diff --git a/flask/helpers.py b/flask/helpers.py
index 3da83854..1bd4ad6e 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -14,6 +14,7 @@ import sys
 import pkgutil
 import posixpath
 import mimetypes
+import warnings
 from time import time
 from zlib import adler32
 from threading import RLock
@@ -504,8 +505,7 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
         a :class:`~datetime.datetime` or timestamp.
         If a file was passed, this overrides its mtime.
     """
-    import warnings
-    if type(filename_or_fp) is str:
+    if isinstance(filename_or_fp, string_types):
         if not is_ascii(filename_or_fp):
             warnings.warn("Non-ASCII filename is being used in send_file", UnicodeWarning)
 
@@ -964,15 +964,12 @@ def total_seconds(td):
     """
     return td.days * 60 * 60 * 24 + td.seconds
 
-def is_ascii(string_to_test):
+def is_ascii(string):
     """Returns if a string object is fully ASCII encoded.
 
-    :param string string_to_test: the string to be evaluated
+    :param string string: the string to be evaluated
 
-    :returns: True if string_to_test is fully ASCII encoded, False if not
+    :returns: True if string is fully ASCII encoded, False if not
     :rtype: boolean
     """
-    if all(ord(char) < 128 for char in string_to_test):
-        return True
-    else:
-        return False
+    return all(ord(char) < 128 for char in string):