LN invoices: support msat precision

fixes #6250

electrum/lnaddr.py

@@ -6,6 +6,7 @@ import time
 from hashlib import sha256
 from binascii import hexlify
 from decimal import Decimal
+from typing import Optional
 
 import bitstring
 
@@ -33,7 +34,7 @@ def shorten_amount(amount):
             break
     return str(amount) + unit
 
-def unshorten_amount(amount):
+def unshorten_amount(amount) -> Decimal:
     """ Given a shortened amount, convert it into a decimal
     """
     # BOLT #11:
@@ -271,12 +272,20 @@ class LnAddr(object):
         self.signature = None
         self.pubkey = None
         self.currency = constants.net.SEGWIT_HRP if currency is None else currency
-        self.amount = amount  # in bitcoins
+        self.amount = amount  # type: Optional[Decimal]  # in bitcoins
         self._min_final_cltv_expiry = 9
 
-    def get_amount_sat(self):
+    def get_amount_sat(self) -> Optional[Decimal]:
+        # note that this has msat resolution potentially
+        if self.amount is None:
+            return None
         return self.amount * COIN
 
+    def get_amount_msat(self) -> Optional[int]:
+        if self.amount is None:
+            return None
+        return int(self.amount * COIN * 1000)
+
     def __str__(self):
         return "LnAddr[{}, amount={}{} tags=[{}]]".format(
             hexlify(self.pubkey.serialize()).decode('utf-8') if self.pubkey else None,