Improvements and notebooks

src/lib/net.py

@@ -3,17 +3,13 @@ import time
 from urllib.error import URLError
 from urllib.request import Request, urlopen
 
-RETRY_DELAYS = (3, 10)
-
 
 def urlopen_with_retry(req: Request, timeout: int = 30):
-    """urlopen wrapper that retries transient network failures."""
-    last_err: Exception | None = None
-    for attempt in range(len(RETRY_DELAYS) + 1):
+    """urlopen wrapper that retries transient network failures (3s, 10s backoff)."""
+    for delay in (3, 10, None):
         try:
             return urlopen(req, timeout=timeout)
-        except (URLError, TimeoutError) as e:
-            last_err = e
-            if attempt < len(RETRY_DELAYS):
-                time.sleep(RETRY_DELAYS[attempt])
-    raise last_err
+        except (URLError, TimeoutError):
+            if delay is None:
+                raise
+            time.sleep(delay)