add is_encoding() method

git-svn-id: https://linkchecker.svn.sourceforge.net/svnroot/linkchecker/trunk/linkchecker@2980 e7d03fd6-7b0d-0410-9947-9c21f3af8025

linkcheck/strformat.py

@@ -21,6 +21,7 @@ necessarily optimised for large strings, so use with care.
 
 import re
 import textwrap
+import codecs
 import os
 import time
 import urlparse
@@ -52,10 +53,10 @@ def ascii_safe (s):
 
     @param s: the Unicode string to be encoded
     @type s: unicode or None
-    @return: encoded ASCII version of s, or s itself if s evaluated to False
+    @return: encoded ASCII version of s, or None if s was None
     @rtype: string
     """
-    if s:
+    if isinstance(s, unicode):
         s = s.encode('ascii', 'ignore')
     return s
 
@@ -71,6 +72,16 @@ def is_ascii (s):
         return False
 
 
+def is_encoding (text):
+    """
+    Check if string is a valid encoding.
+    """
+    try:
+        return codecs.lookup(text)
+    except LookupError:
+        return False
+
+
 def url_unicode_split (url):
     """
     Like urlparse.urlsplit(), but always returning unicode parts.