Add some docstring and type checking

sdk/core/azure-core/azure/core/serialization.py

@@ -6,12 +6,12 @@
 # --------------------------------------------------------------------------
 import base64
 from json import JSONEncoder
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Union
 
 from .utils._utils import _FixedOffset
 
 if TYPE_CHECKING:
-    from datetime import timedelta
+    from datetime import timedelta, date, datetime, time
 
 __all__ = ["NULL", "AzureJSONEncoder"]
 
@@ -84,6 +84,8 @@ def _timedelta_as_str(td):
 
 
 def _datetime_as_isostr(dt):
+    # type: (Union[datetime, date, time, timedelta]) -> str
+    """Converts a datetime.(datetime|date|time|timedelta) object into an ISO 8601 formatted string"""
     # First try datetime.datetime
     if hasattr(dt, "year") and hasattr(dt, "hour"):
         # astimezone() fails for naive times in Python 2.7, so make make sure o is aware (tzinfo is set)
@@ -118,5 +120,6 @@ def default(self, o):  # pylint: disable=too-many-return-statements
         try:
             return _datetime_as_isostr(o)
         except AttributeError:
+            # This will be raised when it hits value.total_seconds in the method above
             pass
         return super(AzureJSONEncoder, self).default(o)