Fix interfaces_file for proper file contents (#37818)

The generated file was completely unusable by the system
therefore the fix which ensures that diffing the file
prior to changes and after only shows diffs

Furthermore the code did not work for Python 3.6
>       f.writelines(to_bytes(lines, errors='surrogate_or_strict'))
E       TypeError: a bytes-like object is required, not 'int'

The other modifications (lambda variable renaming) is to
comply with default flake8 rules

test/units/modules/system/interfaces_file/test_interfaces_file.py

@@ -16,14 +16,13 @@
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
 
 from ansible.compat.tests import unittest
-import ansible.module_utils.basic
 from ansible.modules.system import interfaces_file
 import os
 import json
-import sys
 import io
 import inspect
-import json
+from shutil import copyfile, move
+import difflib
 
 
 class AnsibleFailJson(Exception):
@@ -31,6 +30,14 @@ class AnsibleFailJson(Exception):
 
 
 class ModuleMocked():
+    def atomic_move(self, src, dst):
+        move(src, dst)
+
+    def backup_local(self, path):
+        backupp = os.path.join("/tmp", os.path.basename(path) + ".bak")
+        copyfile(path, backupp)
+        return backupp
+
     def fail_json(self, msg):
         raise AnsibleFailJson(msg)
 
@@ -44,6 +51,18 @@ class TestInterfacesFileModule(unittest.TestCase):
     def getTestFiles(self):
         return next(os.walk(fixture_path))[2]
 
+    def compareFileToBackup(self, path, backup):
+        with open(path) as f1:
+            with open(backup) as f2:
+                diffs = difflib.context_diff(f1.readlines(),
+                                             f2.readlines(),
+                                             fromfile=os.path.basename(path),
+                                             tofile=os.path.basename(backup))
+        # Restore backup
+        move(backup, path)
+        deltas = [d for d in diffs]
+        self.assertTrue(len(deltas) == 0)
+
     def compareInterfacesLinesToFile(self, interfaces_lines, path, testname=None):
         if not testname:
             testname = "%s.%s" % (path, inspect.stack()[1][3])
@@ -137,3 +156,35 @@ class TestInterfacesFileModule(unittest.TestCase):
 
                 self.compareInterfacesLinesToFile(lines, testfile, "%s_%s" % (testfile, testname))
                 self.compareInterfacesToFile(ifaces, testfile, "%s_%s.json" % (testfile, testname))
+
+    def test_revert(self):
+        testcases = {
+            "revert": [
+                {
+                    'iface': 'eth0',
+                    'option': 'mtu',
+                    'value': '1350',
+                }
+            ],
+        }
+        for testname, options_list in testcases.items():
+            for testfile in self.getTestFiles():
+                path = os.path.join(fixture_path, testfile)
+                lines, ifaces = interfaces_file.read_interfaces_file(module, path)
+                backupp = module.backup_local(path)
+                options = options_list[0]
+                for state in ['present', 'absent']:
+                    fail_json_iterations = []
+                    options['state'] = state
+                    try:
+                        _, lines = interfaces_file.setInterfaceOption(module, lines, options['iface'], options['option'], options['value'], options['state'])
+                    except AnsibleFailJson as e:
+                        fail_json_iterations.append("fail_json message: %s\noptions:\n%s" %
+                                                    (str(e), json.dumps(options, sort_keys=True, indent=4, separators=(',', ': '))))
+                    interfaces_file.write_changes(module, [d['line'] for d in lines if 'line' in d], path)
+
+                self.compareStringWithFile("\n=====\n".join(fail_json_iterations), "%s_%s.exceptions.txt" % (testfile, testname))
+
+                self.compareInterfacesLinesToFile(lines, testfile, "%s_%s" % (testfile, testname))
+                self.compareInterfacesToFile(ifaces, testfile, "%s_%s.json" % (testfile, testname))
+                self.compareFileToBackup(path, backupp)