Graceful handling of "ImportError: No module named pkg_resources"

Originally reported by: sleepycal (Bitbucket: sleepycal, GitHub: Unknown)

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The following problem happens sometimes when freezing/installing requirements between different distributions, with and without virtualenv.

ubuntu-precise$ virtualenv --no-site-packages /opt/venv
ubuntu-precise$ /opt/venv/bin/pip freeze > requirements.txt
ubuntu-precise$ cat requirements.txt
Django==1.5
distribute==0.7.3

debian-wheezy$ virtualenv --no-site-packages /opt/venv
debian-wheezy$ /opt/venv/bin/pip install -r requirements.txt

Traceback (most recent call last):
CommandExecutionError: Traceback (most recent call last):
  File "/opt/venv/bin/pip", line 5, in <module>
    from pkg_resources import load_entry_point
ImportError: No module named pkg_resources

This seems to be breaking because it's forcing a version of distribute which different than the one already installed seems to break things. However it's not clear why this happens in the first place, and feels quite user unfriendly (surely this should be handled more gracefully?)

The easiest fix is to remove the distribute line from requirements.txt, but it would be nice if this was handled better?

Any thoughts?

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/138