From 2f7c764f73fe260ae8a5a4f2e9db2209e6862983 Mon Sep 17 00:00:00 2001
From: Ethan Furman <ethan@stoneleaf.us>
Date: Fri, 2 Oct 2020 10:19:56 -0700
Subject: [PATCH] fix format() behavior for IntFlag

---
 Lib/enum.py                                        |  2 +-
 Lib/test/test_enum.py                              | 14 ++++++++++++++
 .../2020-10-02-10-19-49.bpo-41907.wiIEsz.rst       |  1 +
 3 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-10-02-10-19-49.bpo-41907.wiIEsz.rst

diff --git a/Lib/enum.py b/Lib/enum.py
index 40ff25b9cdad37..8a837462c53db0 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -653,7 +653,7 @@ def __format__(self, format_spec):
         # the value
 
         # pure Enum branch, or branch with __str__ explicitly overridden
-        str_overridden = type(self).__str__ != Enum.__str__
+        str_overridden = type(self).__str__ not in (Enum.__str__, Flag.__str__)
         if self._member_type_ is object or str_overridden:
             cls = str
             val = str(self)
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 3431040f98a726..2cfe6cd532a348 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -585,12 +585,15 @@ def hello(self):
         class Test1Enum(MyMethodEnum, int, MyStrEnum):
             One = 1
             Two = 2
+        self.assertTrue(Test1Enum._member_type_ is int)
         self.assertEqual(str(Test1Enum.One), 'MyStr')
+        self.assertEqual(format(Test1Enum.One, ''), 'MyStr')
         #
         class Test2Enum(MyStrEnum, MyMethodEnum):
             One = 1
             Two = 2
         self.assertEqual(str(Test2Enum.One), 'MyStr')
+        self.assertEqual(format(Test1Enum.One, ''), 'MyStr')
 
     def test_inherited_data_type(self):
         class HexInt(int):
@@ -2225,6 +2228,11 @@ def test_repr(self):
         self.assertEqual(repr(~(Open.RO | Open.CE)), '<Open.AC: 3>')
         self.assertEqual(repr(~(Open.WO | Open.CE)), '<Open.RW: 2>')
 
+    def test_format(self):
+        Perm = self.Perm
+        self.assertEqual(format(Perm.R, ''), 'Perm.R')
+        self.assertEqual(format(Perm.R | Perm.X, ''), 'Perm.R|X')
+
     def test_or(self):
         Perm = self.Perm
         for i in Perm:
@@ -2564,6 +2572,7 @@ class Color(IntFlag):
 
     def test_type(self):
         Perm = self.Perm
+        self.assertTrue(Perm._member_type_ is int)
         Open = self.Open
         for f in Perm:
             self.assertTrue(isinstance(f, Perm))
@@ -2643,6 +2652,11 @@ def test_repr(self):
         self.assertEqual(repr(~(Open.WO | Open.CE)), '<Open.RW: -524290>')
         self.assertEqual(repr(Open(~4)), '<Open.CE|AC|RW|WO: -5>')
 
+    def test_format(self):
+        Perm = self.Perm
+        self.assertEqual(format(Perm.R, ''), '4')
+        self.assertEqual(format(Perm.R | Perm.X, ''), '5')
+
     def test_or(self):
         Perm = self.Perm
         for i in Perm:
diff --git a/Misc/NEWS.d/next/Library/2020-10-02-10-19-49.bpo-41907.wiIEsz.rst b/Misc/NEWS.d/next/Library/2020-10-02-10-19-49.bpo-41907.wiIEsz.rst
new file mode 100644
index 00000000000000..aa337b38046e61
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-10-02-10-19-49.bpo-41907.wiIEsz.rst
@@ -0,0 +1 @@
+fix `format()` behavior for `IntFlag`