Allow StaticPairlist in non-first position

closes #5754

docs/includes/pairlists.md

@@ -52,6 +52,8 @@ To skip pair validation against active markets, set `"allow_inactive": true` wit
 This can be useful for backtesting expired pairs (like quarterly spot-markets).
 This option must be configured along with `exchange.skip_pair_validation` in the exchange configuration.
 
+When used in a "follow-up" position (e.g. after VolumePairlist), all pairs in `'pair_whitelist'` will be added to the end of the pairlist.
+
 #### Volume Pair List
 
 `VolumePairList` employs sorting/filtering of pairs by their trading volume. It selects `number_assets` top pairs with sorting based on the `sort_key` (which can only be `quoteVolume`).

freqtrade/plugins/pairlist/StaticPairList.py

@@ -4,9 +4,9 @@ Static Pair List provider
 Provides pair white list as it configured in config
 """
 import logging
+from copy import deepcopy
 from typing import Any, Dict, List
 
-from freqtrade.exceptions import OperationalException
 from freqtrade.plugins.pairlist.IPairList import IPairList
 
 
@@ -20,10 +20,6 @@ class StaticPairList(IPairList):
                  pairlist_pos: int) -> None:
         super().__init__(exchange, pairlistmanager, config, pairlistconfig, pairlist_pos)
 
-        if self._pairlist_pos != 0:
-            raise OperationalException(f"{self.name} can only be used in the first position "
-                                       "in the list of Pairlist Handlers.")
-
         self._allow_inactive = self._pairlistconfig.get('allow_inactive', False)
 
     @property
@@ -64,4 +60,8 @@ class StaticPairList(IPairList):
         :param tickers: Tickers (from exchange.get_tickers()). May be cached.
         :return: new whitelist
         """
-        return pairlist
+        pairlist_ = deepcopy(pairlist)
+        for pair in self._config['exchange']['pair_whitelist']:
+            if pair not in pairlist_:
+                pairlist_.append(pair)
+        return pairlist_

tests/plugins/test_pairlist.py

@@ -415,10 +415,10 @@ def test_VolumePairList_refresh_empty(mocker, markets_empty, whitelist_conf):
     # SpreadFilter only
     ([{"method": "SpreadFilter", "max_spread_ratio": 0.005}],
      "BTC", 'filter_at_the_beginning'),  # OperationalException expected
-    # Static Pairlist after VolumePairList, on a non-first position
-    ([{"method": "VolumePairList", "number_assets": 5, "sort_key": "quoteVolume"},
+    # Static Pairlist after VolumePairList, on a non-first position (appends pairs)
+    ([{"method": "VolumePairList", "number_assets": 2, "sort_key": "quoteVolume"},
       {"method": "StaticPairList"}],
-        "BTC", 'static_in_the_middle'),
+        "BTC", ['ETH/BTC', 'TKN/BTC', 'TRST/BTC', 'SWT/BTC', 'BCC/BTC', 'HOT/BTC']),
     ([{"method": "VolumePairList", "number_assets": 20, "sort_key": "quoteVolume"},
       {"method": "PriceFilter", "low_price_ratio": 0.02}],
         "USDT", ['ETH/USDT', 'NANO/USDT']),
@@ -469,13 +469,6 @@ def test_VolumePairList_whitelist_gen(mocker, whitelist_conf, shitcoinmarkets, t
 
     mocker.patch('freqtrade.exchange.Exchange.exchange_has', MagicMock(return_value=True))
 
-    if whitelist_result == 'static_in_the_middle':
-        with pytest.raises(OperationalException,
-                           match=r"StaticPairList can only be used in the first position "
-                                 r"in the list of Pairlist Handlers."):
-            freqtrade = get_patched_freqtradebot(mocker, whitelist_conf)
-        return
-
     freqtrade = get_patched_freqtradebot(mocker, whitelist_conf)
     mocker.patch.multiple('freqtrade.exchange.Exchange',
                           get_tickers=tickers,