How can python subprocess.Popen see select.poll and then later not? (select 'module' object has no attribute 'poll')

I'm using the (awesome) mrjob library from Yelp to run my python programs in Amazon's Elastic Map Reduce. It depends on subprocess in the standard python library. From my mac running python2.7.2, everything works as expected

However, when I switched to using the exact same code on Ubuntu LTS 11.04 also with python2.7.2, I encountered something strange:

mrjob loads the job, and then attempts to communicate with its child processes using subprocess and generates this error:

      File "/usr/local/lib/python2.7/dist-packages/mrjob-0.3.1-py2.7.egg/mrjob/emr.py", line 1212, in _build_steps
        steps = self._get_steps()
      File "/usr/local/lib/python2.7/dist-packages/mrjob-0.3.1-py2.7.egg/mrjob/runner.py", line 1003, in _get_steps
        stdout, stderr = steps_proc.communicate()
      File "/usr/lib/python2.7/subprocess.py", line 754, in communicate
        return self._communicate(input)
      File "/usr/lib/python2.7/subprocess.py", line 1302, in _communicate
        stdout, stderr = self._communicate_with_poll(input)
      File "/usr/lib/python2.7/subprocess.py", line 1332, in _communicate_with_poll
        poller = select.poll()
    AttributeError: 'module' object has no attribute 'poll'

This appears to be a problem with subprocess and not mrjob.

I dug into /usr/lib/python2.7/subprocess.py and found that during import it runs:

    if mswindows:
        ... snip ...
    else:
        import select
        _has_poll = hasattr(select, 'poll')

By editing that, I verified that it really does set _has_poll==True. And this is correct; easily verified on the command line.

However, when execution progresses to using Popen._communicate_with_poll somehow the select module has changed! This is generated by printing dir(select) right before it attempts to use select.poll().

    ['EPOLLERR', 'EPOLLET', 'EPOLLHUP', 'EP