#2505: allow easier creation of AST nodes.
diff --git a/Doc/library/_ast.rst b/Doc/library/_ast.rst
index e9f4ad4..80b8a37 100644
--- a/Doc/library/_ast.rst
+++ b/Doc/library/_ast.rst
@@ -46,9 +46,32 @@
If these attributes are marked as optional in the grammar (using a question
mark), the value might be ``None``. If the attributes can have zero-or-more
values (marked with an asterisk), the values are represented as Python lists.
+All possible attributes must be present and have valid values when compiling an
+AST with :func:`compile`.
-The constructors of all ``_ast`` classes don't take arguments; instead, if you
-create instances, you must assign the required attributes separately.
+The constructor of a class ``_ast.T`` parses their arguments as follows:
+
+* If there are positional arguments, there must be as many as there are items in
+ ``T._fields``; they will be assigned as attributes of these names.
+* If there are keyword arguments, they will set the attributes of the same names
+ to the given values.
+
+For example, to create and populate a ``UnaryOp`` node, you could use ::
+
+ node = _ast.UnaryOp()
+ node.op = _ast.USub()
+ node.operand = _ast.Num()
+ node.operand.n = 5
+ node.operand.lineno = 0
+ node.operand.col_offset = 0
+ node.lineno = 0
+ node.col_offset = 0
+
+or the more compact ::
+
+ node = _ast.UnaryOp(_ast.USub(), _ast.Num(5, lineno=0, col_offset=0),
+ lineno=0, col_offset=0)
+
Abstract Grammar