cert_audit: Code refinement

This commit is a collection of code refinements
from review comments.

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>

tests/scripts/audit-validity-dates.py

@@ -86,7 +86,12 @@ class X509Parser:
         DataType.CSR: 'CERTIFICATE REQUEST'
     }
 
-    def __init__(self, backends: dict):
+    def __init__(self,
+                 backends:
+                 typing.Dict[DataType,
+                             typing.Dict[DataFormat,
+                                         typing.Callable[[bytes], object]]]) \
+    -> None:
         self.backends = backends
         self.__generate_parsers()
 
@@ -122,7 +127,7 @@ class X509Parser:
         return self.parsers[item]
 
     @staticmethod
-    def pem_data_type(data: bytes) -> str:
+    def pem_data_type(data: bytes) -> typing.Optional[str]:
         """Get the tag from the data in PEM format
 
         :param data: data to be checked in binary mode.
@@ -132,7 +137,7 @@ class X509Parser:
         if m is not None:
             return m.group('type').decode('UTF-8')
         else:
-            return ""
+            return None
 
     @staticmethod
     def check_hex_string(hex_str: str) -> bool:
@@ -165,6 +170,7 @@ class Auditor:
     def __init__(self, verbose):
         self.verbose = verbose
         self.default_files = []
+        # A list to store the parsed audit_data.
         self.audit_data = []
         self.parser = X509Parser({
             DataType.CRT: {
@@ -198,12 +204,12 @@ class Auditor:
         """
         with open(filename, 'rb') as f:
             data = f.read()
-        result_list = []
         result = self.parse_bytes(data)
         if result is not None:
             result.location = filename
-            result_list.append(result)
-        return result_list
+            return [result]
+        else:
+            return []
 
     def parse_bytes(self, data: bytes):
         """Parse AuditData from bytes."""
@@ -218,11 +224,11 @@ class Auditor:
                 return audit_data
         return None
 
-    def walk_all(self, file_list):
+    def walk_all(self, file_list: typing.Optional[typing.List[str]] = None):
         """
         Iterate over all the files in the list and get audit data.
         """
-        if not file_list:
+        if file_list is None:
             file_list = self.default_files
         for filename in file_list:
             data_list = self.parse_file(filename)
@@ -250,11 +256,9 @@ class TestDataAuditor(Auditor):
     def collect_default_files(self):
         """Collect all files in tests/data_files/"""
         test_dir = self.find_test_dir()
-        test_data_folder = os.path.join(test_dir, 'data_files')
-        data_files = []
-        for (dir_path, _, file_names) in os.walk(test_data_folder):
-            data_files.extend(os.path.join(dir_path, file_name)
-                              for file_name in file_names)
+        test_data_glob = os.path.join(test_dir, 'data_files/**')
+        data_files = [f for f in glob.glob(test_data_glob, recursive=True)
+                      if os.path.isfile(f)]
         return data_files
 
 class FileWrapper():