.accounting: type `Transaction.etype` as a `Literal`

Start working out the set of possible "txn types" we want to define in
a simple set.

Relates to #510

piker/accounting/__init__.py

@@ -88,7 +88,7 @@ def get_likely_pair(
 
     '''
     try:
-        src_name_start = bs_mktid.rindex(src)
+        src_name_start: str = bs_mktid.rindex(src)
     except (
         ValueError,   # substr not found
     ):
@@ -99,8 +99,8 @@ def get_likely_pair(
         # log.warning(
         #     f'No src fiat {src} found in {bs_mktid}?'
         # )
-        return
+        return None
 
-    likely_dst = bs_mktid[:src_name_start]
+    likely_dst: str = bs_mktid[:src_name_start]
     if likely_dst == dst:
         return bs_mktid

piker/accounting/_ledger.py

@@ -28,6 +28,7 @@ from typing import (
     Any,
     Callable,
     Generator,
+    Literal,
     TYPE_CHECKING,
 )
 
@@ -51,6 +52,18 @@ if TYPE_CHECKING:
 log = get_logger(__name__)
 
 
+TxnType = Literal[
+    'clear',
+    'transfer',
+
+    # TODO: see https://github.com/pikers/piker/issues/510
+    # 'split',
+    # 'rename',
+    # 'resize',
+    # 'removal',
+]
+
+
 class Transaction(Struct, frozen=True):
 
     # NOTE: this is a unified acronym also used in our `MktPair`
@@ -70,10 +83,9 @@ class Transaction(Struct, frozen=True):
     cost: float  # commisions or other additional costs
     dt: DateTime
 
-    # the "event type" in terms of "market events" see
-    # https://github.com/pikers/piker/issues/510 for where we're
-    # probably going with this.
-    etype: str = 'clear'
+    # the "event type" in terms of "market events" see above and
+    # https://github.com/pikers/piker/issues/510
+    etype: TxnType = 'clear'
 
     # TODO: we can drop this right since we
     # can instead expect the backend to provide this