creational pattern patch

app/data/__init__.py

@@ -3,13 +3,7 @@
 from ..services.location.jhu import JhuLocationService
 from ..services.location.nyt import NYTLocationService
 
-# Mapping of services to data-sources.
-DATA_SOURCES = {
-    "jhu": JhuLocationService(),
-    "csbs": CSBSLocationService(),
-    "nyt": NYTLocationService(),
-}
-
+DATA_SOURCES =  ['jhu', 'csbs', 'nyt']
 
 def data_source(source):
     """
@@ -18,4 +12,13 @@ def data_source(source):
     :returns: The service.
     :rtype: LocationService
     """
-    return DATA_SOURCES.get(source.lower())
+    s = source.lower()
+    if s not in DATA_SOURCES:
+        return None
+    # return the singleton instance of the required service
+    if s == 'jhu': 
+        return JhuLocationService.getInstance()
+    elif s == 'csbs': 
+        return CSBSLocationService.getInstance()
+    elif s == 'nyt': 
+        return NYTLocationService.getInstance()

app/routers/v2.py

@@ -107,4 +107,4 @@ async def sources():
     """
     Retrieves a list of data-sources that are availble to use.
     """
-    return {"sources": list(DATA_SOURCES.keys())}
+    return {"sources": DATA_SOURCES)}

app/services/location/__init__.py

@@ -7,6 +7,10 @@ class LocationService(ABC):
     Service for retrieving locations.
     """
 
+    # this class variable will store the singleton instance for 
+    # the LocationService instance from the child class
+    __instance__ = None
+
     @abstractmethod
     async def get_all(self):
         """
@@ -26,3 +30,13 @@ async def get(self, id):  # pylint: disable=redefined-builtin,invalid-name
         :rtype: Location
         """
         raise NotImplementedError
+
+    @classmethod
+    def getInstance(cls):
+        # singleton pattern
+        if cls.__instance__ is None:
+            # instantiate new object from the class (note that we are 
+            # expecting 0 parameters for the constructor. if parameters 
+            # needed this method should be overridden by the subclass)
+            cls.__instance__ = cls()
+        return cls.__instance__