This seems like a bug to me, but it may be a recognized limitation even though I couldn't find any documentation suggesting that my tests should not work reliably.  I see no reason why my tests should not work reliably.

I have reliably reproduced the flakeyness under Windows XP with Python 2.6.4 and under Xubuntu 14.04 64-bit with Python 2.7.6 and Python 3.4.0.

Ubuntu 14.04 (trusty)  - XFCE desktop (Xubuntu) 64-bit
Gnome:  3.8.4
Kernel:  3.13.0-62-generic
GCC Version:  4.8 (x86_64-linux-gnu)
Xorg Version:  1.15.1 (12 February 2015  02:49:29PM)

Rebooting doesn't help.

I first encountered the flakeyness in a complicated script which I will describe in outline first.  The very, very simple interactive Python sessions that follow are probably of more interest and use to you.
---
Outline of my complicated script:

import email
    
    # The next line works around a flakeyness in Python    
    from email.mime.nonmultipart import MIMENonMultipart
    
    msg = email.mime.nonmultipart.MIMENonMultipart('text', 'plain', charset='utf-8')

Without the above work around I always get Error:

AttributeError: 'module' object has no attribute 'nonmultipart'

Note "import email" is global.  "from email.mime.nonmultipart import MIMENonMultipart" is local to the function containing the "msg = " line which is the line that fails whenever the workaround is absent.
---
XP Interpreter Session:

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\Admin>python
Python 2.6.4 (r264:75708, Oct 26 2009, 08:23:19) [MSC v.1500 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import email
>>> print email.mime.nonmultipart.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> print email.mime.nonmultipart.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> print email.mime.nonmultipart.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> print email.mime.nonmultipart.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> print email.mime.nonmultipart.__file__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> from email.mime import nonmultipart
>>> print email.mime.nonmultipart.__file__
C:\bin\Python26\lib\email\mime\nonmultipart.pyc
>>>

Xubuntu Python 2.7.6 Session

2015-08-25 14:02:46 /home/bob06
$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import email
>>> print(email.mime.nonmultipart.__file__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'nonmultipart'
>>> print(email.mime.nonmultipart.__file__)
/usr/lib/python2.7/email/mime/nonmultipart.pyc


Xubuntu Python 3.4.0 Session

2015-08-27 15:27:39 /home/bob06
$ python3
Python 3.4.0 (default, Jun 19 2015, 14:20:21) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import email
>>> print(email.mime.nonmultipart.__file__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'mime'
>>> print(email.mime.nonmultipart.__file__)
/usr/lib/python3.4/email/mime/nonmultipart.py