Surface inappropriate usage of a FlagHolder object as a runtime error

With the aim of steering users away from potential pitfalls in usage of the `FlagHolder` API, this change introduces implementation overrides for the class `__eq__` and `__bool__` methods which turn any such usage into a `TypeError` at runtime.

This is to help avoid situations where a user neglects to use the `FlagHolder.value` property in cases where they intended to, e.g.,

```
>>> SOME_FLAG = flags.DEFINE_boolean(...)
>>> if SOME_FLAG: do_something()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: bool() not supported for instances of type 'FlagHolder' (did you mean to use 'FlagHolder.value' instead?)
```
PiperOrigin-RevId: 338473759
Change-Id: If3b6505a3bdca03db6d96ec7cc28e7214cb1a5e7

Author: Matt McDonald

absl/CHANGELOG.md

@@ -15,6 +15,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com).
 *   (testing) Multiple flags are now set together before their validators run.
     This resolves an issue where multi-flag validators rely on specific flag
     combinations.
+*   (flags) As a deterrent for misuse, FlagHolder objects will now raise a
+    TypeError exception when used in a conditional statement or equality
+    expression.
 
 ## 0.10.0 (2020-08-19)
 

absl/flags/_flagvalues.py

@@ -1357,6 +1357,20 @@ def __init__(self, flag_values, flag, ensure_non_none_value=False):
     # This allows future use of this for "required flags with None default"
     self._ensure_non_none_value = ensure_non_none_value
 
+  def __eq__(self, other):
+    raise TypeError(
+        "unsupported operand type(s) for ==: '{0}' and '{1}' "
+        "(did you mean to use '{0}.value' instead?)".format(
+            type(self).__name__, type(other).__name__))
+
+  def __bool__(self):
+    raise TypeError(
+        "bool() not supported for instances of type '{0}' "
+        "(did you mean to use '{0}.value' instead?)".format(
+            type(self).__name__))
+
+  __nonzero__ = __bool__
+
   @property
   def name(self):
     return self._name

absl/flags/tests/_flagvalues_test.py

@@ -914,6 +914,18 @@ def test_allow_override(self):
     self.assertEqual(3, first.value)
     self.assertEqual(3, second.value)
 
+  def test_eq(self):
+    with self.assertRaises(TypeError):
+      self.name_flag == 'value'  # pylint: disable=pointless-statement
+
+  def test_eq_reflection(self):
+    with self.assertRaises(TypeError):
+      'value' == self.name_flag  # pylint: disable=pointless-statement
+
+  def test_bool(self):
+    with self.assertRaises(TypeError):
+      bool(self.name_flag)
+
 
 if __name__ == '__main__':
   absltest.main()