pkg_resources' detection of Python2 vs Python3 is brittle

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Originally reported by: anntzer (Bitbucket: anntzer, GitHub: anntzer)

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The following part of pkg_resources

#!python
try:
    basestring
    next = lambda o: o.next()
    from cStringIO import StringIO as BytesIO
except NameError:
    basestring = str
    from io import BytesIO
    def execfile(fn, globs=None, locs=None):
        if globs is None:
            globs = globals()
        if locs is None:
            locs = globs
        exec(compile(open(fn).read(), fn, 'exec'), globs, locs)

tries to provide some standard Python2 functions when run with Python3, but is broken if other packages already do the same, but for a different subset. Specifically, if another package already defined (in Python3) builtins.basestring = str, the import of cStringIO will fail with an ImportError, and later calls to next will fail too.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/237

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Original comment by flutefreak7 (Bitbucket: flutefreak7, GitHub: flutefreak7):

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This issue caused the cStringIO import error when I imported pandas after importing pyqtgraph on Python 3.4. Pyqtgraph has a python2_3 compatibility module in which builtins.basestring = str is implemented for Python 3. Pandas, during initialization, imports pkg_resources which then throws " ImportError: No module named 'cStringIO' ". For me this means if pyqtgraph and pandas are both imported, pandas should be imported first if possible. That or some kind of check that deletes basestr.

Better yet.... I think I'll just add ImportError to the structure shown above so that the except is triggered when cStringIO isn't found in Python 3.

This needs to utilize sys.version_info or at least handle the ImportError.

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Original comment by flutefreak7 (Bitbucket: flutefreak7, GitHub: flutefreak7):

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Dealing with the setuptools egg was a pain so I did this around my pandas import as a workaround for the pkg_resources bug:

#!python

if hasattr(builtins, 'basestring'):
    delattr(builtins, 'basestring')
    import pandas as pd
    builtins.basestring = str
else:
    import pandas as pd

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Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Use PY3/PY2 indicators to reliably select behavior. Fixes #237

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Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Released as 5.8.