Josh Rosenberg <[email protected]> added the comment:
basestring in Python 2 means "thing that is logically text", because in Python
2, str can mean *either* logical text *or* binary data, and unicode is always
logical text. str and unicode can kinda sorta interoperate on Python 2, so it
can make sense to test for basestring if you're planning to use it as logical
text; if you do 'foo' + u'bar', that's fine in Python 2. In Python 3, only str
is logically text; b'foo' + 'bar' is completely illegal, so it doesn't make
sense to convert it to recognize both bytes and str.
Your problem is that you're using basestring incorrectly in Python 2, and it
happens to work only because Python 2 did a bad job of separating text and
binary data. Your original example code should actually have been written in
Python 2 as:
if isinstance(value, bytes): # bytes is an alias of str, and only str, on 2.7
value = value.decode(encoding)
elif not isinstance(value, unicode):
some other code
which 2to3 would convert correctly (changing unicode to str, and leaving
everything else untouched) because you actually tested what you meant to test
to control the actions taken:
1. If it was binary data (which you interpret all Py2 strs to be), then it is
decoded to text (Py2 unicode/Py3 str)
2. If it wasn't binary data and it wasn't text, you did something else
Point is, the converter is doing the right thing. You misunderstood the logical
meaning of basestring, and wrote code that depended on your misinterpretation,
that's all.
Your try/except to try to detect Python 3-ness was doomed from the start; you
referenced basestring, and 2to3 (reasonably) converts that to str, which breaks
your logic. You wrote cross-version code that can't be 2to3-ed because it's
*already* Python 3 code; Python 3 code should never be subjected to 2to3,
because it'll do dumb things (e.g. change print(1, 2) to print((1, 2))); it's
2to3, not 2or3to3 after all.
----------
nosy: +josh.r
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<https://bugs.python.org/issue38003>
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