bpo-39981: Default values for AST nodes

Doc/whatsnew/3.9.rst

@@ -159,6 +159,10 @@ Added the *indent* option to :func:`~ast.dump` which allows it to produce a
 multiline indented output.
 (Contributed by Serhiy Storchaka in :issue:`37995`.)
 
+Added the *omit_defaults* option to :func:`~ast.dump` which allows to
+produce a representation without printing default values.
+(Contributed by Serhiy Storchaka and Batuhan Taskaya in :issue:`36287`)
+
 Added :func:`ast.unparse` as a function in the :mod:`ast` module that can
 be used to unparse an :class:`ast.AST` object and produce a string with code
 that would produce an equivalent :class:`ast.AST` object when parsed.
@@ -167,6 +171,9 @@ that would produce an equivalent :class:`ast.AST` object when parsed.
 Added docstrings to AST nodes that contains the ASDL signature used to
 construct that node. (Contributed by Batuhan Taskaya in :issue:`39638`.)
 
+Added default values to AST node classes.
+(Contributed by Batuhan Taskaya in :issue:`39981`)
+
 asyncio
 -------
 

Lib/ast.py

@@ -28,6 +28,7 @@
 from _ast import *
 from contextlib import contextmanager, nullcontext
 from enum import IntEnum, auto
+from functools import lru_cache
 
 
 def parse(source, filename='<unknown>', mode='exec', *,
@@ -102,7 +103,7 @@ def _convert(node):
     return _convert(node_or_string)
 
 
-def dump(node, annotate_fields=True, include_attributes=False, *, indent=None):
+def dump(node, annotate_fields=True, include_attributes=False, *, indent=None, omit_defaults=True):
     """
     Return a formatted dump of the tree in node.  This is mainly useful for
     debugging purposes.  If annotate_fields is true (by default),
@@ -112,7 +113,9 @@ def dump(node, annotate_fields=True, include_attributes=False, *, indent=None):
     numbers and column offsets are not dumped by default.  If this is wanted,
     include_attributes can be set to true.  If indent is a non-negative
     integer or string, then the tree will be pretty-printed with that indent
-    level. None (the default) selects the single line representation.
+    level. None (the default) selects the single line representation. If
+    omit_defaults is true, fields that have their default values will be
+    omitted.
     """
     def _format(node, level=0):
         if indent is not None:
@@ -127,13 +130,18 @@ def _format(node, level=0):
             args = []
             allsimple = True
             keywords = annotate_fields
+            field_defaults = cls._field_defaults
             for name in node._fields:
                 try:
                     value = getattr(node, name)
                 except AttributeError:
                     keywords = True
                     continue
-                if value is None and getattr(cls, name, ...) is None:
+                if (
+                    omit_defaults
+                    and name in field_defaults
+                    and field_defaults[name] == value
+                ):
                     keywords = True
                     continue
                 value, simple = _format(value, level)
@@ -148,7 +156,11 @@ def _format(node, level=0):
                         value = getattr(node, name)
                     except AttributeError:
                         continue
-                    if value is None and getattr(cls, name, ...) is None:
+                    if (
+                        omit_defaults
+                        and value is None
+                        and getattr(cls, name, ...) is None
+                    ):
                         continue
                     value, simple = _format(value, level)
                     allsimple = allsimple and simple

Lib/test/test_ast.py

@@ -352,23 +352,42 @@ def test_field_attr_existence(self):
                     self.assertEqual(type(x._fields), tuple)
 
     def test_arguments(self):
-        x = ast.arguments()
-        self.assertEqual(x._fields, ('posonlyargs', 'args', 'vararg', 'kwonlyargs',
-                                     'kw_defaults', 'kwarg', 'defaults'))
+        x = ast.FunctionDef()
+        self.assertEqual(x._fields, ('name', 'args', 'body', 'decorator_list', 'returns', 'type_comment'))
+        self.assertEqual(x._field_defaults, {'body': [], 'decorator_list': [], 'returns': None, 'type_comment': None})
 
         with self.assertRaises(AttributeError):
-            x.args
-        self.assertIsNone(x.vararg)
+            x.name
 
-        x = ast.arguments(*range(1, 8))
+        self.assertIsNone(x.returns)
+        self.assertEqual(x.body, [])
+
+        x = ast.FunctionDef(*range(1, 4))
         self.assertEqual(x.args, 2)
-        self.assertEqual(x.vararg, 3)
+        self.assertEqual(x.body, 3)
+        self.assertEqual(x.decorator_list, [])
+        self.assertIsNone(x.returns)
+        self.assertIsNone(x.type_comment)
+
+        x = ast.Module(body=1)
+        self.assertEqual(x.body, 1)
+        self.assertEqual(x.type_ignores, [])
+
+        x = ast.Module(body=1, type_ignores=2)
+        self.assertEqual(x.body, 1)
+        self.assertEqual(x.type_ignores, 2)
+
+        x = ast.Module(type_ignores=2)
+        self.assertEqual(x.body, [])
+        self.assertEqual(x.type_ignores, 2)
 
     def test_field_attr_writable(self):
         x = ast.Num()
         # We can assign to _fields
         x._fields = 666
+        x._field_defaults = 999
         self.assertEqual(x._fields, 666)
+        self.assertEqual(x._field_defaults, 999)
 
     def test_classattrs(self):
         x = ast.Num()
@@ -666,21 +685,21 @@ def test_dump(self):
         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')], "
-            "keywords=[]))], type_ignores=[])"
+            "args=[Name(id='eggs', ctx=Load()), Constant(value='and cheese')"
+            "]))])"
         )
         self.assertEqual(ast.dump(node, annotate_fields=False),
             "Module([Expr(Call(Name('spam', Load()), [Name('eggs', Load()), "
-            "Constant('and cheese')], []))], [])"
+            "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', "
-            "lineno=1, col_offset=11, end_lineno=1, end_col_offset=23)], keywords=[], "
+            "lineno=1, col_offset=11, end_lineno=1, end_col_offset=23)], "
             "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=[])"
+            "lineno=1, col_offset=0, end_lineno=1, end_col_offset=24)])"
         )
 
     def test_dump_indent(self):
@@ -693,9 +712,7 @@ def test_dump_indent(self):
             func=Name(id='spam', ctx=Load()),
             args=[
                Name(id='eggs', ctx=Load()),
-               Constant(value='and cheese')],
-            keywords=[]))],
-   type_ignores=[])""")
+               Constant(value='and cheese')]))])""")
         self.assertEqual(ast.dump(node, annotate_fields=False, indent='\t'), """\
 Module(
 \t[
@@ -704,9 +721,7 @@ def test_dump_indent(self):
 \t\t\t\tName('spam', Load()),
 \t\t\t\t[
 \t\t\t\t\tName('eggs', Load()),
-\t\t\t\t\tConstant('and cheese')],
-\t\t\t\t[]))],
-\t[])""")
+\t\t\t\t\tConstant('and cheese')]))])""")
         self.assertEqual(ast.dump(node, include_attributes=True, indent=3), """\
 Module(
    body=[
@@ -733,16 +748,14 @@ def test_dump_indent(self):
                   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=[])""")
+         end_col_offset=24)])""")
 
     def test_dump_incomplete(self):
         node = ast.Raise(lineno=3, col_offset=4)
@@ -791,16 +804,13 @@ def test_fix_missing_locations(self):
         self.maxDiff = None
         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', 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(), "
-            "lineno=1, col_offset=0, end_lineno=1, end_col_offset=0), "
-            "args=[Constant(value='eggs', lineno=1, col_offset=0, end_lineno=1, "
-            "end_col_offset=0)], keywords=[], lineno=1, col_offset=0, end_lineno=1, "
-            "end_col_offset=0), lineno=1, col_offset=0, end_lineno=1, end_col_offset=0)], "
-            "type_ignores=[])"
+            "lineno=1, col_offset=0, end_lineno=1, end_col_offset=5), args=["
+            "Constant(value='spam', lineno=1, col_offset=6, end_lineno=1, end_col_offset=12)], "
+            "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(), "
+            "lineno=1, col_offset=0, end_lineno=1, end_col_offset=0), args=[Constant(value='eggs', "
+            "lineno=1, col_offset=0, end_lineno=1, end_col_offset=0)], lineno=1, col_offset=0, "
+            "end_lineno=1, end_col_offset=0), lineno=1, col_offset=0, end_lineno=1, end_col_offset=0)])"
         )
 
     def test_increment_lineno(self):

Misc/NEWS.d/next/Library/2020-03-16-22-18-30.bpo-39981.5TKqVR.rst

@@ -0,0 +1 @@
+Add default values for :mod:`ast` node classes. Patch by Batuhan Taskaya.

Parser/asdl.py

@@ -71,16 +71,15 @@ def __init__(self, type, name=None, seq=False, opt=False):
         self.name = name
         self.seq = seq
         self.opt = opt
-
-    def __str__(self):
-        if self.seq:
-            extra = "*"
-        elif self.opt:
-            extra = "?"
+        if seq:
+            self.extra = "*"
+        elif opt:
+            self.extra = "?"
         else:
-            extra = ""
+            self.extra = ""
 
-        return "{}{} {}".format(self.type, extra, self.name)
+    def __str__(self):
+        return "{}{} {}".format(self.type, self.extra, self.name)
 
     def __repr__(self):
         if self.seq: