Issue 25180: Fix Tools/parser/unparse.py for f-strings. Patch by Martin Panter.

Lib/test/test_tools/test_unparse.py

@@ -134,6 +134,15 @@ class ASTTestCase(unittest.TestCase):
 class UnparseTestCase(ASTTestCase):
     # Tests for specific bugs found in earlier versions of unparse
 
+    def test_fstrings(self):
+        # See issue 25180
+        self.check_roundtrip(r"""f'{f"{0}"*3}'""")
+        self.check_roundtrip(r"""f'{f"{y}"*3}'""")
+        self.check_roundtrip(r"""f'{f"{\'x\'}"*3}'""")
+
+        self.check_roundtrip(r'''f"{r'x' f'{\"s\"}'}"''')
+        self.check_roundtrip(r'''f"{r'x'rf'{\"s\"}'}"''')
+
     def test_del_statement(self):
         self.check_roundtrip("del x, y, z")
 
@@ -264,8 +273,6 @@ class DirectoryTestCase(ASTTestCase):
         for d in self.test_directories:
             test_dir = os.path.join(basepath, d)
             for n in os.listdir(test_dir):
-                if n == 'test_fstring.py':
-                    continue
                 if n.endswith('.py') and not n.startswith('bad'):
                     names.append(os.path.join(test_dir, n))
 

Tools/parser/unparse.py

@@ -322,6 +322,45 @@ class Unparser:
     def _Str(self, tree):
         self.write(repr(tree.s))
 
+    def _JoinedStr(self, t):
+        self.write("f")
+        string = io.StringIO()
+        self._fstring_JoinedStr(t, string.write)
+        self.write(repr(string.getvalue()))
+
+    def _FormattedValue(self, t):
+        self.write("f")
+        string = io.StringIO()
+        self._fstring_FormattedValue(t, string.write)
+        self.write(repr(string.getvalue()))
+
+    def _fstring_JoinedStr(self, t, write):
+        for value in t.values:
+            meth = getattr(self, "_fstring_" + type(value).__name__)
+            meth(value, write)
+
+    def _fstring_Str(self, t, write):
+        value = t.s.replace("{", "{{").replace("}", "}}")
+        write(value)
+
+    def _fstring_FormattedValue(self, t, write):
+        write("{")
+        expr = io.StringIO()
+        Unparser(t.value, expr)
+        expr = expr.getvalue().rstrip("\n")
+        if expr.startswith("{"):
+            write(" ")  # Separate pair of opening brackets as "{ {"
+        write(expr)
+        if t.conversion != -1:
+            conversion = chr(t.conversion)
+            assert conversion in "sra"
+            write(f"!{conversion}")
+        if t.format_spec:
+            write(":")
+            meth = getattr(self, "_fstring_" + type(t.format_spec).__name__)
+            meth(t.format_spec, write)
+        write("}")
+
     def _Name(self, t):
         self.write(t.id)