bpo-36287: Make ast.dump() not output optional fields and attributes with default values. (GH-18843)

The default values for optional fields and attributes of AST nodes are now set
as class attributes (e.g. Constant.kind is set to None).

2 files changed, 41 insertions, 34 deletions

diff --git a/Lib/ast.py b/Lib/ast.py
index 9a3d380..0bce4a4 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -123,31 +123,36 @@ def dump(node, annotate_fields=True, include_attributes=False, *, indent=None):
             prefix = ''
             sep = ', '
         if isinstance(node, AST):
+            cls = type(node)
             args = []
             allsimple = True
             keywords = annotate_fields
-            for field in node._fields:
+            for name in node._fields:
                 try:
-                    value = getattr(node, field)
+                    value = getattr(node, name)
                 except AttributeError:
                     keywords = True
+                    continue
+                if value is None and getattr(cls, name, ...) is None:
+                    keywords = True
+                    continue
+                value, simple = _format(value, level)
+                allsimple = allsimple and simple
+                if keywords:
+                    args.append('%s=%s' % (name, value))
                 else:
-                    value, simple = _format(value, level)
-                    allsimple = allsimple and simple
-                    if keywords:
-                        args.append('%s=%s' % (field, value))
-                    else:
-                        args.append(value)
+                    args.append(value)
             if include_attributes and node._attributes:
-                for attr in node._attributes:
+                for name in node._attributes:
                     try:
-                        value = getattr(node, attr)
+                        value = getattr(node, name)
                     except AttributeError:
-                        pass
-                    else:
-                        value, simple = _format(value, level)
-                        allsimple = allsimple and simple
-                        args.append('%s=%s' % (attr, value))
+                        continue
+                    if value is None and getattr(cls, name, ...) is None:
+                        continue
+                    value, simple = _format(value, level)
+                    allsimple = allsimple and simple
+                    args.append('%s=%s' % (name, value))
             if allsimple and len(args) <= 3:
                 return '%s(%s)' % (node.__class__.__name__, ', '.join(args)), not args
             return '%s(%s%s)' % (node.__class__.__name__, prefix, sep.join(args)), False
@@ -170,9 +175,10 @@ def copy_location(new_node, old_node):
     attributes) from *old_node* to *new_node* if possible, and return *new_node*.
     """
     for attr in 'lineno', 'col_offset', 'end_lineno', 'end_col_offset':
-        if attr in old_node._attributes and attr in new_node._attributes \
-           and hasattr(old_node, attr):
-            setattr(new_node, attr, getattr(old_node, attr))
+        if attr in old_node._attributes and attr in new_node._attributes:
+            value = getattr(old_node, attr, None)
+            if value is not None:
+                setattr(new_node, attr, value)
     return new_node
 
 
@@ -191,7 +197,7 @@ def fix_missing_locations(node):
             else:
                 lineno = node.lineno
         if 'end_lineno' in node._attributes:
-            if not hasattr(node, 'end_lineno'):
+            if getattr(node, 'end_lineno', None) is None:
                 node.end_lineno = end_lineno
             else:
                 end_lineno = node.end_lineno
@@ -201,7 +207,7 @@ def fix_missing_locations(node):
             else:
                 col_offset = node.col_offset
         if 'end_col_offset' in node._attributes:
-            if not hasattr(node, 'end_col_offset'):
+            if getattr(node, 'end_col_offset', None) is None:
                 node.end_col_offset = end_col_offset
             else:
                 end_col_offset = node.end_col_offset
diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index e788485..c1e9f00 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -353,9 +353,11 @@ class AST_Tests(unittest.TestCase):
                                      'kw_defaults', 'kwarg', 'defaults'))
 
         with self.assertRaises(AttributeError):
-            x.vararg
+            x.args
+        self.assertIsNone(x.vararg)
 
         x = ast.arguments(*range(1, 8))
+        self.assertEqual(x.args, 2)
         self.assertEqual(x.vararg, 3)
 
     def test_field_attr_writable(self):
@@ -650,18 +652,18 @@ class ASTHelpers_Test(unittest.TestCase):
         node = ast.parse('spam(eggs, "and cheese")')
         self.assertEqual(ast.dump(node),
             "Module(body=[Expr(value=Call(func=Name(id='spam', ctx=Load()), "
-            "args=[Name(id='eggs', ctx=Load()), Constant(value='and cheese', kind=None)], "
+            "args=[Name(id='eggs', ctx=Load()), Constant(value='and cheese')], "
             "keywords=[]))], type_ignores=[])"
         )
         self.assertEqual(ast.dump(node, annotate_fields=False),
             "Module([Expr(Call(Name('spam', Load()), [Name('eggs', Load()), "
-            "Constant('and cheese', None)], []))], [])"
+            "Constant('and cheese')], []))], [])"
         )
         self.assertEqual(ast.dump(node, include_attributes=True),
             "Module(body=[Expr(value=Call(func=Name(id='spam', ctx=Load(), "
             "lineno=1, col_offset=0, end_lineno=1, end_col_offset=4), "
             "args=[Name(id='eggs', ctx=Load(), lineno=1, col_offset=5, "
-            "end_lineno=1, end_col_offset=9), Constant(value='and cheese', kind=None, "
+            "end_lineno=1, end_col_offset=9), Constant(value='and cheese', "
             "lineno=1, col_offset=11, end_lineno=1, end_col_offset=23)], keywords=[], "
             "lineno=1, col_offset=0, end_lineno=1, end_col_offset=24), "
             "lineno=1, col_offset=0, end_lineno=1, end_col_offset=24)], type_ignores=[])"
@@ -677,7 +679,7 @@ Module(
             func=Name(id='spam', ctx=Load()),
             args=[
                Name(id='eggs', ctx=Load()),
-               Constant(value='and cheese', kind=None)],
+               Constant(value='and cheese')],
             keywords=[]))],
    type_ignores=[])""")
         self.assertEqual(ast.dump(node, annotate_fields=False, indent='\t'), """\
@@ -688,7 +690,7 @@ Module(
 \t\t\t\tName('spam', Load()),
 \t\t\t\t[
 \t\t\t\t\tName('eggs', Load()),
-\t\t\t\t\tConstant('and cheese', None)],
+\t\t\t\t\tConstant('and cheese')],
 \t\t\t\t[]))],
 \t[])""")
         self.assertEqual(ast.dump(node, include_attributes=True, indent=3), """\
@@ -713,7 +715,6 @@ Module(
                   end_col_offset=9),
                Constant(
                   value='and cheese',
-                  kind=None,
                   lineno=1,
                   col_offset=11,
                   end_lineno=1,
@@ -762,7 +763,7 @@ Module(
         src = ast.parse('1 + 1', mode='eval')
         src.body.right = ast.copy_location(ast.Num(2), src.body.right)
         self.assertEqual(ast.dump(src, include_attributes=True),
-            'Expression(body=BinOp(left=Constant(value=1, kind=None, lineno=1, col_offset=0, '
+            'Expression(body=BinOp(left=Constant(value=1, lineno=1, col_offset=0, '
             'end_lineno=1, end_col_offset=1), op=Add(), right=Constant(value=2, '
             'lineno=1, col_offset=4, end_lineno=1, end_col_offset=5), lineno=1, '
             'col_offset=0, end_lineno=1, end_col_offset=5))'
@@ -777,7 +778,7 @@ Module(
         self.assertEqual(ast.dump(src, include_attributes=True),
             "Module(body=[Expr(value=Call(func=Name(id='write', ctx=Load(), "
             "lineno=1, col_offset=0, end_lineno=1, end_col_offset=5), "
-            "args=[Constant(value='spam', kind=None, lineno=1, col_offset=6, end_lineno=1, "
+            "args=[Constant(value='spam', lineno=1, col_offset=6, end_lineno=1, "
             "end_col_offset=12)], keywords=[], lineno=1, col_offset=0, end_lineno=1, "
             "end_col_offset=13), lineno=1, col_offset=0, end_lineno=1, "
             "end_col_offset=13), Expr(value=Call(func=Name(id='spam', ctx=Load(), "
@@ -792,8 +793,8 @@ Module(
         src = ast.parse('1 + 1', mode='eval')
         self.assertEqual(ast.increment_lineno(src, n=3), src)
         self.assertEqual(ast.dump(src, include_attributes=True),
-            'Expression(body=BinOp(left=Constant(value=1, kind=None, lineno=4, col_offset=0, '
-            'end_lineno=4, end_col_offset=1), op=Add(), right=Constant(value=1, kind=None, '
+            'Expression(body=BinOp(left=Constant(value=1, lineno=4, col_offset=0, '
+            'end_lineno=4, end_col_offset=1), op=Add(), right=Constant(value=1, '
             'lineno=4, col_offset=4, end_lineno=4, end_col_offset=5), lineno=4, '
             'col_offset=0, end_lineno=4, end_col_offset=5))'
         )
@@ -801,8 +802,8 @@ Module(
         src = ast.parse('1 + 1', mode='eval')
         self.assertEqual(ast.increment_lineno(src.body, n=3), src.body)
         self.assertEqual(ast.dump(src, include_attributes=True),
-            'Expression(body=BinOp(left=Constant(value=1, kind=None, lineno=4, col_offset=0, '
-            'end_lineno=4, end_col_offset=1), op=Add(), right=Constant(value=1, kind=None, '
+            'Expression(body=BinOp(left=Constant(value=1, lineno=4, col_offset=0, '
+            'end_lineno=4, end_col_offset=1), op=Add(), right=Constant(value=1, '
             'lineno=4, col_offset=4, end_lineno=4, end_col_offset=5), lineno=4, '
             'col_offset=0, end_lineno=4, end_col_offset=5))'
         )
@@ -821,7 +822,7 @@ Module(
         self.assertEqual(next(iterator).value, 23)
         self.assertEqual(next(iterator).value, 42)
         self.assertEqual(ast.dump(next(iterator)),
-            "keyword(arg='eggs', value=Constant(value='leek', kind=None))"
+            "keyword(arg='eggs', value=Constant(value='leek'))"
         )
 
     def test_get_docstring(self):