gh-118465: Add __firstlineno__ attribute to class (GH-118475)

It is set by compiler with the line number of the first line of
the class definition.

Author: serhiy-storchaka

Doc/reference/datamodel.rst

@@ -971,6 +971,7 @@ A class object can be called (see above) to yield a class instance (see below).
    single: __annotations__ (class attribute)
    single: __type_params__ (class attribute)
    single: __static_attributes__ (class attribute)
+   single: __firstlineno__ (class attribute)
 
 Special attributes:
 
@@ -1005,6 +1006,9 @@ Special attributes:
       A tuple containing names of attributes of this class which are accessed
       through ``self.X`` from any function in its body.
 
+   :attr:`__firstlineno__`
+      The line number of the first line of the class definition, including decorators.
+
 
 Class instances
 ---------------

Doc/whatsnew/3.13.rst

@@ -328,6 +328,11 @@ Other Language Changes
   class scopes are not inlined into their parent scope. (Contributed by
   Jelle Zijlstra in :gh:`109118` and :gh:`118160`.)
 
+* Classes have a new :attr:`!__firstlineno__` attribute,
+  populated by the compiler, with the line number of the first line
+  of the class definition.
+  (Contributed by Serhiy Storchaka in :gh:`118465`.)
+
 * ``from __future__ import ...`` statements are now just normal
   relative imports if dots are present before the module name.
   (Contributed by Jeremiah Gabriel Pascual in :gh:`118216`.)

Include/internal/pycore_global_objects_fini_generated.h

@@ -113,6 +113,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(__eq__)
         STRUCT_FOR_ID(__exit__)
         STRUCT_FOR_ID(__file__)
+        STRUCT_FOR_ID(__firstlineno__)
         STRUCT_FOR_ID(__float__)
         STRUCT_FOR_ID(__floordiv__)
         STRUCT_FOR_ID(__format__)

Include/internal/pycore_global_strings.h

@@ -2035,7 +2035,7 @@ def _test_simple_enum(checked_enum, simple_enum):
                 )
         for key in set(checked_keys + simple_keys):
             if key in ('__module__', '_member_map_', '_value2member_map_', '__doc__',
-                       '__static_attributes__'):
+                       '__static_attributes__', '__firstlineno__'):
                 # keys known to be different, or very long
                 continue
             elif key in member_names:

Include/internal/pycore_runtime_init_generated.h

@@ -471,6 +471,7 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.13a1 3567 (Reimplement line number propagation by the compiler)
 #     Python 3.13a1 3568 (Change semantics of END_FOR)
 #     Python 3.13a5 3569 (Specialize CONTAINS_OP)
+#     Python 3.13a6 3570 (Add __firstlineno__ class attribute)
 
 #     Python 3.14 will start with 3600
 
@@ -487,7 +488,7 @@ def _write_atomic(path, data, mode=0o666):
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3569).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3570).to_bytes(2, 'little') + b'\r\n'
 
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 

Include/internal/pycore_unicodeobject_generated.h

@@ -1035,79 +1035,6 @@ class ClassFoundException(Exception):
     pass
 
 
-class _ClassFinder(ast.NodeVisitor):
-
-    def __init__(self, cls, tree, lines, qualname):
-        self.stack = []
-        self.cls = cls
-        self.tree = tree
-        self.lines = lines
-        self.qualname = qualname
-        self.lineno_found = []
-
-    def visit_FunctionDef(self, node):
-        self.stack.append(node.name)
-        self.stack.append('<locals>')
-        self.generic_visit(node)
-        self.stack.pop()
-        self.stack.pop()
-
-    visit_AsyncFunctionDef = visit_FunctionDef
-
-    def visit_ClassDef(self, node):
-        self.stack.append(node.name)
-        if self.qualname == '.'.join(self.stack):
-            # Return the decorator for the class if present
-            if node.decorator_list:
-                line_number = node.decorator_list[0].lineno
-            else:
-                line_number = node.lineno
-
-            # decrement by one since lines starts with indexing by zero
-            self.lineno_found.append((line_number - 1, node.end_lineno))
-        self.generic_visit(node)
-        self.stack.pop()
-
-    def get_lineno(self):
-        self.visit(self.tree)
-        lineno_found_number = len(self.lineno_found)
-        if lineno_found_number == 0:
-            raise OSError('could not find class definition')
-        elif lineno_found_number == 1:
-            return self.lineno_found[0][0]
-        else:
-            # We have multiple candidates for the class definition.
-            # Now we have to guess.
-
-            # First, let's see if there are any method definitions
-            for member in self.cls.__dict__.values():
-                if (isinstance(member, types.FunctionType) and
-                    member.__module__ == self.cls.__module__):
-                    for lineno, end_lineno in self.lineno_found:
-                        if lineno <= member.__code__.co_firstlineno <= end_lineno:
-                            return lineno
-
-            class_strings = [(''.join(self.lines[lineno: end_lineno]), lineno)
-                             for lineno, end_lineno in self.lineno_found]
-
-            # Maybe the class has a docstring and it's unique?
-            if self.cls.__doc__:
-                ret = None
-                for candidate, lineno in class_strings:
-                    if self.cls.__doc__.strip() in candidate:
-                        if ret is None:
-                            ret = lineno
-                        else:
-                            break
-                else:
-                    if ret is not None:
-                        return ret
-
-            # We are out of ideas, just return the last one found, which is
-            # slightly better than previous ones
-            return self.lineno_found[-1][0]
-
-
 def findsource(object):
     """Return the entire source file and starting line number for an object.
 
@@ -1140,11 +1067,11 @@ def findsource(object):
         return lines, 0
 
     if isclass(object):
-        qualname = object.__qualname__
-        source = ''.join(lines)
-        tree = ast.parse(source)
-        class_finder = _ClassFinder(object, tree, lines, qualname)
-        return lines, class_finder.get_lineno()
+        try:
+            firstlineno = object.__firstlineno__
+        except AttributeError:
+            raise OSError('source code not available')
+        return lines, object.__firstlineno__ - 1
 
     if ismethod(object):
         object = object.__func__

Lib/enum.py

@@ -326,7 +326,7 @@ def visiblename(name, all=None, obj=None):
                 '__date__', '__doc__', '__file__', '__spec__',
                 '__loader__', '__module__', '__name__', '__package__',
                 '__path__', '__qualname__', '__slots__', '__version__',
-                '__static_attributes__'}:
+                '__static_attributes__', '__firstlineno__'}:
         return 0
     # Private names are hidden, but special names are displayed.
     if name.startswith('__') and name.endswith('__'): return 1