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This is a more general version of #10496: os.path.expanduser is documented as returning the unmodified string if it cannot be expanded (http://docs.python.org/3/library/os.path.html#os.path.expanduser) but there's one edge case where this can fail: when running without HOME in the environment:
https://github.com/python/cpython/blob/3.3/Lib/posixpath.py#L259-L263
In this case, pwd.getpwuid is called and although it's not documented as such, it raises a KeyError anytime the underlying POSIX getpwuid() call fails:
https://github.com/python/cpython/blob/3.3/Modules/pwdmodule.c#L119-L120
Reproducing this deliberately:
cadams@Io:~ $ sudo -H python
Python 2.7.5 (default, Aug 25 2013, 00:04:04) 
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import posix
>>> posix.setuid(1001)
>>> posix.seteuid(1001)
>>> os.environ.pop('HOME')
'/var/root'
>>> os.path.expanduser("~")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 269, in expanduser
 userhome = pwd.getpwuid(os.getuid()).pw_dir
KeyError: 'getpwuid(): uid not found: 1001'
Context:
* https://github.com/toastdriven/django-haystack/issues/924
* https://github.com/kennethreitz/requests/issues/1846.