From 5af5cfdd75c4247620f9e592d86aa290dd180f4b Mon Sep 17 00:00:00 2001
From: isidentical <batuhanosmantaskaya@gmail.com>
Date: Sun, 15 Dec 2019 14:00:47 +0300
Subject: [PATCH 1/5] bpo-38870: refactor delimiting with context managers

---
 Lib/ast.py                                    | 292 +++++++++---------
 .../2019-12-15-14-07-16.bpo-38870.8D28DB.rst  |   1 +
 2 files changed, 146 insertions(+), 147 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst

diff --git a/Lib/ast.py b/Lib/ast.py
index 77eb24971ed24cb..a2b8815c79f3da2 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -613,6 +613,30 @@ def __exit__(self, exc_type, exc_value, traceback):
     def block(self):
         return self._Block(self)
 
+    class _NoDelimit:
+        def __enter__(self): pass
+        def __exit__(self, *args): pass
+
+    class _Delimit:
+        """A context manager for preparing the source for expressions. It adds
+        start of the delimiter  and enters, after exit it adds delimiter end."""
+        def __init__(self, unparser, delimiter):
+            self.unparser = unparser
+            self.delimiter = delimiter
+        def __enter__(self):
+            self.unparser.write(self.delimiter[0])
+        def __exit__(self, exc_type, exc_value, traceback):
+            self.unparser.write(self.delimiter[-1])
+
+    def delimit_if(self, condition, delimiter = "()"):
+        if condition:
+            return self._Delimit(self, delimiter)
+        else:
+            return self._NoDelimit()
+
+    def delimit(self, delimiter = "()"):
+        return self._Delimit(self, delimiter)
+
     def traverse(self, node):
         if isinstance(node, list):
             for item in node:
@@ -636,11 +660,10 @@ def visit_Expr(self, node):
         self.traverse(node.value)
 
     def visit_NamedExpr(self, node):
-        self.write("(")
-        self.traverse(node.target)
-        self.write(" := ")
-        self.traverse(node.value)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.target)
+            self.write(" := ")
+            self.traverse(node.value)
 
     def visit_Import(self, node):
         self.fill("import ")
@@ -669,11 +692,8 @@ def visit_AugAssign(self, node):
 
     def visit_AnnAssign(self, node):
         self.fill()
-        if not node.simple and isinstance(node.target, Name):
-            self.write("(")
-        self.traverse(node.target)
-        if not node.simple and isinstance(node.target, Name):
-            self.write(")")
+        with self.delimit_if(not node.simple and isinstance(node.target, Name)):
+            self.traverse(node.target)
         self.write(": ")
         self.traverse(node.annotation)
         if node.value:
@@ -715,28 +735,25 @@ def visit_Nonlocal(self, node):
         self.interleave(lambda: self.write(", "), self.write, node.names)
 
     def visit_Await(self, node):
-        self.write("(")
-        self.write("await")
-        if node.value:
-            self.write(" ")
-            self.traverse(node.value)
-        self.write(")")
+        with self.delimit():
+            self.write("await")
+            if node.value:
+                self.write(" ")
+                self.traverse(node.value)
 
     def visit_Yield(self, node):
-        self.write("(")
-        self.write("yield")
-        if node.value:
-            self.write(" ")
-            self.traverse(node.value)
-        self.write(")")
+        with self.delimit():
+            self.write("yield")
+            if node.value:
+                self.write(" ")
+                self.traverse(node.value)
 
     def visit_YieldFrom(self, node):
-        self.write("(")
-        self.write("yield from")
-        if node.value:
-            self.write(" ")
-            self.traverse(node.value)
-        self.write(")")
+        with self.delimit():
+            self.write("yield from")
+            if node.value:
+                self.write(" ")
+                self.traverse(node.value)
 
     def visit_Raise(self, node):
         self.fill("raise")
@@ -782,21 +799,20 @@ def visit_ClassDef(self, node):
             self.fill("@")
             self.traverse(deco)
         self.fill("class " + node.name)
-        self.write("(")
-        comma = False
-        for e in node.bases:
-            if comma:
-                self.write(", ")
-            else:
-                comma = True
-            self.traverse(e)
-        for e in node.keywords:
-            if comma:
-                self.write(", ")
-            else:
-                comma = True
-            self.traverse(e)
-        self.write(")")
+        with self.delimit():
+            comma = False
+            for e in node.bases:
+                if comma:
+                    self.write(", ")
+                else:
+                    comma = True
+                self.traverse(e)
+            for e in node.keywords:
+                if comma:
+                    self.write(", ")
+                else:
+                    comma = True
+                self.traverse(e)
 
         with self.block():
             self.traverse(node.body)
@@ -812,10 +828,10 @@ def __FunctionDef_helper(self, node, fill_suffix):
         for deco in node.decorator_list:
             self.fill("@")
             self.traverse(deco)
-        def_str = fill_suffix + " " + node.name + "("
+        def_str = fill_suffix + " " + node.name
         self.fill(def_str)
-        self.traverse(node.args)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.args)
         if node.returns:
             self.write(" -> ")
             self.traverse(node.returns)
@@ -931,13 +947,12 @@ def _write_constant(self, value):
     def visit_Constant(self, node):
         value = node.value
         if isinstance(value, tuple):
-            self.write("(")
-            if len(value) == 1:
-                self._write_constant(value[0])
-                self.write(",")
-            else:
-                self.interleave(lambda: self.write(", "), self._write_constant, value)
-            self.write(")")
+            with self.delimit():
+                if len(value) == 1:
+                    self._write_constant(value[0])
+                    self.write(",")
+                else:
+                    self.interleave(lambda: self.write(", "), self._write_constant, value)
         elif value is ...:
             self.write("...")
         else:
@@ -946,39 +961,34 @@ def visit_Constant(self, node):
             self._write_constant(node.value)
 
     def visit_List(self, node):
-        self.write("[")
-        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
-        self.write("]")
+        with self.delimit("[]"):
+            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
 
     def visit_ListComp(self, node):
-        self.write("[")
-        self.traverse(node.elt)
-        for gen in node.generators:
-            self.traverse(gen)
-        self.write("]")
+        with self.delimit("[]"):
+            self.traverse(node.elt)
+            for gen in node.generators:
+                self.traverse(gen)
 
     def visit_GeneratorExp(self, node):
-        self.write("(")
-        self.traverse(node.elt)
-        for gen in node.generators:
-            self.traverse(gen)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.elt)
+            for gen in node.generators:
+                self.traverse(gen)
 
     def visit_SetComp(self, node):
-        self.write("{")
-        self.traverse(node.elt)
-        for gen in node.generators:
-            self.traverse(gen)
-        self.write("}")
+        with self.delimit("{}"):
+            self.traverse(node.elt)
+            for gen in node.generators:
+                self.traverse(gen)
 
     def visit_DictComp(self, node):
-        self.write("{")
-        self.traverse(node.key)
-        self.write(": ")
-        self.traverse(node.value)
-        for gen in node.generators:
-            self.traverse(gen)
-        self.write("}")
+        with self.delimit("{}"):
+            self.traverse(node.key)
+            self.write(": ")
+            self.traverse(node.value)
+            for gen in node.generators:
+                self.traverse(gen)
 
     def visit_comprehension(self, node):
         if node.is_async:
@@ -993,24 +1003,20 @@ def visit_comprehension(self, node):
             self.traverse(if_clause)
 
     def visit_IfExp(self, node):
-        self.write("(")
-        self.traverse(node.body)
-        self.write(" if ")
-        self.traverse(node.test)
-        self.write(" else ")
-        self.traverse(node.orelse)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.body)
+            self.write(" if ")
+            self.traverse(node.test)
+            self.write(" else ")
+            self.traverse(node.orelse)
 
     def visit_Set(self, node):
         if not node.elts:
             raise ValueError("Set node should has at least one item")
-        self.write("{")
-        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
-        self.write("}")
+        with self.delimit("{}"):
+            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
 
     def visit_Dict(self, node):
-        self.write("{")
-
         def write_key_value_pair(k, v):
             self.traverse(k)
             self.write(": ")
@@ -1026,29 +1032,27 @@ def write_item(item):
             else:
                 write_key_value_pair(k, v)
 
-        self.interleave(
-            lambda: self.write(", "), write_item, zip(node.keys, node.values)
-        )
-        self.write("}")
+        with self.delimit("{}"):
+            self.interleave(
+                lambda: self.write(", "), write_item, zip(node.keys, node.values)
+            )
 
     def visit_Tuple(self, node):
-        self.write("(")
-        if len(node.elts) == 1:
-            elt = node.elts[0]
-            self.traverse(elt)
-            self.write(",")
-        else:
-            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
-        self.write(")")
+        with self.delimit():
+            if len(node.elts) == 1:
+                elt = node.elts[0]
+                self.traverse(elt)
+                self.write(",")
+            else:
+                self.interleave(lambda: self.write(", "), self.traverse, node.elts)
 
     unop = {"Invert": "~", "Not": "not", "UAdd": "+", "USub": "-"}
 
     def visit_UnaryOp(self, node):
-        self.write("(")
-        self.write(self.unop[node.op.__class__.__name__])
-        self.write(" ")
-        self.traverse(node.operand)
-        self.write(")")
+        with self.delimit():
+            self.write(self.unop[node.op.__class__.__name__])
+            self.write(" ")
+            self.traverse(node.operand)
 
     binop = {
         "Add": "+",
@@ -1067,11 +1071,10 @@ def visit_UnaryOp(self, node):
     }
 
     def visit_BinOp(self, node):
-        self.write("(")
-        self.traverse(node.left)
-        self.write(" " + self.binop[node.op.__class__.__name__] + " ")
-        self.traverse(node.right)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.left)
+            self.write(" " + self.binop[node.op.__class__.__name__] + " ")
+            self.traverse(node.right)
 
     cmpops = {
         "Eq": "==",
@@ -1087,20 +1090,18 @@ def visit_BinOp(self, node):
     }
 
     def visit_Compare(self, node):
-        self.write("(")
-        self.traverse(node.left)
-        for o, e in zip(node.ops, node.comparators):
-            self.write(" " + self.cmpops[o.__class__.__name__] + " ")
-            self.traverse(e)
-        self.write(")")
+        with self.delimit():
+            self.traverse(node.left)
+            for o, e in zip(node.ops, node.comparators):
+                self.write(" " + self.cmpops[o.__class__.__name__] + " ")
+                self.traverse(e)
 
-    boolops = {And: "and", Or: "or"}
+    boolops = {"And": "and", "Or": "or"}
 
     def visit_BoolOp(self, node):
-        self.write("(")
-        s = " %s " % self.boolops[node.op.__class__]
-        self.interleave(lambda: self.write(s), self.traverse, node.values)
-        self.write(")")
+        with self.delimit():
+            s = " %s " % self.boolops[node.op.__class__.__name__]
+            self.interleave(lambda: self.write(s), self.traverse, node.values)
 
     def visit_Attribute(self, node):
         self.traverse(node.value)
@@ -1114,27 +1115,25 @@ def visit_Attribute(self, node):
 
     def visit_Call(self, node):
         self.traverse(node.func)
-        self.write("(")
-        comma = False
-        for e in node.args:
-            if comma:
-                self.write(", ")
-            else:
-                comma = True
-            self.traverse(e)
-        for e in node.keywords:
-            if comma:
-                self.write(", ")
-            else:
-                comma = True
-            self.traverse(e)
-        self.write(")")
+        with self.delimit():
+            comma = False
+            for e in node.args:
+                if comma:
+                    self.write(", ")
+                else:
+                    comma = True
+                self.traverse(e)
+            for e in node.keywords:
+                if comma:
+                    self.write(", ")
+                else:
+                    comma = True
+                self.traverse(e)
 
     def visit_Subscript(self, node):
         self.traverse(node.value)
-        self.write("[")
-        self.traverse(node.slice)
-        self.write("]")
+        with self.delimit("[]"):
+            self.traverse(node.slice)
 
     def visit_Starred(self, node):
         self.write("*")
@@ -1228,12 +1227,11 @@ def visit_keyword(self, node):
         self.traverse(node.value)
 
     def visit_Lambda(self, node):
-        self.write("(")
-        self.write("lambda ")
-        self.traverse(node.args)
-        self.write(": ")
-        self.traverse(node.body)
-        self.write(")")
+        with self.delimit():
+            self.write("lambda ")
+            self.traverse(node.args)
+            self.write(": ")
+            self.traverse(node.body)
 
     def visit_alias(self, node):
         self.write(node.name)
diff --git a/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst b/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst
new file mode 100644
index 000000000000000..1287081d0bfba89
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst
@@ -0,0 +1 @@
+Refactor delimiting in ``ast._Unparse`` with context managers.

From ccbafcce0e600e860eaf63ceaee41e632693bf3a Mon Sep 17 00:00:00 2001
From: isidentical <batuhanosmantaskaya@gmail.com>
Date: Mon, 16 Dec 2019 10:38:10 +0300
Subject: [PATCH 2/5] remove delimit default

---
 Lib/ast.py | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/Lib/ast.py b/Lib/ast.py
index a2b8815c79f3da2..387e8021e04a7e7 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -628,13 +628,13 @@ def __enter__(self):
         def __exit__(self, exc_type, exc_value, traceback):
             self.unparser.write(self.delimiter[-1])
 
-    def delimit_if(self, condition, delimiter = "()"):
+    def delimit_if(self, condition, delimiter):
         if condition:
             return self._Delimit(self, delimiter)
         else:
             return self._NoDelimit()
 
-    def delimit(self, delimiter = "()"):
+    def delimit(self, delimiter):
         return self._Delimit(self, delimiter)
 
     def traverse(self, node):
@@ -660,7 +660,7 @@ def visit_Expr(self, node):
         self.traverse(node.value)
 
     def visit_NamedExpr(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.target)
             self.write(" := ")
             self.traverse(node.value)
@@ -735,21 +735,21 @@ def visit_Nonlocal(self, node):
         self.interleave(lambda: self.write(", "), self.write, node.names)
 
     def visit_Await(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.write("await")
             if node.value:
                 self.write(" ")
                 self.traverse(node.value)
 
     def visit_Yield(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.write("yield")
             if node.value:
                 self.write(" ")
                 self.traverse(node.value)
 
     def visit_YieldFrom(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.write("yield from")
             if node.value:
                 self.write(" ")
@@ -799,7 +799,7 @@ def visit_ClassDef(self, node):
             self.fill("@")
             self.traverse(deco)
         self.fill("class " + node.name)
-        with self.delimit():
+        with self.delimit("()"):
             comma = False
             for e in node.bases:
                 if comma:
@@ -830,7 +830,7 @@ def __FunctionDef_helper(self, node, fill_suffix):
             self.traverse(deco)
         def_str = fill_suffix + " " + node.name
         self.fill(def_str)
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.args)
         if node.returns:
             self.write(" -> ")
@@ -947,7 +947,7 @@ def _write_constant(self, value):
     def visit_Constant(self, node):
         value = node.value
         if isinstance(value, tuple):
-            with self.delimit():
+            with self.delimit("()"):
                 if len(value) == 1:
                     self._write_constant(value[0])
                     self.write(",")
@@ -971,7 +971,7 @@ def visit_ListComp(self, node):
                 self.traverse(gen)
 
     def visit_GeneratorExp(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.elt)
             for gen in node.generators:
                 self.traverse(gen)
@@ -1003,7 +1003,7 @@ def visit_comprehension(self, node):
             self.traverse(if_clause)
 
     def visit_IfExp(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.body)
             self.write(" if ")
             self.traverse(node.test)
@@ -1038,7 +1038,7 @@ def write_item(item):
             )
 
     def visit_Tuple(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             if len(node.elts) == 1:
                 elt = node.elts[0]
                 self.traverse(elt)
@@ -1049,7 +1049,7 @@ def visit_Tuple(self, node):
     unop = {"Invert": "~", "Not": "not", "UAdd": "+", "USub": "-"}
 
     def visit_UnaryOp(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.write(self.unop[node.op.__class__.__name__])
             self.write(" ")
             self.traverse(node.operand)
@@ -1071,7 +1071,7 @@ def visit_UnaryOp(self, node):
     }
 
     def visit_BinOp(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.left)
             self.write(" " + self.binop[node.op.__class__.__name__] + " ")
             self.traverse(node.right)
@@ -1090,7 +1090,7 @@ def visit_BinOp(self, node):
     }
 
     def visit_Compare(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.traverse(node.left)
             for o, e in zip(node.ops, node.comparators):
                 self.write(" " + self.cmpops[o.__class__.__name__] + " ")
@@ -1099,7 +1099,7 @@ def visit_Compare(self, node):
     boolops = {"And": "and", "Or": "or"}
 
     def visit_BoolOp(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             s = " %s " % self.boolops[node.op.__class__.__name__]
             self.interleave(lambda: self.write(s), self.traverse, node.values)
 
@@ -1115,7 +1115,7 @@ def visit_Attribute(self, node):
 
     def visit_Call(self, node):
         self.traverse(node.func)
-        with self.delimit():
+        with self.delimit("()"):
             comma = False
             for e in node.args:
                 if comma:
@@ -1227,7 +1227,7 @@ def visit_keyword(self, node):
         self.traverse(node.value)
 
     def visit_Lambda(self, node):
-        with self.delimit():
+        with self.delimit("()"):
             self.write("lambda ")
             self.traverse(node.args)
             self.write(": ")

From e1cb63f36a5a718ddaa89a4cddc54608ae828459 Mon Sep 17 00:00:00 2001
From: isidentical <batuhanosmantaskaya@gmail.com>
Date: Tue, 17 Dec 2019 00:48:19 +0300
Subject: [PATCH 3/5] refactor context manager with contextlib

---
 Lib/ast.py | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/Lib/ast.py b/Lib/ast.py
index 387e8021e04a7e7..db06dff07750935 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -26,6 +26,7 @@
 """
 import sys
 from _ast import *
+from contextlib import contextmanager, nullcontext
 
 
 def parse(source, filename='<unknown>', mode='exec', *,
@@ -613,29 +614,20 @@ def __exit__(self, exc_type, exc_value, traceback):
     def block(self):
         return self._Block(self)
 
-    class _NoDelimit:
-        def __enter__(self): pass
-        def __exit__(self, *args): pass
-
-    class _Delimit:
+    @contextmanager
+    def delimit(self, delimiter):
         """A context manager for preparing the source for expressions. It adds
         start of the delimiter  and enters, after exit it adds delimiter end."""
-        def __init__(self, unparser, delimiter):
-            self.unparser = unparser
-            self.delimiter = delimiter
-        def __enter__(self):
-            self.unparser.write(self.delimiter[0])
-        def __exit__(self, exc_type, exc_value, traceback):
-            self.unparser.write(self.delimiter[-1])
+
+        self.write(delimiter[0])
+        yield
+        self.write(delimiter[-1])
 
     def delimit_if(self, condition, delimiter):
         if condition:
-            return self._Delimit(self, delimiter)
+            return self.delimit(delimiter)
         else:
-            return self._NoDelimit()
-
-    def delimit(self, delimiter):
-        return self._Delimit(self, delimiter)
+            return nullcontext()
 
     def traverse(self, node):
         if isinstance(node, list):

From a4cc6305206daf2522d9868ace0a997fb3c6126d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Batuhan=20Ta=C5=9Fkaya?=
 <47358913+isidentical@users.noreply.github.com>
Date: Tue, 17 Dec 2019 11:11:57 +0300
Subject: [PATCH 4/5] documentaton improvements

Co-Authored-By: Victor Stinner <vstinner@python.org>
---
 Lib/ast.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/ast.py b/Lib/ast.py
index db06dff07750935..3dbe314563c51b3 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -617,7 +617,7 @@ def block(self):
     @contextmanager
     def delimit(self, delimiter):
         """A context manager for preparing the source for expressions. It adds
-        start of the delimiter  and enters, after exit it adds delimiter end."""
+        the first *delimiter* character at enters, adds the last *delimiter* character at exit."""
 
         self.write(delimiter[0])
         yield

From 602704065a7a7ca7af9c2acbe6792338068f558d Mon Sep 17 00:00:00 2001
From: isidentical <batuhanosmantaskaya@gmail.com>
Date: Mon, 23 Dec 2019 18:34:27 +0300
Subject: [PATCH 5/5] instead of a whole string, use start= and end= parameters
 to take delimiter, remove news entry

---
 Lib/ast.py                                    | 60 +++++++++----------
 .../2019-12-15-14-07-16.bpo-38870.8D28DB.rst  |  1 -
 2 files changed, 30 insertions(+), 31 deletions(-)
 delete mode 100644 Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst

diff --git a/Lib/ast.py b/Lib/ast.py
index db06dff07750935..355afc9a6250392 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -615,17 +615,17 @@ def block(self):
         return self._Block(self)
 
     @contextmanager
-    def delimit(self, delimiter):
+    def delimit(self, start, end):
         """A context manager for preparing the source for expressions. It adds
-        start of the delimiter  and enters, after exit it adds delimiter end."""
+        ``start`` to the buffer and enters, after exit it adds ``end``."""
 
-        self.write(delimiter[0])
+        self.write(start)
         yield
-        self.write(delimiter[-1])
+        self.write(end)
 
-    def delimit_if(self, condition, delimiter):
+    def delimit_if(self, start, end, condition):
         if condition:
-            return self.delimit(delimiter)
+            return self.delimit(start, end)
         else:
             return nullcontext()
 
@@ -652,7 +652,7 @@ def visit_Expr(self, node):
         self.traverse(node.value)
 
     def visit_NamedExpr(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.target)
             self.write(" := ")
             self.traverse(node.value)
@@ -684,7 +684,7 @@ def visit_AugAssign(self, node):
 
     def visit_AnnAssign(self, node):
         self.fill()
-        with self.delimit_if(not node.simple and isinstance(node.target, Name)):
+        with self.delimit_if("(", ")", not node.simple and isinstance(node.target, Name)):
             self.traverse(node.target)
         self.write(": ")
         self.traverse(node.annotation)
@@ -727,21 +727,21 @@ def visit_Nonlocal(self, node):
         self.interleave(lambda: self.write(", "), self.write, node.names)
 
     def visit_Await(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.write("await")
             if node.value:
                 self.write(" ")
                 self.traverse(node.value)
 
     def visit_Yield(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.write("yield")
             if node.value:
                 self.write(" ")
                 self.traverse(node.value)
 
     def visit_YieldFrom(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.write("yield from")
             if node.value:
                 self.write(" ")
@@ -791,7 +791,7 @@ def visit_ClassDef(self, node):
             self.fill("@")
             self.traverse(deco)
         self.fill("class " + node.name)
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             comma = False
             for e in node.bases:
                 if comma:
@@ -822,7 +822,7 @@ def __FunctionDef_helper(self, node, fill_suffix):
             self.traverse(deco)
         def_str = fill_suffix + " " + node.name
         self.fill(def_str)
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.args)
         if node.returns:
             self.write(" -> ")
@@ -939,7 +939,7 @@ def _write_constant(self, value):
     def visit_Constant(self, node):
         value = node.value
         if isinstance(value, tuple):
-            with self.delimit("()"):
+            with self.delimit("(", ")"):
                 if len(value) == 1:
                     self._write_constant(value[0])
                     self.write(",")
@@ -953,29 +953,29 @@ def visit_Constant(self, node):
             self._write_constant(node.value)
 
     def visit_List(self, node):
-        with self.delimit("[]"):
+        with self.delimit("[", "]"):
             self.interleave(lambda: self.write(", "), self.traverse, node.elts)
 
     def visit_ListComp(self, node):
-        with self.delimit("[]"):
+        with self.delimit("[", "]"):
             self.traverse(node.elt)
             for gen in node.generators:
                 self.traverse(gen)
 
     def visit_GeneratorExp(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.elt)
             for gen in node.generators:
                 self.traverse(gen)
 
     def visit_SetComp(self, node):
-        with self.delimit("{}"):
+        with self.delimit("{", "}"):
             self.traverse(node.elt)
             for gen in node.generators:
                 self.traverse(gen)
 
     def visit_DictComp(self, node):
-        with self.delimit("{}"):
+        with self.delimit("{", "}"):
             self.traverse(node.key)
             self.write(": ")
             self.traverse(node.value)
@@ -995,7 +995,7 @@ def visit_comprehension(self, node):
             self.traverse(if_clause)
 
     def visit_IfExp(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.body)
             self.write(" if ")
             self.traverse(node.test)
@@ -1005,7 +1005,7 @@ def visit_IfExp(self, node):
     def visit_Set(self, node):
         if not node.elts:
             raise ValueError("Set node should has at least one item")
-        with self.delimit("{}"):
+        with self.delimit("{", "}"):
             self.interleave(lambda: self.write(", "), self.traverse, node.elts)
 
     def visit_Dict(self, node):
@@ -1024,13 +1024,13 @@ def write_item(item):
             else:
                 write_key_value_pair(k, v)
 
-        with self.delimit("{}"):
+        with self.delimit("{", "}"):
             self.interleave(
                 lambda: self.write(", "), write_item, zip(node.keys, node.values)
             )
 
     def visit_Tuple(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             if len(node.elts) == 1:
                 elt = node.elts[0]
                 self.traverse(elt)
@@ -1041,7 +1041,7 @@ def visit_Tuple(self, node):
     unop = {"Invert": "~", "Not": "not", "UAdd": "+", "USub": "-"}
 
     def visit_UnaryOp(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.write(self.unop[node.op.__class__.__name__])
             self.write(" ")
             self.traverse(node.operand)
@@ -1063,7 +1063,7 @@ def visit_UnaryOp(self, node):
     }
 
     def visit_BinOp(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.left)
             self.write(" " + self.binop[node.op.__class__.__name__] + " ")
             self.traverse(node.right)
@@ -1082,7 +1082,7 @@ def visit_BinOp(self, node):
     }
 
     def visit_Compare(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.traverse(node.left)
             for o, e in zip(node.ops, node.comparators):
                 self.write(" " + self.cmpops[o.__class__.__name__] + " ")
@@ -1091,7 +1091,7 @@ def visit_Compare(self, node):
     boolops = {"And": "and", "Or": "or"}
 
     def visit_BoolOp(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             s = " %s " % self.boolops[node.op.__class__.__name__]
             self.interleave(lambda: self.write(s), self.traverse, node.values)
 
@@ -1107,7 +1107,7 @@ def visit_Attribute(self, node):
 
     def visit_Call(self, node):
         self.traverse(node.func)
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             comma = False
             for e in node.args:
                 if comma:
@@ -1124,7 +1124,7 @@ def visit_Call(self, node):
 
     def visit_Subscript(self, node):
         self.traverse(node.value)
-        with self.delimit("[]"):
+        with self.delimit("[", "]"):
             self.traverse(node.slice)
 
     def visit_Starred(self, node):
@@ -1219,7 +1219,7 @@ def visit_keyword(self, node):
         self.traverse(node.value)
 
     def visit_Lambda(self, node):
-        with self.delimit("()"):
+        with self.delimit("(", ")"):
             self.write("lambda ")
             self.traverse(node.args)
             self.write(": ")
diff --git a/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst b/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst
deleted file mode 100644
index 1287081d0bfba89..000000000000000
--- a/Misc/NEWS.d/next/Library/2019-12-15-14-07-16.bpo-38870.8D28DB.rst
+++ /dev/null
@@ -1 +0,0 @@
-Refactor delimiting in ``ast._Unparse`` with context managers.