.
 [^5]: This somewhat confusing operator is useful when you want to include hierachical features in your data, and where certain interaction terms do not make sense (particularly in ANOVA contexts). For example, if `a` represents countries, and `b` represents cities, then the full product of terms from `a * b === a + b + a:b` does not make sense, because any value of `b` is guaranteed to coincide with a value in `a`, and does not independently add value. Thus, the operation `a / b === a + a:b` results in more sensible dataset. As a result, the `/` operator is right-distributive, since if `b` and `c` were both nested in `a`, you would want `a/(b+c) === a + a:b + a:c`. Likewise, the operator is not left-distributive, since if `c` is nested under both `a` and `b` separately, then you want `(a + b)/c === a + b + a:b:c`. Lastly, if `c` is nested in `b`, and `b` is nested in `a`, then you would want `a/b/c === a + a:(b/c) === a + a:b + a:b:c`.
 [^6]: Implemented by an R package called [Formula](https://cran.r-project.org/web/packages/Formula/index.html) that extends the default formula syntax.
 [^7]: Patsy uses the `rescale` keyword rather than `scale`, but provides the same functionality.
-[^8]: For increased compatibility with patsy, we use patsy's signature for `standardize`.
+[^8]: For increased compatibility with patsy, we use patsy's signature for `standardize`.
+[^9]: Requires additional context to be passed in when directly using the `Formula` constructor. e.g. `Formula("y ~ .", context={"__formulaic_variables_available__": ["x", "y", "z"]})`; or you can use `model_matrix`, `ModelSpec.get_model_matrix()`, or `FormulaMaterializer.get_model_matrix()` without further specification.

formulaic/parser/algos/sanitize_tokens.py

@@ -15,6 +15,8 @@ def sanitize_tokens(tokens: Iterable[Token]) -> Iterable[Token]:
         - possible more in the future
     """
     for token in tokens:
+        if token.token == ".":  # noqa: S105
+            token.kind = Token.Kind.OPERATOR
         if token.kind is Token.Kind.PYTHON:
             token.token = sanitize_python_code(token.token)
         yield token

formulaic/parser/parser.py

@@ -11,6 +11,7 @@
     Generator,
     Iterable,
     List,
+    Mapping,
     MutableMapping,
     Set,
     Tuple,
@@ -20,11 +21,13 @@
 
 from typing_extensions import Self
 
-from formulaic.parser.types.ast_node import ASTNode
+from formulaic.errors import FormulaParsingError
+from formulaic.utils.layered_mapping import LayeredMapping
 
 from .algos.sanitize_tokens import sanitize_tokens
 from .algos.tokenize import tokenize
 from .types import (
+    ASTNode,
     Factor,
     FormulaParser,
     Operator,
@@ -149,6 +152,7 @@ def get_tokens_from_formula(
                     [token_one],
                     kind=Token.Kind.OPERATOR,
                     join_operator="+",
+                    no_join_for_operators={"+", "-"},
                 )
             )
 
@@ -191,9 +195,16 @@ def find_rhs_index(tokens: List[Token]) -> int:
                     [token_one],
                     kind=Token.Kind.OPERATOR,
                     join_operator="+",
+                    no_join_for_operators={"+", "-"},
                 ),
             ]
 
+            context["__formulaic_variables_used_lhs__"] = [
+                variable
+                for token in tokens[:rhs_index]
+                for variable in token.required_variables
+            ]
+
         # Collapse inserted "+" and "-" operators to prevent unary issues.
         tokens = merge_operator_tokens(tokens, symbols={"+", "-"})
 
@@ -356,6 +367,37 @@ def get_terms(terms: OrderedSet[Term]) -> List[Term]:
 
             return Structured(get_terms(lhs), deps=(Structured(lhs=lhs, rhs=rhs),))
 
+        def insert_unused_terms(context: Mapping[str, Any]) -> OrderedSet[Term]:
+            available_variables: OrderedSet[str]
+            used_variables: Set[str] = set(context["__formulaic_variables_used_lhs__"])
+
+            # Populate `available_variables` or raise.
+            if "__formulaic_variables_available__" in context:
+                available_variables = OrderedSet(
+                    context["__formulaic_variables_available__"]
+                )
+            elif isinstance(context, LayeredMapping) and "data" in context.named_layers:
+                available_variables = OrderedSet(context.named_layers["data"])
+            else:
+                raise FormulaParsingError(
+                    "The `.` operator requires additional context about which "
+                    "variables are available to use. This can be provided by "
+                    "passing in a value for `__formulaic_variables_available__`"
+                    "in the context while parsing the formula; by passing the "
+                    "formula to the materializer's `.get_model_matrix()` method; "
+                    "or by passing a `LayeredMapping` instance as the context "
+                    "with a `data` layer containing the available variables "
+                    "(such as the `.layered_context` from a "
+                    "`FormulaMaterializer` instance)."
+                )
+
+            unused_variables = available_variables - used_variables
+
+            return OrderedSet(
+                Term([Factor(variable, eval_method="lookup")])
+                for variable in unused_variables
+            )
+
         return [
             Operator(
                 "~",
@@ -474,6 +516,13 @@ def get_terms(terms: OrderedSet[Term]) -> List[Term]:
             Operator(
                 "^", arity=2, precedence=500, associativity="right", to_terms=power
             ),
+            Operator(
+                ".",
+                arity=0,
+                precedence=1000,
+                fixity="postfix",
+                to_terms=insert_unused_terms,
+            ),
         ]
 
     def resolve(

formulaic/parser/utils.py

@@ -71,9 +71,11 @@ def __get_token_for_ast(ast: Union[Token, ASTNode]) -> Token:  # pragma: no cove
     while isinstance(rhs_token, ASTNode):
         rhs_token = rhs_token.args[-1]  # type: ignore
     return Token(
-        token=lhs_token.source[lhs_token.source_start : rhs_token.source_end + 1]
-        if lhs_token.source
-        else "",
+        token=(
+            lhs_token.source[lhs_token.source_start : rhs_token.source_end + 1]
+            if lhs_token.source
+            else ""
+        ),
         source=lhs_token.source,
         source_start=lhs_token.source_start,
         source_end=rhs_token.source_end,
@@ -93,19 +95,29 @@ def __get_tokens_for_gap(
     """
     lhs_token = lhs
     while isinstance(lhs_token, ASTNode):
-        lhs_token = lhs_token.args[-1]  # type: ignore
+        lhs_token = (
+            lhs_token.args[-1]  # type: ignore
+            if lhs_token.args
+            else Token(lhs_token.operator.symbol)
+        )
     rhs_token = rhs or lhs
     while isinstance(rhs_token, ASTNode):
-        rhs_token = rhs_token.args[0]  # type: ignore
+        rhs_token = (
+            rhs_token.args[0]  # type: ignore
+            if rhs_token.args
+            else Token(rhs_token.operator.symbol)
+        )
     return (
         lhs_token,
         rhs_token,
         Token(
-            lhs_token.source[lhs_token.source_start : rhs_token.source_end + 1]
-            if lhs_token.source
-            and lhs_token.source_start is not None
-            and rhs_token.source_end is not None
-            else "",
+            (
+                lhs_token.source[lhs_token.source_start : rhs_token.source_end + 1]
+                if lhs_token.source
+                and lhs_token.source_start is not None
+                and rhs_token.source_end is not None
+                else ""
+            ),
             source=lhs_token.source,
             source_start=lhs_token.source_start,
             source_end=rhs_token.source_end,
@@ -154,6 +166,7 @@ def insert_tokens_after(
     *,
     kind: Optional[Token.Kind] = None,
     join_operator: Optional[str] = None,
+    no_join_for_operators: Union[bool, Set[str]] = True,
 ) -> Iterable[Token]:
     """
     Insert additional tokens into a sequence of tokens after (within token)
@@ -177,6 +190,10 @@ def insert_tokens_after(
             the added tokens with existing tokens, the value set here will be
             used to create a joining operator token. If not provided, not
             additional operators are added.
+        no_join_for_operators: Whether to use the join operator when the next
+            token is an operator token; or a set of operator symbols for which
+            to skip adding the join token.
+
     """
     tokens = list(tokens)
 
@@ -205,9 +222,11 @@ def insert_tokens_after(
                         next_token = split_tokens[j + 1]
                     elif i < len(tokens) - 1:
                         next_token = tokens[i + 1]
-                    if (
-                        next_token is not None
-                        and next_token.kind is not Token.Kind.OPERATOR
+                    if next_token is not None and (
+                        next_token.kind is not Token.Kind.OPERATOR
+                        or no_join_for_operators is False
+                        or isinstance(no_join_for_operators, set)
+                        and next_token.token not in no_join_for_operators
                     ):
                         yield Token(join_operator, kind=Token.Kind.OPERATOR)
 

tests/materializers/test_pandas.py

@@ -67,6 +67,23 @@
         ["Intercept"],
         1,
     ),
+    ".": (
+        ["Intercept", "a", "b", "A[T.b]", "A[T.c]", "B[T.b]", "B[T.c]"],
+        [
+            "Intercept",
+            "a",
+            "b",
+            "A[a]",
+            "A[b]",
+            "A[c]",
+            "B[a]",
+            "B[b]",
+            "B[c]",
+            "D[a]",
+        ],
+        ["Intercept", "a", "b"],
+        1,
+    ),
 }
 
 
@@ -86,7 +103,13 @@ def data(self):
     @pytest.fixture
     def data_with_nulls(self):
         return pandas.DataFrame(
-            {"a": [1, 2, None], "A": ["a", None, "c"], "B": ["a", "b", None]}
+            {
+                "a": [1, 2, None],
+                "b": [1, 2, 3],
+                "A": ["a", None, "c"],
+                "B": ["a", "b", None],
+                "D": ["a", "a", "a"],
+            }
         )
 
     @pytest.fixture
@@ -182,7 +205,10 @@ def test_na_handling(self, data_with_nulls, formula, tests, output):
             formula, na_action="ignore"
         )
         assert isinstance(mm, pandas.DataFrame)
-        assert mm.shape == (3, len(tests[0]) + (-1 if "A" in formula else 0))
+        if formula == ".":
+            assert mm.shape == (3, 5)
+        else:
+            assert mm.shape == (3, len(tests[0]) + (-1 if "A" in formula else 0))
 
         if formula != "C(A)":  # C(A) pre-encodes the data, stripping out nulls.
             with pytest.raises(ValueError):

tests/parser/test_parser.py

@@ -10,6 +10,7 @@
 from formulaic.parser import DefaultFormulaParser, DefaultOperatorResolver
 from formulaic.parser.types import Structured, Token
 from formulaic.parser.types.term import Term
+from formulaic.utils.layered_mapping import LayeredMapping
 
 FORMULA_TO_TOKENS = {
     "": ["1"],
@@ -133,12 +134,21 @@
     # Quoting
     "`a|b~c*d`": ["1", "a|b~c*d"],
     "{a | b | c}": ["1", "a | b | c"],
+    # Wildcards
+    ".": ["1", "a", "b", "c"],
+    ".^2": ["1", "a", "a:b", "a:c", "b", "b:c", "c"],
+    ".^2 - a:b": ["1", "a", "a:c", "b", "b:c", "c"],
+    "a ~ .": {
+        "lhs": ["a"],
+        "rhs": ["1", "b", "c"],
+    },
 }
 
 PARSER = DefaultFormulaParser(feature_flags={"all"})
 PARSER_NO_INTERCEPT = DefaultFormulaParser(
     include_intercept=False, feature_flags={"all"}
 )
+PARSER_CONTEXT = {"__formulaic_variables_available__": ["a", "b", "c"]}
 
 
 class TestFormulaParser:
@@ -148,7 +158,9 @@ class TestFormulaParser:
 
     @pytest.mark.parametrize("formula,terms", FORMULA_TO_TERMS.items())
     def test_to_terms(self, formula, terms):
-        generated_terms: Structured[List[Term]] = PARSER.get_terms(formula)
+        generated_terms: Structured[List[Term]] = PARSER.get_terms(
+            formula, context=PARSER_CONTEXT
+        )
         if generated_terms._has_keys:
             comp = generated_terms._map(list)._to_dict()
         elif generated_terms._has_root and isinstance(generated_terms.root, tuple):
@@ -280,6 +292,19 @@ def test_invalid_multistage_formula(self):
         ):
             DefaultFormulaParser(feature_flags={"all"}).get_terms("[[a ~ b] ~ c]")
 
+    def test_alternative_wildcard_usage(self):
+        PARSER.get_terms(
+            ".", context=LayeredMapping({"a": 1, "b": 2}, name="data")
+        ) == ["1", "a", "b"]
+
+        with pytest.raises(
+            FormulaParsingError,
+            match=re.escape(
+                "The `.` operator requires additional context about which "
+            ),
+        ):
+            PARSER.get_terms(".")
+
 
 class TestDefaultOperatorResolver:
     @pytest.fixture

tests/parser/test_utils.py

@@ -1,9 +1,9 @@
-from ntpath import join
-
 import pytest
 
+from formulaic.errors import FormulaSyntaxError
 from formulaic.parser.types import Token
 from formulaic.parser.utils import (
+    exc_for_token,
     insert_tokens_after,
     merge_operator_tokens,
     replace_tokens,
@@ -29,6 +29,15 @@ def test_replace_tokens(tokens):
     ]
 
 
+def test_exc_for_token(tokens):
+    with pytest.raises(FormulaSyntaxError, match="Hello World"):
+        raise exc_for_token(tokens[0], "Hello World")
+    with pytest.raises(FormulaSyntaxError, match="Hello World"):
+        raise exc_for_token(
+            Token("h", source="hi", source_start=0, source_end=1), "Hello World"
+        )
+
+
 def test_insert_tokens_after(tokens):
     assert list(
         insert_tokens_after(
@@ -50,6 +59,32 @@ def test_insert_tokens_after(tokens):
             join_operator="+",
         )
     ) == ["1", "+|", "hi", "-", "field"]
+    assert list(
+        insert_tokens_after(
+            [
+                Token("1", kind=Token.Kind.VALUE),
+                Token("+|-", kind=Token.Kind.OPERATOR),
+                Token("field", kind=Token.Kind.NAME),
+            ],
+            r"\|",
+            [Token("hi", kind=Token.Kind.NAME)],
+            join_operator="+",
+            no_join_for_operators=False,
+        )
+    ) == ["1", "+|", "hi", "+", "-", "field"]
+    assert list(
+        insert_tokens_after(
+            [
+                Token("1", kind=Token.Kind.VALUE),
+                Token("+|-", kind=Token.Kind.OPERATOR),
+                Token("field", kind=Token.Kind.NAME),
+            ],
+            r"\|",
+            [Token("hi", kind=Token.Kind.NAME)],
+            join_operator="+",
+            no_join_for_operators={"+", "-"},
+        )
+    ) == ["1", "+|", "hi", "-", "field"]
     assert list(
         insert_tokens_after(
             [