style(core): improve types in rlp.py

core/src/trezor/crypto/rlp.py

@@ -5,10 +5,16 @@
     from typing import Union
     from trezor.utils import Writer
 
-    # what we want:
-    # RLPItem = Union[list["RLPItem"], bytes, int]
-    # what mypy can process:
-    RLPItem = Union[list, bytes, int]
+    # The intention below is basically:
+    # RLPItem = int | bytes | list[RLPItem]
+    # That will not typecheck though. Type `list` is invariant in its parameter, meaning
+    # that we cannot pass list[bytes] into a list[RLPItem] parameter (what if the
+    # function wanted to append an int?). We do want to enforce that it's a `list`, not
+    # a generic `Sequence` (because we do isinstance checks for a list). We are however
+    # only reading from the list and passing into things that consume a RLPItem. Hence
+    # we have to enumerate single-type lists as well as the universal list[RLPItem].
+    RLPList = Union[list[int], list[bytes], list["RLPItem"]]
+    RLPItem = Union[RLPList, bytes, int]
 
 
 STRING_HEADER_BYTE = const(0x80)
@@ -80,7 +86,7 @@ def write_string(w: Writer, string: bytes) -> None:
     w.extend(string)
 
 
-def write_list(w: Writer, lst: list[RLPItem]) -> None:
+def write_list(w: Writer, lst: RLPList) -> None:
     payload_length = sum(length(item) for item in lst)
     write_header(w, payload_length, LIST_HEADER_BYTE)
     for item in lst: