Terry J. Reedy <tjre...@udel.edu> added the comment:
Replacing 'basestring' with 'str' is not a bug in the behavioral sense because it is intended and documented. https://docs.python.org/3/library/2to3.html#2to3fixer-basestring How the current behavior is correct: 2to3 converts syntactically valid 2.x code to syntactically valid 3.x code. It cannot, however, guarantee semantic correctness. A particular problem is that str is semantically ambiguous in 2.x, as it is used both for (encoded) text and binary data. To resolve the ambiguity, 2.6 introduced 'bytes' as a synonym for 'str'. 2to3 assumes that 'bytes' means binary data, including text that will still be encoded in 3.x, while 'str' means text that is encoded bytes in 2.x but *will be unicode* in 3.x. Hence it changes 'unicode' to unambiguous 'str' and 'basestring' == Union(unicode, str) to Union(str, str) == 'str'. If you fool 2to3 by applying isinstance(value, basestring) to a value that will still be bytes at that point in 3.x, you get a semantic change. Possible fixes: 1. Since you decode value after the check, do it before the check. if isinstance(value, bytes): value = value.decode(encoding) if not isinstance(value, unicode): some other code 2. Replace 'basestring' with '(unicode, basestring)' In both cases, the 'unicode' to 'str' replacement should result in correct 3.x code. 3. Edit Lib/lib2to3/fixes/fix_basestring.py to replace with '(str, bytes)'. This should be straightforward, but ask on python-list if you need help. As for your second example, 2to3 is not meant for 2&3 code using exception tricks and six/future imports. Turning 2&3 code into idiomatic 3-only code is a separate subject. Since other have and will run into the same issues, I intend to post a revised version of the explanation above, with fixes for a revised example, to python-list as "2to3, str, and basestring". Any further discussion should go there. ---------- resolution: rejected -> not a bug _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38003> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
