Fix #199: Correctly checking _inherited_absl_flags by comparing to …

…`None` instead of implicit bool.

The documented contract is to skip absl flags when `inherited_absl_flags` is passed `None`. A `FlagValues` instance without any flags evaluates to `False` and shouldn't let it skip.

PiperOrigin-RevId: 578025536

CHANGELOG.md

@@ -16,6 +16,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com).
     tests equality of `dataclass.dataclass` objects with better error messages
     when the assert fails.
 
+### Changed
+
+*   (flags) `absl.flags.argparse_flags.ArgumentParser` now correctly inherits
+    an empty instance of `FlagValues` and ensure absl flags, such as
+    `--flagfile`, `--undefok` are supported.
+
 ### Fixed
 
 *   The flag `foo` no longer retains the value `bar` after `FLAGS.foo = bar`

absl/flags/argparse_flags.py

@@ -145,15 +145,15 @@ def __init__(self, **kwargs):
           '--helpfull', action=_HelpFullAction,
           default=argparse.SUPPRESS, help='show full help message and exit')
 
-    if self._inherited_absl_flags:
+    if self._inherited_absl_flags is not None:
       self.add_argument(
           '--undefok', default=argparse.SUPPRESS, help=argparse.SUPPRESS)
       self._define_absl_flags(self._inherited_absl_flags)
 
   def parse_known_args(self, args=None, namespace=None):
     if args is None:
       args = sys.argv[1:]
-    if self._inherited_absl_flags:
+    if self._inherited_absl_flags is not None:
       # Handle --flagfile.
       # Explicitly specify force_gnu=True, since argparse behaves like
       # gnu_getopt: flags can be specified after positional arguments.
@@ -172,7 +172,7 @@ def parse_known_args(self, args=None, namespace=None):
     if undefok is not undefok_missing:
       namespace.undefok = undefok
 
-    if self._inherited_absl_flags:
+    if self._inherited_absl_flags is not None:
       # Handle --undefok. At this point, `args` only contains unknown flags,
       # so it won't strip defined flags that are also specified with --undefok.
       # For Python <= 2.7.8: https://bugs.python.org/issue9351, a bug where
@@ -350,7 +350,7 @@ def __call__(self, parser, namespace, values, option_string=None):
     parser.print_help()
 
     absl_flags = parser._inherited_absl_flags  # pylint: disable=protected-access
-    if absl_flags:
+    if absl_flags is not None:
       modules = sorted(absl_flags.flags_by_module_dict())
       main_module = sys.argv[0]
       if main_module in modules:

absl/flags/tests/argparse_flags_test.py

@@ -406,6 +406,21 @@ def test_argument_default(self):
     args = parser.parse_args([])
     self.assertEqual(args.magic_number, 23)
 
+  def test_empty_inherited_absl_flags(self):
+    parser = argparse_flags.ArgumentParser(
+        inherited_absl_flags=flags.FlagValues()
+    )
+    parser.add_argument('--foo')
+    flagfile = self.create_tempfile(content='--foo=bar').full_path
+    # Make sure these flags are still available when inheriting an empty
+    # FlagValues instance.
+    ns = parser.parse_args([
+        '--undefok=undefined_flag',
+        '--undefined_flag=value',
+        '--flagfile=' + flagfile,
+    ])
+    self.assertEqual(ns.foo, 'bar')
+
 
 class ArgparseWithAppRunTest(parameterized.TestCase):