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.

Lib/inspect.py

@@ -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__