Make flags_into_string deterministic.

This is (1) to match the c++ behavior and (2) so that build based upon the flag
values are also deterministic.

PiperOrigin-RevId: 222884822

absl/flags/_flagvalues.py

@@ -1178,11 +1178,15 @@ def flags_into_string(self):
 
     Returns:
       str, the string with the flags assignments from this FlagValues object.
+      The flags are ordered by (module_name, flag_name).
     """
+    module_flags = sorted(self.flags_by_module_dict().items())
     s = ''
-    for flag in self._flags().values():
-      if flag.value is not None:
-        s += flag.serialize() + '\n'
+    for unused_module_name, flags in module_flags:
+      flags = sorted(flags, key=lambda f: f.name)
+      for flag in flags:
+        if flag.value is not None:
+          s += flag.serialize() + '\n'
     return s
 
   def append_flags_into_file(self, filename):

absl/flags/tests/_flagvalues_test.py

@@ -18,6 +18,7 @@
 from __future__ import division
 from __future__ import print_function
 
+import collections
 import copy
 import pickle
 import types
@@ -526,6 +527,33 @@ def test_flags_dir(self):
     self.assertEqual(
         sorted([flag_name1, flag_name2, flag_name3]), dir(flag_values))
 
+  def test_flags_into_string_deterministic(self):
+    flag_values = _flagvalues.FlagValues()
+    _defines.DEFINE_string(
+        'fa', 'x', '', flag_values=flag_values, module_name='mb')
+    _defines.DEFINE_string(
+        'fb', 'x', '', flag_values=flag_values, module_name='mb')
+    _defines.DEFINE_string(
+        'fc', 'x', '', flag_values=flag_values, module_name='ma')
+    _defines.DEFINE_string(
+        'fd', 'x', '', flag_values=flag_values, module_name='ma')
+
+    expected = ('--fc=x\n'
+                '--fd=x\n'
+                '--fa=x\n'
+                '--fb=x\n')
+
+    flags_by_module_items = sorted(
+        flag_values.flags_by_module_dict().items(), reverse=True)
+    for _, module_flags in flags_by_module_items:
+      module_flags.sort(reverse=True)
+
+    flag_values.__dict__['__flags_by_module'] = collections.OrderedDict(
+        flags_by_module_items)
+
+    actual = flag_values.flags_into_string()
+    self.assertEqual(expected, actual)
+
 
 class FlagValuesLoggingTest(absltest.TestCase):
   """Test to make sure logging.* functions won't recurse.