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Well apparently nt.environ doesn't reflect os.environ:
>>> os.environ['PYTHONCASEOK'] = '1'
>>> nt.environ['PYTHONCASEOK']
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
KeyError: 'PYTHONCASEOK'
>>> nt.environ[b'PYTHONCASEOK']
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
KeyError: b'PYTHONCASEOK'
>>> os.environ['PYTHONCASEOK']
'1'
>>> os.environ[b'PYTHONCASEOK']
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "C:\t\cpython\lib\os.py", line 450, in __getitem__
 value = self._data[self.encodekey(key)]
 File "C:\t\cpython\lib\os.py", line 508, in encodekey
 return encode(key).upper()
 File "C:\t\cpython\lib\os.py", line 503, in check_str
 raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not bytes
This is silly and is because of how the environ mapping is implemented in Lib/os.py: under POXIX, os.environ reflects posix.environ (it uses the same underlying dict), while under Windows, os.environ uses a distinct dict from nt.environ.