python 3.6 compatibility: reimplement str.isascii()

langcodes/tag_parser.py

@@ -95,7 +95,6 @@
     ...
 langcodes.tag_parser.LanguageTagError: Language tags must be made of ASCII characters
 """
-from __future__ import print_function, unicode_literals
 
 # These tags should not be parsed by the usual parser; they're grandfathered
 # in from RFC 3066. The 'irregular' ones don't fit the syntax at all; the
@@ -147,6 +146,18 @@
 ]
 
 
+def _is_ascii(s):
+    """
+    Determine whether a tag consists of ASCII characters.
+    """
+    # When Python 3.6 support is dropped, we can replace this with str.isascii().
+    try:
+        s.encode('ascii')
+        return True
+    except UnicodeEncodeError:
+        return False
+
+
 def normalize_characters(tag):
     """
     BCP 47 is case-insensitive, and CLDR's use of it considers underscores
@@ -167,7 +178,7 @@ def parse_tag(tag):
     registry, yet. Returns a list of (type, value) tuples indicating what
     information will need to be looked up.
     """
-    if not tag.isascii():
+    if not _is_ascii(tag):
         raise LanguageTagError("Language tags must be made of ASCII characters")
 
     tag = normalize_characters(tag)