transaction.serialize_preimage: trivial clean-up

electrum/transaction.py

@@ -958,14 +958,13 @@ class Transaction:
         s += script
         return s
 
-    def serialize_preimage(self, i):
+    def serialize_preimage(self, txin_index: int) -> str:
         nVersion = int_to_hex(self.version, 4)
-        nHashType = int_to_hex(1, 4)
+        nHashType = int_to_hex(1, 4)  # SIGHASH_ALL
         nLocktime = int_to_hex(self.locktime, 4)
         inputs = self.inputs()
         outputs = self.outputs()
-        txin = inputs[i]
-        # TODO: py3 hex
+        txin = inputs[txin_index]
         if self.is_segwit_input(txin):
             hashPrevouts = bh2u(sha256d(bfh(''.join(self.serialize_outpoint(txin) for txin in inputs))))
             hashSequence = bh2u(sha256d(bfh(''.join(int_to_hex(txin.get('sequence', 0xffffffff - 1), 4) for txin in inputs))))
@@ -977,7 +976,8 @@ class Transaction:
             nSequence = int_to_hex(txin.get('sequence', 0xffffffff - 1), 4)
             preimage = nVersion + hashPrevouts + hashSequence + outpoint + scriptCode + amount + nSequence + hashOutputs + nLocktime + nHashType
         else:
-            txins = var_int(len(inputs)) + ''.join(self.serialize_input(txin, self.get_preimage_script(txin) if i==k else '') for k, txin in enumerate(inputs))
+            txins = var_int(len(inputs)) + ''.join(self.serialize_input(txin, self.get_preimage_script(txin) if txin_index==k else '')
+                                                   for k, txin in enumerate(inputs))
             txouts = var_int(len(outputs)) + ''.join(self.serialize_output(o) for o in outputs)
             preimage = nVersion + txins + txouts + nLocktime + nHashType
         return preimage