bpo-43521: Allow ast.unparse with empty sets and NaN

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):

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))
 

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.