Don't type check most function bodies if ignoring errors (python#14150)

If errors are ignored, type checking function bodies often can have no
effect. Remove function bodies after parsing to speed up type checking.

Methods that define attributes have an externally visible effect even if
errors are ignored. The body of any method that assigns to any attribute
is preserved to deal with this (even if it doesn't actually define a new
attribute). Most methods don't assign to an attribute, so stripping
bodies is still effective for methods.

There are a couple of additional interesting things in the
implementation:

1. We need to know whether an abstract method has a trivial body (e.g.
just `...`) to check `super()` method calls. The approach here is to
preserve such trivial bodies and treat them differently from no body at
all.
2. Stubgen analyzes the bodies of functions to e.g. infer some return
types. As a workaround, explicitly preserve full ASTs when using
stubgen.

The main benefit is faster type checking when using installed packages
with inline type information (PEP 561). Errors are ignored in this case,
and it's common to have a large number of third-party code to type
check. For example, a self check (code under `mypy/`) is now about **20%
faster**, with a compiled mypy on Python 3.11.

Another, more subtle benefit is improved reliability. A third-party
library may have some code that triggers a mypy crash or an invalid
blocking error. If bodies are stripped, often the error will no longer
be triggered, since the amount code to type check is much lower.

---------

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/build.py

@@ -835,6 +835,8 @@ def parse_file(
         Raise CompileError if there is a parse error.
         """
         t0 = time.time()
+        if ignore_errors:
+            self.errors.ignored_files.add(path)
         tree = parse(source, path, id, self.errors, options=options)
         tree._fullname = id
         self.add_stats(

mypy/config_parser.py

@@ -538,10 +538,7 @@ def split_directive(s: str) -> tuple[list[str], list[str]]:
 
 
 def mypy_comments_to_config_map(line: str, template: Options) -> tuple[dict[str, str], list[str]]:
-    """Rewrite the mypy comment syntax into ini file syntax.
-
-    Returns
-    """
+    """Rewrite the mypy comment syntax into ini file syntax."""
     options = {}
     entries, errors = split_directive(line)
     for entry in entries:

mypy/fastparse.py

@@ -99,6 +99,7 @@
 )
 from mypy.reachability import infer_reachability_of_if_statement, mark_block_unreachable
 from mypy.sharedparse import argument_elide_name, special_function_elide_names
+from mypy.traverser import TraverserVisitor
 from mypy.types import (
     AnyType,
     CallableArgument,
@@ -260,6 +261,11 @@ def parse(
     Return the parse tree. If errors is not provided, raise ParseError
     on failure. Otherwise, use the errors object to report parse errors.
     """
+    ignore_errors = (options is not None and options.ignore_errors) or (
+        errors is not None and fnam in errors.ignored_files
+    )
+    # If errors are ignored, we can drop many function bodies to speed up type checking.
+    strip_function_bodies = ignore_errors and (options is None or not options.preserve_asts)
     raise_on_error = False
     if options is None:
         options = Options()
@@ -281,7 +287,13 @@ def parse(
             warnings.filterwarnings("ignore", category=DeprecationWarning)
             ast = ast3_parse(source, fnam, "exec", feature_version=feature_version)
 
-        tree = ASTConverter(options=options, is_stub=is_stub_file, errors=errors).visit(ast)
+        tree = ASTConverter(
+            options=options,
+            is_stub=is_stub_file,
+            errors=errors,
+            ignore_errors=ignore_errors,
+            strip_function_bodies=strip_function_bodies,
+        ).visit(ast)
         tree.path = fnam
         tree.is_stub = is_stub_file
     except SyntaxError as e:
@@ -400,14 +412,24 @@ def is_no_type_check_decorator(expr: ast3.expr) -> bool:
 
 
 class ASTConverter:
-    def __init__(self, options: Options, is_stub: bool, errors: Errors) -> None:
-        # 'C' for class, 'F' for function
-        self.class_and_function_stack: list[Literal["C", "F"]] = []
+    def __init__(
+        self,
+        options: Options,
+        is_stub: bool,
+        errors: Errors,
+        *,
+        ignore_errors: bool,
+        strip_function_bodies: bool,
+    ) -> None:
+        # 'C' for class, 'D' for function signature, 'F' for function, 'L' for lambda
+        self.class_and_function_stack: list[Literal["C", "D", "F", "L"]] = []
         self.imports: list[ImportBase] = []
 
         self.options = options
         self.is_stub = is_stub
         self.errors = errors
+        self.ignore_errors = ignore_errors
+        self.strip_function_bodies = strip_function_bodies
 
         self.type_ignores: dict[int, list[str]] = {}
 
@@ -475,7 +497,12 @@ def get_lineno(self, node: ast3.expr | ast3.stmt) -> int:
         return node.lineno
 
     def translate_stmt_list(
-        self, stmts: Sequence[ast3.stmt], ismodule: bool = False
+        self,
+        stmts: Sequence[ast3.stmt],
+        *,
+        ismodule: bool = False,
+        can_strip: bool = False,
+        is_coroutine: bool = False,
     ) -> list[Statement]:
         # A "# type: ignore" comment before the first statement of a module
         # ignores the whole module:
@@ -504,11 +531,41 @@ def translate_stmt_list(
             mark_block_unreachable(block)
             return [block]
 
+        stack = self.class_and_function_stack
+        if self.strip_function_bodies and len(stack) == 1 and stack[0] == "F":
+            return []
+
         res: list[Statement] = []
         for stmt in stmts:
             node = self.visit(stmt)
             res.append(node)
 
+        if (
+            self.strip_function_bodies
+            and can_strip
+            and stack[-2:] == ["C", "F"]
+            and not is_possible_trivial_body(res)
+        ):
+            # We only strip method bodies if they don't assign to an attribute, as
+            # this may define an attribute which has an externally visible effect.
+            visitor = FindAttributeAssign()
+            for s in res:
+                s.accept(visitor)
+                if visitor.found:
+                    break
+            else:
+                if is_coroutine:
+                    # Yields inside an async function affect the return type and should not
+                    # be stripped.
+                    yield_visitor = FindYield()
+                    for s in res:
+                        s.accept(yield_visitor)
+                        if yield_visitor.found:
+                            break
+                    else:
+                        return []
+                else:
+                    return []
         return res
 
     def translate_type_comment(
@@ -573,9 +630,20 @@ def as_block(self, stmts: list[ast3.stmt], lineno: int) -> Block | None:
             b.set_line(lineno)
         return b
 
-    def as_required_block(self, stmts: list[ast3.stmt], lineno: int) -> Block:
+    def as_required_block(
+        self,
+        stmts: list[ast3.stmt],
+        lineno: int,
+        *,
+        can_strip: bool = False,
+        is_coroutine: bool = False,
+    ) -> Block:
         assert stmts  # must be non-empty
-        b = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
+        b = Block(
+            self.fix_function_overloads(
+                self.translate_stmt_list(stmts, can_strip=can_strip, is_coroutine=is_coroutine)
+            )
+        )
         # TODO: in most call sites line is wrong (includes first line of enclosing statement)
         # TODO: also we need to set the column, and the end position here.
         b.set_line(lineno)
@@ -831,9 +899,6 @@ def _is_stripped_if_stmt(self, stmt: Statement) -> bool:
         # For elif, IfStmt are stored recursively in else_body
         return self._is_stripped_if_stmt(stmt.else_body.body[0])
 
-    def in_method_scope(self) -> bool:
-        return self.class_and_function_stack[-2:] == ["C", "F"]
-
     def translate_module_id(self, id: str) -> str:
         """Return the actual, internal module id for a source text id."""
         if id == self.options.custom_typing_module:
@@ -868,7 +933,7 @@ def do_func_def(
         self, n: ast3.FunctionDef | ast3.AsyncFunctionDef, is_coroutine: bool = False
     ) -> FuncDef | Decorator:
         """Helper shared between visit_FunctionDef and visit_AsyncFunctionDef."""
-        self.class_and_function_stack.append("F")
+        self.class_and_function_stack.append("D")
         no_type_check = bool(
             n.decorator_list and any(is_no_type_check_decorator(d) for d in n.decorator_list)
         )
@@ -915,7 +980,8 @@ def do_func_def(
                 return_type = TypeConverter(self.errors, line=lineno).visit(func_type_ast.returns)
 
                 # add implicit self type
-                if self.in_method_scope() and len(arg_types) < len(args):
+                in_method_scope = self.class_and_function_stack[-2:] == ["C", "D"]
+                if in_method_scope and len(arg_types) < len(args):
                     arg_types.insert(0, AnyType(TypeOfAny.special_form))
             except SyntaxError:
                 stripped_type = n.type_comment.split("#", 2)[0].strip()
@@ -965,7 +1031,10 @@ def do_func_def(
         end_line = getattr(n, "end_lineno", None)
         end_column = getattr(n, "end_col_offset", None)
 
-        func_def = FuncDef(n.name, args, self.as_required_block(n.body, lineno), func_type)
+        self.class_and_function_stack.pop()
+        self.class_and_function_stack.append("F")
+        body = self.as_required_block(n.body, lineno, can_strip=True, is_coroutine=is_coroutine)
+        func_def = FuncDef(n.name, args, body, func_type)
         if isinstance(func_def.type, CallableType):
             # semanal.py does some in-place modifications we want to avoid
             func_def.unanalyzed_type = func_def.type.copy_modified()
@@ -1409,9 +1478,11 @@ def visit_Lambda(self, n: ast3.Lambda) -> LambdaExpr:
         body.lineno = n.body.lineno
         body.col_offset = n.body.col_offset
 
+        self.class_and_function_stack.append("L")
         e = LambdaExpr(
             self.transform_args(n.args, n.lineno), self.as_required_block([body], n.lineno)
         )
+        self.class_and_function_stack.pop()
         e.set_line(n.lineno, n.col_offset)  # Overrides set_line -- can't use self.set_line
         return e
 
@@ -2081,3 +2152,85 @@ def stringify_name(n: AST) -> str | None:
         if sv is not None:
             return f"{sv}.{n.attr}"
     return None  # Can't do it.
+
+
+class FindAttributeAssign(TraverserVisitor):
+    """Check if an AST contains attribute assignments (e.g. self.x = 0)."""
+
+    def __init__(self) -> None:
+        self.lvalue = False
+        self.found = False
+
+    def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
+        self.lvalue = True
+        for lv in s.lvalues:
+            lv.accept(self)
+        self.lvalue = False
+
+    def visit_with_stmt(self, s: WithStmt) -> None:
+        self.lvalue = True
+        for lv in s.target:
+            if lv is not None:
+                lv.accept(self)
+        self.lvalue = False
+        s.body.accept(self)
+
+    def visit_for_stmt(self, s: ForStmt) -> None:
+        self.lvalue = True
+        s.index.accept(self)
+        self.lvalue = False
+        s.body.accept(self)
+        if s.else_body:
+            s.else_body.accept(self)
+
+    def visit_expression_stmt(self, s: ExpressionStmt) -> None:
+        # No need to look inside these
+        pass
+
+    def visit_call_expr(self, e: CallExpr) -> None:
+        # No need to look inside these
+        pass
+
+    def visit_index_expr(self, e: IndexExpr) -> None:
+        # No need to look inside these
+        pass
+
+    def visit_member_expr(self, e: MemberExpr) -> None:
+        if self.lvalue:
+            self.found = True
+
+
+class FindYield(TraverserVisitor):
+    """Check if an AST contains yields or yield froms."""
+
+    def __init__(self) -> None:
+        self.found = False
+
+    def visit_yield_expr(self, e: YieldExpr) -> None:
+        self.found = True
+
+    def visit_yield_from_expr(self, e: YieldFromExpr) -> None:
+        self.found = True
+
+
+def is_possible_trivial_body(s: list[Statement]) -> bool:
+    """Could the statements form a "trivial" function body, such as 'pass'?
+
+    This mimics mypy.semanal.is_trivial_body, but this runs before
+    semantic analysis so some checks must be conservative.
+    """
+    l = len(s)
+    if l == 0:
+        return False
+    i = 0
+    if isinstance(s[0], ExpressionStmt) and isinstance(s[0].expr, StrExpr):
+        # Skip docstring
+        i += 1
+    if i == l:
+        return True
+    if l > i + 1:
+        return False
+    stmt = s[i]
+    return isinstance(stmt, (PassStmt, RaiseStmt)) or (
+        isinstance(stmt, ExpressionStmt) and isinstance(stmt.expr, EllipsisExpr)
+    )

mypy/semanal.py

@@ -6684,7 +6684,7 @@ def is_trivial_body(block: Block) -> bool:
     "..." (ellipsis), or "raise NotImplementedError()". A trivial body may also
     start with a statement containing just a string (e.g. a docstring).
 
-    Note: functions that raise other kinds of exceptions do not count as
+    Note: Functions that raise other kinds of exceptions do not count as
     "trivial". We use this function to help us determine when it's ok to
     relax certain checks on body, but functions that raise arbitrary exceptions
     are more likely to do non-trivial work. For example:
@@ -6694,11 +6694,18 @@ def halt(self, reason: str = ...) -> NoReturn:
 
     A function that raises just NotImplementedError is much less likely to be
     this complex.
+
+    Note: If you update this, you may also need to update
+    mypy.fastparse.is_possible_trivial_body!
     """
     body = block.body
+    if not body:
+        # Functions have empty bodies only if the body is stripped or the function is
+        # generated or deserialized. In these cases the body is unknown.
+        return False
 
     # Skip a docstring
-    if body and isinstance(body[0], ExpressionStmt) and isinstance(body[0].expr, StrExpr):
+    if isinstance(body[0], ExpressionStmt) and isinstance(body[0].expr, StrExpr):
         body = block.body[1:]
 
     if len(body) == 0:

mypy/stubgen.py

@@ -1588,6 +1588,7 @@ def mypy_options(stubgen_options: Options) -> MypyOptions:
     options.python_version = stubgen_options.pyversion
     options.show_traceback = True
     options.transform_source = remove_misplaced_type_comments
+    options.preserve_asts = True
     return options
 
 

mypy/test/testparse.py

@@ -7,11 +7,13 @@
 from pytest import skip
 
 from mypy import defaults
+from mypy.config_parser import parse_mypy_comments
 from mypy.errors import CompileError
 from mypy.options import Options
 from mypy.parse import parse
 from mypy.test.data import DataDrivenTestCase, DataSuite
 from mypy.test.helpers import assert_string_arrays_equal, find_test_files, parse_options
+from mypy.util import get_mypy_comments
 
 
 class ParserSuite(DataSuite):
@@ -40,13 +42,16 @@ def test_parser(testcase: DataDrivenTestCase) -> None:
     else:
         options.python_version = defaults.PYTHON3_VERSION
 
+    source = "\n".join(testcase.input)
+
+    # Apply mypy: comments to options.
+    comments = get_mypy_comments(source)
+    changes, _ = parse_mypy_comments(comments, options)
+    options = options.apply_changes(changes)
+
     try:
         n = parse(
-            bytes("\n".join(testcase.input), "ascii"),
-            fnam="main",
-            module="__main__",
-            errors=None,
-            options=options,
+            bytes(source, "ascii"), fnam="main", module="__main__", errors=None, options=options
         )
         a = n.str_with_options(options).split("\n")
     except CompileError as e: