Enhance iterlen() and add doctest to Bio/_utils.py

Bio/_utils.py

@@ -12,11 +12,27 @@
 def iterlen(items):
     """Count the number of items in an iterable.
 
+    If the argument supports len(items), and some iterators do, then
+    this returns len(items). Otherwise it will scan over the entries
+    in order to count them.
+
     Exhausts a generator, but doesn't require creating a full list.
+
+    >>> iterlen("abcde")
+    5
+    >>> iterlen(iter("abcde"))
+    5
+    >>> iterlen(xrange(5))
+    5
+
     """
-    for i, x in enumerate(items):
-        count = i
-    return count + 1
+    try:
+        #e.g. Under Python 2, the xrange iterator defines __len__
+        return len(items)
+    except TypeError:
+        for i, x in enumerate(items):
+            count = i
+        return count + 1
 
 
 def read_forward(handle):
@@ -101,3 +117,6 @@ def run_doctest(target_dir=None, *args, **kwargs):
         # and revert back to initial directory
         os.chdir(cur_dir)
     print "Done"
+
+if __name__ == "__main__":
+    run_doctest()