Replaced unsafe loop with a list comprehension & added docstring in refresh_pairlist()

freqtrade/plugins/pairlistmanager.py

@@ -134,7 +134,19 @@ class PairListManager(LoggingMixin):
         return self._exchange.get_tickers()
 
     def refresh_pairlist(self, only_first: bool = False, pairs: list[str] | None = None) -> None:
-        """Run pairlist through all configured Pairlist Handlers."""
+        """
+        Run pairlist through all configured Pairlist Handlers.
+
+        :param only_first: If True, only run the first PairList handler (the generator)
+            and skip all subsequent filters. Used during backtesting startup to ensure
+            historic data is loaded for the complete universe of pairs that the
+            generator can produce (even if later filters would reduce the list size).
+            Prevents missing data when a filter returns a variable number of pairs
+            across refresh cycles.
+        :param pairs: Optional list of pairs to intersect with the generated pairlist.
+            Only pairs present both in the generated list and this parameter are kept.
+            Used in backtesting to filter out pairs with no available data.
+        """
         # Tickers should be cached to avoid calling the exchange on each call.
         tickers: dict = {}
         if self._tickers_needed:
@@ -143,10 +155,9 @@ class PairListManager(LoggingMixin):
         # Generate the pairlist with first Pairlist Handler in the chain
         pairlist = self._pairlist_handlers[0].gen_pairlist(tickers)
 
-        if pairs:
-            for pair in pairlist:
-                if pair not in pairs:
-                    pairlist.remove(pair)
+        # Optional intersection with an explicit list of pairs (used in backtesting)
+        if pairs is not None:
+            pairlist = [p for p in pairlist if p in pairs]
 
         if not only_first:
             # Process all Pairlist Handlers in the chain