mark/expression: support compiling once and reusing for multiple evaluations

In current pytest, the same expression is matched against all items. But
it is re-parsed for every match.

Add support for "compiling" an expression and reusing the result. Errors
may only occur during compilation.

This is done by parsing the expression into a Python `ast.Expression`,
then `compile()`ing it into a code object. Evaluation is then done using
`eval()`.

Note: historically we used to use `eval` directly on the user input --
this is not the case here, the expression is entirely under our control
according to our grammar, we just JIT-compile it to Python as a
(completely safe) optimization.

src/_pytest/mark/expression.py

@@ -15,10 +15,13 @@ The semantics are:
 - ident evaluates to True of False according to a provided matcher function.
 - or/and/not evaluate according to the usual boolean semantics.
 """
+import ast
 import enum
 import re
+import types
 from typing import Callable
 from typing import Iterator
+from typing import Mapping
 from typing import Optional
 from typing import Sequence
 
@@ -31,7 +34,7 @@ if TYPE_CHECKING:
 
 
 __all__ = [
-    "evaluate",
+    "Expression",
     "ParseError",
 ]
 
@@ -124,50 +127,92 @@ class Scanner:
         )
 
 
-def expression(s: Scanner, matcher: Callable[[str], bool]) -> bool:
+def expression(s: Scanner) -> ast.Expression:
     if s.accept(TokenType.EOF):
-        return False
+        ret = ast.NameConstant(False)  # type: ast.expr
-    ret = expr(s, matcher)
+    else:
-    s.accept(TokenType.EOF, reject=True)
+        ret = expr(s)
-    return ret
+        s.accept(TokenType.EOF, reject=True)
+    return ast.fix_missing_locations(ast.Expression(ret))
 
 
-def expr(s: Scanner, matcher: Callable[[str], bool]) -> bool:
+def expr(s: Scanner) -> ast.expr:
-    ret = and_expr(s, matcher)
+    ret = and_expr(s)
     while s.accept(TokenType.OR):
-        rhs = and_expr(s, matcher)
+        rhs = and_expr(s)
-        ret = ret or rhs
+        ret = ast.BoolOp(ast.Or(), [ret, rhs])
     return ret
 
 
-def and_expr(s: Scanner, matcher: Callable[[str], bool]) -> bool:
+def and_expr(s: Scanner) -> ast.expr:
-    ret = not_expr(s, matcher)
+    ret = not_expr(s)
     while s.accept(TokenType.AND):
-        rhs = not_expr(s, matcher)
+        rhs = not_expr(s)
-        ret = ret and rhs
+        ret = ast.BoolOp(ast.And(), [ret, rhs])
     return ret
 
 
-def not_expr(s: Scanner, matcher: Callable[[str], bool]) -> bool:
+def not_expr(s: Scanner) -> ast.expr:
     if s.accept(TokenType.NOT):
-        return not not_expr(s, matcher)
+        return ast.UnaryOp(ast.Not(), not_expr(s))
     if s.accept(TokenType.LPAREN):
-        ret = expr(s, matcher)
+        ret = expr(s)
         s.accept(TokenType.RPAREN, reject=True)
         return ret
     ident = s.accept(TokenType.IDENT)
     if ident:
-        return matcher(ident.value)
+        return ast.Name(ident.value, ast.Load())
     s.reject((TokenType.NOT, TokenType.LPAREN, TokenType.IDENT))
 
 
-def evaluate(input: str, matcher: Callable[[str], bool]) -> bool:
+class MatcherAdapter(Mapping[str, bool]):
-    """Evaluate a match expression as used by -k and -m.
+    """Adapts a matcher function to a locals mapping as required by eval()."""
 
-    :param input: The input expression - one line.
+    def __init__(self, matcher: Callable[[str], bool]) -> None:
-    :param matcher: Given an identifier, should return whether it matches or not.
+        self.matcher = matcher
-                    Should be prepared to handle arbitrary strings as input.
 
-    Returns whether the entire expression matches or not.
+    def __getitem__(self, key: str) -> bool:
+        return self.matcher(key)
+
+    def __iter__(self) -> Iterator[str]:
+        raise NotImplementedError()
+
+    def __len__(self) -> int:
+        raise NotImplementedError()
+
+
+class Expression:
+    """A compiled match expression as used by -k and -m.
+
+    The expression can be evaulated against different matchers.
     """
-    return expression(Scanner(input), matcher)
+
+    __slots__ = ("code",)
+
+    def __init__(self, code: types.CodeType) -> None:
+        self.code = code
+
+    @classmethod
+    def compile(self, input: str) -> "Expression":
+        """Compile a match expression.
+
+        :param input: The input expression - one line.
+        """
+        astexpr = expression(Scanner(input))
+        code = compile(
+            astexpr, filename="<pytest match expression>", mode="eval",
+        )  # type: types.CodeType
+        return Expression(code)
+
+    def evaluate(self, matcher: Callable[[str], bool]) -> bool:
+        """Evaluate the match expression.
+
+        :param matcher: Given an identifier, should return whether it matches or not.
+                        Should be prepared to handle arbitrary strings as input.
+
+        Returns whether the expression matches or not.
+        """
+        ret = eval(
+            self.code, {"__builtins__": {}}, MatcherAdapter(matcher)
+        )  # type: bool
+        return ret

src/_pytest/mark/legacy.py

@@ -8,7 +8,7 @@ import attr
 
 from _pytest.compat import TYPE_CHECKING
 from _pytest.config import UsageError
-from _pytest.mark.expression import evaluate
+from _pytest.mark.expression import Expression
 from _pytest.mark.expression import ParseError
 
 if TYPE_CHECKING:
@@ -77,11 +77,12 @@ class KeywordMatcher:
 def matchmark(colitem, markexpr: str) -> bool:
     """Tries to match on any marker names, attached to the given colitem."""
     try:
-        return evaluate(markexpr, MarkMatcher.from_item(colitem))
+        expression = Expression.compile(markexpr)
     except ParseError as e:
         raise UsageError(
             "Wrong expression passed to '-m': {}: {}".format(markexpr, e)
         ) from None
+    return expression.evaluate(MarkMatcher.from_item(colitem))
 
 
 def matchkeyword(colitem, keywordexpr: str) -> bool:
@@ -94,8 +95,9 @@ def matchkeyword(colitem, keywordexpr: str) -> bool:
     any item, as well as names directly assigned to test functions.
     """
     try:
-        return evaluate(keywordexpr, KeywordMatcher.from_item(colitem))
+        expression = Expression.compile(keywordexpr)
     except ParseError as e:
         raise UsageError(
             "Wrong expression passed to '-k': {}: {}".format(keywordexpr, e)
         ) from None
+    return expression.evaluate(KeywordMatcher.from_item(colitem))

testing/test_mark_expression.py

@@ -1,8 +1,14 @@
+from typing import Callable
+
 import pytest
-from _pytest.mark.expression import evaluate
+from _pytest.mark.expression import Expression
 from _pytest.mark.expression import ParseError
 
 
+def evaluate(input: str, matcher: Callable[[str], bool]) -> bool:
+    return Expression.compile(input).evaluate(matcher)
+
+
 def test_empty_is_false() -> None:
     assert not evaluate("", lambda ident: False)
     assert not evaluate("", lambda ident: True)