Reverse sort order of listed orders

On listing orders the API returns newest orders first. To print newest
orders at the bottom of the list the sort order is reversed. To keep
memory usage within limits the reversal is only performed within chunks.

Author: cwasicki

src/frequenz/client/electricity_trading/cli/etrading.py

@@ -3,9 +3,11 @@
 
 """CLI tool to interact with the trading API."""
 
+from collections import deque
 from datetime import datetime, timedelta, timezone
 from decimal import Decimal
 from enum import Enum
+from typing import AsyncIterator
 
 from frequenz.client.electricity_trading import (
     Client,
@@ -84,6 +86,9 @@ async def list_orders(
     for the 15 minute delivery period starting at delivery_start.
     If no delivery_start is provided, stream new orders for any delivery period.
 
+    Note that retrieved sort order for listed orders (starting from the newest)
+    is reversed in chunks trying to bring more recent orders to the bottom.
+
     Args:
         url: URL of the trading API.
         key: API key.
@@ -104,7 +109,7 @@ async def list_orders(
         )
     lst = client.list_gridpool_orders(gid, delivery_period=delivery_period)
 
-    async for order in lst:
+    async for order in reverse_iterator(lst):
         print_order(order)
 
     if delivery_start and delivery_start <= datetime.now(timezone.utc):
@@ -282,3 +287,27 @@ def print_order(order: OrderDetail) -> None:
         order.state_detail.state,
     ]
     print(",".join(v.name if isinstance(v, Enum) else str(v) for v in values))
+
+
+async def reverse_iterator(
+    iterator: AsyncIterator[OrderDetail], chunk_size: int = 100_000
+) -> AsyncIterator[OrderDetail]:
+    """Reverse an async iterator in chunks to avoid loading all elements into memory.
+
+    Args:
+        iterator: Async iterator to reverse.
+        chunk_size: Size of the buffer to store elements.
+
+    Yields:
+        Elements of the iterator in reverse order.
+    """
+    buffer: deque[OrderDetail] = deque(maxlen=chunk_size)
+    async for item in iterator:
+        buffer.append(item)
+        if len(buffer) == chunk_size:
+            for item in reversed(buffer):
+                yield item
+            buffer.clear()
+    if buffer:
+        for item in reversed(buffer):
+            yield item