[vs] Incorporate python3 best practices into DVSLib (sonic-net#1357)

- Use fstrings instead of `format`
- Add type hints
- Simplify some of the comments

Signed-off-by: Danny Allen <daall@microsoft.com>

tests/dvslib/dvs_common.py

@@ -1,47 +1,41 @@
-"""
-    dvs_common contains common infrastructure for writing tests for the
-    virtual switch.
-"""
+"""Common infrastructure for writing VS tests."""
 
 import collections
 import time
 
+from typing import Any, Callable, Tuple
+
 _PollingConfig = collections.namedtuple('PollingConfig', 'polling_interval timeout strict')
 
+
 class PollingConfig(_PollingConfig):
-    """
-        PollingConfig provides parameters that are used to control the behavior
-        for polling functions.
-
-        Params:
-            polling_interval (int): How often to poll, in seconds.
-            timeout (int): The maximum amount of time to wait, in seconds.
-            strict (bool): If the strict flag is set, reaching the timeout
-                will cause tests to fail (e.g. assert False)
+    """PollingConfig provides parameters that are used to control polling behavior.
+
+    Attributes:
+        polling_interval (int): How often to poll, in seconds.
+        timeout (int): The maximum amount of time to wait, in seconds.
+        strict (bool): If the strict flag is set, reaching the timeout will cause tests to fail.
     """
 
-    pass
 
-def wait_for_result(polling_function, polling_config):
-    """
-        wait_for_result will periodically run `polling_function`
-        using the parameters described in `polling_config` and return the
-        output of the polling function.
-
-        Args:
-            polling_config (PollingConfig): The parameters to use to poll
-                the db.
-            polling_function (Callable[[], (bool, Any)]): The function being
-                polled. The function takes no arguments and must return a
-                status which indicates if the function was succesful or
-                not, as well as some return value.
-
-        Returns:
-            (bool, Any): If the polling function succeeds, then this method
-            will return True and the output of the polling function. If it
-            does not succeed within the provided timeout, it will return False
-            and whatever the output of the polling function was on the final
-            attempt.
+def wait_for_result(
+    polling_function: Callable[[], Tuple[bool, Any]],
+    polling_config: PollingConfig,
+) -> Tuple[bool, Any]:
+    """Run `polling_function` periodically using the specified `polling_config`.
+
+    Args:
+        polling_function: The function being polled. The function cannot take any arguments and
+            must return a status which indicates if the function was succesful or not, as well as
+            some return value.
+        polling_config: The parameters to use to poll the polling function.
+
+    Returns:
+        If the polling function succeeds, then this method will return True and the output of the
+        polling function.
+
+        If it does not succeed within the provided timeout, it will return False and whatever the
+        output of the polling function was on the final attempt.
     """
     if polling_config.polling_interval == 0:
         iterations = 1
@@ -57,6 +51,6 @@ def wait_for_result(polling_function, polling_config):
         time.sleep(polling_config.polling_interval)
 
     if polling_config.strict:
-        assert False, "Operation timed out after {}s".format(polling_config.timeout)
+        assert False, f"Operation timed out after {polling_config.timeout} seconds"
 
     return (False, result)