Speed up the PageIterator by evaluating items once per page (#572)

The `firebase_admin.auth.list_users().iterate_all()` method uses a sub-class
of PageIterator, which happens to access the .items computed property more
than once for every page of results. This has been changed so we take care
not to access the `self.items` property more than once per page.

This is a lot faster.

Author: davidwtbuxton

firebase_admin/_auth_utils.py

@@ -43,30 +43,32 @@ class PageIterator:
     def __init__(self, current_page):
         if not current_page:
             raise ValueError('Current page must not be None.')
+
         self._current_page = current_page
-        self._index = 0
+        self._iter = None
+
+    def __next__(self):
+        if self._iter is None:
+            self._iter = iter(self.items)
 
-    def next(self):
-        if self._index == len(self.items):
+        try:
+            return next(self._iter)
+        except StopIteration:
             if self._current_page.has_next_page:
                 self._current_page = self._current_page.get_next_page()
-                self._index = 0
-        if self._index < len(self.items):
-            result = self.items[self._index]
-            self._index += 1
-            return result
-        raise StopIteration
+                self._iter = iter(self.items)
 
-    @property
-    def items(self):
-        raise NotImplementedError
+                return next(self._iter)
 
-    def __next__(self):
-        return self.next()
+            raise
 
     def __iter__(self):
         return self
 
+    @property
+    def items(self):
+        raise NotImplementedError
+
 
 def get_emulator_host():
     emulator_host = os.getenv(EMULATOR_HOST_ENV_VAR, '')