some pr comments

ops/pebble.py

@@ -97,7 +97,8 @@ class _Writable(Protocol):
         # but we can't import that type
         def write(self, buf: Union[bytes, str, bytearray]) -> int: ...  # noqa
 
-    _AnyStrFileLikeIO = typing.Union[_FileLikeIO[bytes], _FileLikeIO[str]]
+    _AnyStrFileLikeIO = Union[_FileLikeIO[bytes], _FileLikeIO[str]]
+    _TextOrBinaryIO = Union[TextIO, BinaryIO]
     _IOSource = Union[str, bytes, _AnyStrFileLikeIO]
     _PartType = Literal['response', 'files']
     _Ctype = Literal['multipart/form-data', 'application/json']
@@ -2184,8 +2185,8 @@ def exec(
         group_id: Optional[int] = None,
         group: Optional[str] = None,
         stdin: Optional['_IOSource'] = None,
-        stdout: Optional[Union[TextIO, BinaryIO]] = None,
-        stderr: Optional[Union[TextIO, BinaryIO]] = None,
+        stdout: Optional['_TextOrBinaryIO'] = None,
+        stderr: Optional['_TextOrBinaryIO'] = None,
         encoding: Optional[str] = 'utf-8',
         combine_stderr: bool = False
     ) -> ExecProcess:
@@ -2584,24 +2585,24 @@ def _prepare_tempfile(self, filename: str):
     def _get_open_tempfile(self):
         return self._files[self.current_filename]
 
-    def get_response(self) -> '_FilesResponse':
+    def get_response(self) -> Optional['_FilesResponse']:
         """Return the deserialized JSON object from the multipart "response" field."""
-        assert self._response, 'response unset'
         return self._response
 
     def filenames(self):
         """Return a list of filenames from the "files" parts of the response."""
         return list(self._files.keys())
 
-    def get_file(self, path: str, encoding: Optional[str]) -> Union[TextIO, BinaryIO]:
+    def get_file(self, path: str, encoding: Optional[str]) -> '_TextOrBinaryIO':
         """Return an open file object containing the data."""
         mode = 'r' if encoding else 'rb'
         # We're using text-based file I/O purely for file encoding purposes, not for
         # newline normalization.  newline='' serves the line endings as-is.
         newline = '' if encoding else None
-        return open(self._files[path].name, mode,  # type: ignore
-                    encoding=encoding, newline=newline)
-
+        file_io = open(self._files[path].name, mode,
+                       encoding=encoding, newline=newline)
+        # open() returns IO[Any]
+        return typing.cast('_TextOrBinaryIO', file_io)
 
 class _MultipartParser:
     def __init__(