Speed up the PageIterator by evaluating items once per page

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 change keeps most of the iterator
state in the class init, meaning we can avoid accessing the computed property
until the current page is exhausted.

This is a lot faster.

Author: davidwtbuxton

firebase_admin/_auth_utils.py

@@ -43,30 +43,29 @@ 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 = iter(self.items)
 
-    def next(self):
-        if self._index == len(self.items):
+    def __next__(self):
+        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
-
-    def __next__(self):
-        return self.next()
+                return next(self._iter)
+            else:
+                raise
 
     def __iter__(self):
         return self
 
+    @property
+    def items(self):
+        raise NotImplementedError
+
 
 def get_emulator_host():
     emulator_host = os.getenv(EMULATOR_HOST_ENV_VAR, '')