Allow waiting for zero units on wait_for_idle

fixes #721

Author: cderici

juju/model.py

@@ -2532,7 +2532,7 @@ async def _get_source_api(self, url, controller_name=None):
 
     async def wait_for_idle(self, apps=None, raise_on_error=True, raise_on_blocked=False,
                             wait_for_active=False, timeout=10 * 60, idle_period=15, check_freq=0.5,
-                            status=None, wait_for_units=1, wait_for_exact_units=-1):
+                            status=None, wait_for_units=1, wait_for_exact_units=None):
         """Wait for applications in the model to settle into an idle state.
 
         :param apps (list[str]): Optional list of specific app names to wait on.
@@ -2574,8 +2574,7 @@ async def wait_for_idle(self, apps=None, raise_on_error=True, raise_on_blocked=F
 
         :param wait_for_exact_units (int): The exact number of units to be expected before
             going into the idle state. (e.g. useful for scaling down).
-            The default is -1 unit.
-            When positive, takes precedence over the `wait_for_units` parameter.
+            When set, takes precedence over the `wait_for_units` parameter.
         """
         if wait_for_active:
             warnings.warn("wait_for_active is deprecated; use status", DeprecationWarning)
@@ -2606,6 +2605,10 @@ def _raise_for_status(entities, status):
                     ", ".join(errored),
                 ))
 
+        if wait_for_exact_units is not None:
+            assert type(wait_for_exact_units) == int and wait_for_exact_units >= 0, \
+                f'Invalid value for wait_for_exact_units {wait_for_exact_units}'
+
         while True:
             busy = []
             errors = {}
@@ -2619,7 +2622,7 @@ def _raise_for_status(entities, status):
                     errors.setdefault("App", []).append(app.name)
                 if raise_on_blocked and app.status == "blocked":
                     blocks.setdefault("App", []).append(app.name)
-                if wait_for_exact_units > 0:
+                if wait_for_exact_units is not None:
                     if len(app.units) != wait_for_exact_units:
                         busy.append(app.name + " (waiting for exactly %s units, current : %s)" %
                                     (wait_for_exact_units, len(app.units)))