Make SlowRetrievalError parameter optional

SlowRetrievalError is raised from RequestsFetcher where
average_download_speed is not calculated.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/exceptions.py

@@ -24,7 +24,7 @@
 
 from urllib import parse
 
-from typing import Any, Dict
+from typing import Any, Dict, Optional
 
 import logging
 logger = logging.getLogger(__name__)
@@ -206,16 +206,19 @@ def __repr__(self) -> str:
 class SlowRetrievalError(DownloadError):
   """"Indicate that downloading a file took an unreasonably long time."""
 
-  def __init__(self, average_download_speed: int):
+  def __init__(self, average_download_speed: Optional[int] = None):
     super(SlowRetrievalError, self).__init__()
 
     self.__average_download_speed = average_download_speed #bytes/second
 
   def __str__(self) -> str:
-    return (
-        'Download was too slow. Average speed: ' +
+    msg = 'Download was too slow.'
+    if self.__average_download_speed is not None:
+      msg = ('Download was too slow. Average speed: ' +
          repr(self.__average_download_speed) + ' bytes per second.')
 
+    return msg
+
   def __repr__(self) -> str:
     return self.__class__.__name__ + ' : ' + str(self)
 

tuf/ngclient/_internal/requests_fetcher.py

@@ -141,7 +141,7 @@ def _chunks(
             )
 
         except urllib3.exceptions.ReadTimeoutError as e:
-            raise exceptions.SlowRetrievalError(str(e))
+            raise exceptions.SlowRetrievalError from e
 
         finally:
             response.close()