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Being able to reuse some "heavy" systemwide packages (like ipython, numpy, etc.) would improve the speed of creating environment and decrease environment size.
Using custom bootstrap as suggested at:
doesn't remove the need for download and longish compile ...
Thanks for the report. This is a good idea, and has been raised before, but the implementation is non-trivial and no-one has ever supplied one.
Although it is related to virtualenv, it would actually need to be a pip feature, as pip deals with packages. It is tracked at pypa/pip#300
Thanks. I subscribed to the pip issue as per above.