From d8115844b20491c464b64cdf51ad7a456e1a6c6f Mon Sep 17 00:00:00 2001
From: Kodi Arfer <git@arfer.net>
Date: Tue, 16 Mar 2021 15:58:46 -0400
Subject: [PATCH] bpo-43521: Allow ast.unparse with empty sets and NaN

---
 Lib/ast.py                                    | 20 +++++++++++++------
 Lib/test/test_unparse.py                      | 15 +++++++++++---
 .../2021-03-16-16-05-02.bpo-43521.mRT6fh.rst  |  1 +
 3 files changed, 27 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2021-03-16-16-05-02.bpo-43521.mRT6fh.rst

diff --git a/Lib/ast.py b/Lib/ast.py
index 03ae2ec44e0992..e46ab43d5cfd30 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -1199,8 +1199,13 @@ def _write_docstring(self, node):
 
     def _write_constant(self, value):
         if isinstance(value, (float, complex)):
-            # Substitute overflowing decimal literal for AST infinities.
-            self.write(repr(value).replace("inf", _INFSTR))
+            # Substitute overflowing decimal literal for AST infinities,
+            # and inf - inf for NaNs.
+            self.write(
+                repr(value)
+                .replace("inf", _INFSTR)
+                .replace("nan", f"({_INFSTR}-{_INFSTR})")
+            )
         elif self._avoid_backslashes and isinstance(value, str):
             self._write_str_avoiding_backslashes(value)
         else:
@@ -1273,10 +1278,13 @@ def visit_IfExp(self, node):
             self.traverse(node.orelse)
 
     def visit_Set(self, node):
-        if not node.elts:
-            raise ValueError("Set node should have at least one item")
-        with self.delimit("{", "}"):
-            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
+        if node.elts:
+            with self.delimit("{", "}"):
+                self.interleave(lambda: self.write(", "), self.traverse, node.elts)
+        else:
+            # `{}` would be interpreted as a dictionary literal, and
+            # `set` might be shadowed. Thus:
+            self.write('{*()}')
 
     def visit_Dict(self, node):
         def write_key_value_pair(k, v):
diff --git a/Lib/test/test_unparse.py b/Lib/test/test_unparse.py
index c7c8613ea2793f..ce03272ad30b2d 100644
--- a/Lib/test/test_unparse.py
+++ b/Lib/test/test_unparse.py
@@ -199,6 +199,12 @@ def test_huge_float(self):
         self.check_ast_roundtrip("1e1000j")
         self.check_ast_roundtrip("-1e1000j")
 
+    def test_nan(self):
+        self.assertASTEqual(
+            ast.parse(ast.unparse(ast.Constant(value=float('nan')))),
+            ast.parse('1e1000 - 1e1000')
+        )
+
     def test_min_int(self):
         self.check_ast_roundtrip(str(-(2 ** 31)))
         self.check_ast_roundtrip(str(-(2 ** 63)))
@@ -252,6 +258,12 @@ def test_annotations(self):
     def test_set_literal(self):
         self.check_ast_roundtrip("{'a', 'b', 'c'}")
 
+    def test_empty_set(self):
+        self.assertASTEqual(
+            ast.parse(ast.unparse(ast.Set(elts=[]))),
+            ast.parse('{*()}')
+        )
+
     def test_set_comprehension(self):
         self.check_ast_roundtrip("{x for x in range(5)}")
 
@@ -326,9 +338,6 @@ def test_invalid_fstring_conversion(self):
     def test_invalid_fstring_backslash(self):
         self.check_invalid(ast.FormattedValue(value=ast.Constant(value="\\\\")))
 
-    def test_invalid_set(self):
-        self.check_invalid(ast.Set(elts=[]))
-
     def test_invalid_yield_from(self):
         self.check_invalid(ast.YieldFrom(value=None))
 
diff --git a/Misc/NEWS.d/next/Library/2021-03-16-16-05-02.bpo-43521.mRT6fh.rst b/Misc/NEWS.d/next/Library/2021-03-16-16-05-02.bpo-43521.mRT6fh.rst
new file mode 100644
index 00000000000000..e6895674437a59
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-03-16-16-05-02.bpo-43521.mRT6fh.rst
@@ -0,0 +1 @@
+``ast.unparse`` can now render NaNs and empty sets.