resolves freqtrade/freqtrade#4650

docs/includes/pairlists.md

@@ -167,7 +167,7 @@ If the trading range over the last 10 days is <1%, remove the pair from the whit
 
 #### VolatilityFilter
 
-Volatily is the degree of historical variation of a pairs over time, is is measured by the standard deviation of logarithmic daily returns.
+Volatily is the degree of historical variation of a pairs over time, is is measured by the standard deviation of logarithmic daily returns. Returns are assumed to be normally distributed, although actual distribution might be different. In a normal distribution, 68% of observations fall within one standard deviation and 95% of observations fall within two standard deviations. Assuming a volatilty of 0.05 means that the expected returns for 20 out of 30 days is expected to be less than 5% (one standard deviation). Volatility is a positive ratio of the expected deviation of return and can be greater than 1.00. Please refer to the wikipedia definition of [`volatility`](https://en.wikipedia.org/wiki/Volatility_(finance)).
 
 Removes pairs where the average volatility over a `lookback_days` days is below `min_volatility` and above `max_volatility`. Since this is a filter that requires additional data, the results are cached for `refresh_period`.
 
@@ -181,8 +181,8 @@ If the volatilty over the last 10 days is not in the range of 0.20-0.30, remove
     {
         "method": "VolatilityFilter",
         "lookback_days": 10,
-        "min_volatilty": 0.20,
-        "max_volatilty": 0.30,
+        "min_volatility": 0.05,
+        "max_volatility": 0.50,
         "refresh_period": 86400
     }
 ]

freqtrade/plugins/pairlist/VolatilityFilter.py

@@ -94,8 +94,9 @@ class VolatilityFilter(IPairList):
         :return: True if the pair can stay, false if it should be removed
         """
         # Check symbol in cache
-        if pair in self._pair_cache:
-            return self._pair_cache[pair]
+        cached_res = self._pair_cache.get(pair, None)
+        if cached_res is not None:
+            return cached_res
 
         result = False
         if daily_candles is not None and not daily_candles.empty: