[New-bugs-announce] [issue38003] Incorrect "fixing" of isinstance tests for basestring

[Bob Kline]

New submission from Bob Kline <bkline at rksystems.com>:

We are attempting to convert a large Python 2 code base. Following the guidance of the official documentation (https://docs.python.org/2/library/functions.html#basestring) we created tests in many, many places that look like this:

if isinstance(value, basestring):
    if not isinstance(value, unicode):
        value = value.decode(encoding)
else:
    some other code

It seems that the 2to3 tool is unaware that replacing basestring with str in such cases will break the software.

Here's an example.

$ 2to3 repro.py
...
--- repro.py	(original)
+++ repro.py	(refactored)
@@ -1,8 +1,8 @@
 from frobnitz import transform

 def foo(value, encoding=None):
-    if isinstance(value, basestring):
-        if not isinstance(value, unicode):
+    if isinstance(value, str):
+        if not isinstance(value, str):
             value = value.decode(encoding or "utf-8")
         return value
     else:

Help me understand how this "fix" results in the correct behavior.

----------
components: 2to3 (2.x to 3.x conversion tool)
messages: 350964
nosy: bkline
priority: normal
severity: normal
status: open
title: Incorrect "fixing" of isinstance tests for basestring
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38003>
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