@@ -770,7 +770,7 @@ def find_cache_meta(id: str, path: str, manager: BuildManager) -> Optional[Cache
     return m
 
 
-def is_meta_fresh(meta: CacheMeta, id: str, path: str, manager: BuildManager) -> bool:
+def is_meta_fresh(meta: Optional[CacheMeta], id: str, path: str, manager: BuildManager) -> bool:
     if meta is None:
         return False
 
 
@@ -53,11 +53,11 @@ def parse_test_cases(
             while i < len(p) and p[i].id != 'case':
                 if p[i].id == 'file':
                     # Record an extra file needed for the test case.
-                    files.append((os.path.join(base_path, p[i].arg),
+                    files.append((os.path.join(base_path, p[i].arg or ''),
                                   '\n'.join(p[i].data)))
                 elif p[i].id in ('builtins', 'builtins_py2'):
                     # Use a custom source file for the std module.
-                    mpath = os.path.join(os.path.dirname(path), p[i].arg)
+                    mpath = os.path.join(os.path.dirname(path), p[i].arg or '')
                     if p[i].id == 'builtins':
                         fnam = 'builtins.pyi'
                     else:
@@ -66,15 +66,17 @@ def parse_test_cases(
                     with open(mpath) as f:
                         files.append((os.path.join(base_path, fnam), f.read()))
                 elif p[i].id == 'stale':
-                    if p[i].arg is None:
+                    arg = p[i].arg
+                    if arg is None:
                         stale_modules = set()
                     else:
-                        stale_modules = {item.strip() for item in p[i].arg.split(',')}
+                        stale_modules = {item.strip() for item in arg.split(',')}
                 elif p[i].id == 'rechecked':
-                    if p[i].arg is None:
+                    arg = p[i].arg
+                    if arg is None:
                         rechecked_modules = set()
                     else:
-                        rechecked_modules = {item.strip() for item in p[i].arg.split(',')}
+                        rechecked_modules = {item.strip() for item in arg.split(',')}
                 elif p[i].id == 'out' or p[i].id == 'out1':
                     tcout = p[i].data
                     if native_sep and os.path.sep == '\\':
@@ -95,7 +97,9 @@ def parse_test_cases(
                 # If the set of rechecked modules isn't specified, make it the same as the set of
                 # modules with a stale public interface.
                 rechecked_modules = stale_modules
-            if stale_modules is not None and not stale_modules.issubset(rechecked_modules):
+            if (stale_modules is not None
+                    and rechecked_modules is not None
+                    and not stale_modules.issubset(rechecked_modules)):
                 raise ValueError(
                     'Stale modules must be a subset of rechecked modules ({})'.format(path))
 
@@ -225,15 +229,15 @@ class TestItem:
     """
 
     id = ''
-    arg = ''
+    arg = ''  # type: Optional[str]
 
     # Text data, array of 8-bit strings
     data = None  # type: List[str]
 
     file = ''
     line = 0  # Line number in file
 
-    def __init__(self, id: str, arg: str, data: List[str], file: str,
+    def __init__(self, id: str, arg: Optional[str], data: List[str], file: str,
                  line: int) -> None:
         self.id = id
         self.arg = arg
@@ -248,8 +252,8 @@ def parse_test_data(l: List[str], fnam: str) -> List[TestItem]:
     ret = []  # type: List[TestItem]
     data = []  # type: List[str]
 
-    id = None  # type: str
-    arg = None  # type: str
+    id = None  # type: Optional[str]
+    arg = None  # type: Optional[str]
 
     i = 0
     i0 = 0
 
@@ -8,7 +8,7 @@
 import typed_ast
 import typed_ast.ast35
 
-from typing import Tuple, List, Dict, Set
+from typing import Dict, List, Optional, Set, Tuple
 
 from mypy import build, defaults
 from mypy.main import parse_version, process_options
@@ -149,15 +149,15 @@ def run_case_once(self, testcase: DataDrivenTestCase, incremental=0) -> None:
         sources = []
         for module_name, program_path, program_text in module_data:
             # Always set to none so we're forced to reread the module in incremental mode
-            program_text = None if incremental else program_text
-            sources.append(BuildSource(program_path, module_name, program_text))
+            sources.append(BuildSource(program_path, module_name,
+                                       None if incremental else program_text))
+        res = None
         try:
             res = build.build(sources=sources,
                               options=options,
                               alt_lib_path=test_temp_dir)
             a = res.errors
         except CompileError as e:
-            res = None
             a = e.messages
         a = normalize_error_messages(a)
 
@@ -191,7 +191,8 @@ def run_case_once(self, testcase: DataDrivenTestCase, incremental=0) -> None:
                     testcase.expected_stale_modules,
                     res.manager.stale_modules)
 
-    def check_module_equivalence(self, name: str, expected: Set[str], actual: Set[str]) -> None:
+    def check_module_equivalence(self, name: str,
+                                 expected: Optional[Set[str]], actual: Set[str]) -> None:
         if expected is not None:
             assert_string_arrays_equal(
                 list(sorted(expected)),
 
@@ -560,7 +560,7 @@ class CallableType(FunctionLike):
     def __init__(self,
                  arg_types: List[Type],
                  arg_kinds: List[int],
-                 arg_names: List[str],
+                 arg_names: List[Optional[str]],
                  ret_type: Type,
                  fallback: Instance,
                  name: str = None,