diff --git a/electrum/exchange_rate.py b/electrum/exchange_rate.py
index fe9d800e7..89d041016 100644
--- a/electrum/exchange_rate.py
+++ b/electrum/exchange_rate.py
@@ -8,7 +8,7 @@ import time
 import csv
 import decimal
 from decimal import Decimal
-from typing import Sequence, Optional
+from typing import Sequence, Optional, Mapping, Dict, Union
 
 from aiorpcx.curio import timeout_after, TaskTimeout
 import aiohttp
@@ -41,8 +41,8 @@ class ExchangeBase(Logger):
 
     def __init__(self, on_quotes, on_history):
         Logger.__init__(self)
-        self.history = {}
-        self.quotes = {}
+        self.history = {}  # type: Dict[str, Dict[str, Union[str, float, Decimal]]]
+        self.quotes = {}  # type: Dict[str, Union[str, float, Decimal, None]]
         self.on_quotes = on_quotes
         self.on_history = on_history
 
@@ -75,7 +75,7 @@ class ExchangeBase(Logger):
     def name(self):
         return self.__class__.__name__
 
-    async def update_safe(self, ccy):
+    async def update_safe(self, ccy: str) -> None:
         try:
             self.logger.info(f"getting fx quotes for {ccy}")
             self.quotes = await self.get_rates(ccy)
@@ -88,7 +88,7 @@ class ExchangeBase(Logger):
             self.quotes = {}
         self.on_quotes()
 
-    def read_historical_rates(self, ccy, cache_dir) -> Optional[dict]:
+    def read_historical_rates(self, ccy: str, cache_dir: str) -> Optional[dict]:
         filename = os.path.join(cache_dir, self.name() + '_'+ ccy)
         if not os.path.exists(filename):
             return None
@@ -106,7 +106,7 @@ class ExchangeBase(Logger):
         return h
 
     @log_exceptions
-    async def get_historical_rates_safe(self, ccy, cache_dir):
+    async def get_historical_rates_safe(self, ccy: str, cache_dir: str) -> None:
         try:
             self.logger.info(f"requesting fx history for {ccy}")
             h = await self.request_history(ccy)
@@ -124,7 +124,7 @@ class ExchangeBase(Logger):
         self.history[ccy] = h
         self.on_history()
 
-    def get_historical_rates(self, ccy, cache_dir):
+    def get_historical_rates(self, ccy: str, cache_dir: str) -> None:
         if ccy not in self.history_ccys():
             return
         h = self.history.get(ccy)
@@ -133,19 +133,19 @@ class ExchangeBase(Logger):
         if h is None or h['timestamp'] < time.time() - 24*3600:
             asyncio.get_event_loop().create_task(self.get_historical_rates_safe(ccy, cache_dir))
 
-    def history_ccys(self):
+    def history_ccys(self) -> Sequence[str]:
         return []
 
-    def historical_rate(self, ccy, d_t):
+    def historical_rate(self, ccy: str, d_t: datetime) -> Union[str, float, Decimal]:
         return self.history.get(ccy, {}).get(d_t.strftime('%Y-%m-%d'), 'NaN')
 
-    async def request_history(self, ccy):
+    async def request_history(self, ccy: str) -> Dict[str, Union[str, float, Decimal]]:
         raise NotImplementedError()  # implemented by subclasses
 
-    async def get_rates(self, ccy):
+    async def get_rates(self, ccy: str) -> Mapping[str, Union[str, float, Decimal, None]]:
         raise NotImplementedError()  # implemented by subclasses
 
-    async def get_currencies(self):
+    async def get_currencies(self) -> Sequence[str]:
         rates = await self.get_rates('')
         return sorted([str(a) for (a, b) in rates.items() if b is not None and len(a)==3])
 
@@ -489,7 +489,7 @@ class FxThread(ThreadJob):
         self.history_used_spot = False
         self.ccy_combo = None
         self.hist_checkbox = None
-        self.cache_dir = os.path.join(config.path, 'cache')
+        self.cache_dir = os.path.join(config.path, 'cache')  # type: str
         self._trigger = asyncio.Event()
         self._trigger.set()
         self.set_exchange(self.config_exchange())