Offsetfilter: add number_assets parameter

closes #6824

docs/includes/pairlists.md

@@ -160,17 +160,17 @@ This filter allows freqtrade to ignore pairs until they have been listed for at
 
 Offsets an incoming pairlist by a given `offset` value.
 
-As an example it can be used in conjunction with `VolumeFilter` to remove the top X volume pairs. Or to split
+As an example it can be used in conjunction with `VolumeFilter` to remove the top X volume pairs. Or to split a larger pairlist on two bot instances.
-a larger pairlist on two bot instances.
 
-Example to remove the first 10 pairs from the pairlist:
+Example to remove the first 10 pairs from the pairlist, and takes the next 20 (taking items 10-30 of the initial list):
 
 ```json
 "pairlists": [
     // ...
     {
         "method": "OffsetFilter",
-        "offset": 10
+        "offset": 10,
+        "number_assets": 20
     }
 ],
 ```

freqtrade/plugins/pairlist/OffsetFilter.py

@@ -19,6 +19,7 @@ class OffsetFilter(IPairList):
         super().__init__(exchange, pairlistmanager, config, pairlistconfig, pairlist_pos)
 
         self._offset = pairlistconfig.get('offset', 0)
+        self._number_pairs = pairlistconfig.get('number_assets', 0)
 
         if self._offset < 0:
             raise OperationalException("OffsetFilter requires offset to be >= 0")
@@ -36,7 +37,9 @@ class OffsetFilter(IPairList):
         """
         Short whitelist method description - used for startup-messages
         """
-        return f"{self.name} - Offseting pairs by {self._offset}."
+        if self._number_pairs:
+            return f"{self.name} - Taking {self._number_pairs} Pairs, starting from {self._offset}."
+        return f"{self.name} - Offsetting pairs by {self._offset}."
 
     def filter_pairlist(self, pairlist: List[str], tickers: Dict) -> List[str]:
         """
@@ -50,5 +53,9 @@ class OffsetFilter(IPairList):
             self.log_once(f"Offset of {self._offset} is larger than " +
                           f"pair count of {len(pairlist)}", logger.warning)
         pairs = pairlist[self._offset:]
+        if self._number_pairs:
+            pairs = pairs[:self._number_pairs]
+
         self.log_once(f"Searching {len(pairs)} pairs: {pairs}", logger.info)
+
         return pairs

tests/plugins/test_pairlist.py

@@ -470,12 +470,16 @@ def test_VolumePairList_refresh_empty(mocker, markets_empty, whitelist_conf):
      "BTC", ['ETH/BTC', 'TKN/BTC']),
     # VolumePairList with no offset = unchanged pairlist
     ([{"method": "VolumePairList", "number_assets": 20, "sort_key": "quoteVolume"},
-      {"method": "OffsetFilter", "offset": 0}],
+      {"method": "OffsetFilter", "offset": 0, "number_assets": 0}],
      "USDT", ['ETH/USDT', 'NANO/USDT', 'ADAHALF/USDT', 'ADADOUBLE/USDT']),
     # VolumePairList with offset = 2
     ([{"method": "VolumePairList", "number_assets": 20, "sort_key": "quoteVolume"},
       {"method": "OffsetFilter", "offset": 2}],
      "USDT", ['ADAHALF/USDT', 'ADADOUBLE/USDT']),
+    # VolumePairList with offset and limit
+    ([{"method": "VolumePairList", "number_assets": 20, "sort_key": "quoteVolume"},
+      {"method": "OffsetFilter", "offset": 1, "number_assets": 2}],
+     "USDT", ['NANO/USDT', 'ADAHALF/USDT']),
     # VolumePairList with higher offset, than total pairlist
     ([{"method": "VolumePairList", "number_assets": 20, "sort_key": "quoteVolume"},
       {"method": "OffsetFilter", "offset": 100}],
@@ -1152,6 +1156,10 @@ def test_spreadfilter_invalid_data(mocker, default_conf, markets, tickers, caplo
      "0.01 and above 0.99 over the last days.'}]",
         None
      ),
+    ({"method": "OffsetFilter", "offset": 5, "number_assets": 10},
+     "[{'OffsetFilter': 'OffsetFilter - Taking 10 Pairs, starting from 5.'}]",
+     None
+     ),
 ])
 def test_pricefilter_desc(mocker, whitelist_conf, markets, pairlistconfig,
                           desc_expected, exception_expected):