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Getting bitten by this with numpy/scipy installations
Having previous scipy installed,
 pip install -I scipy
creates a broken scipy installation, because the previous one is not removed and gets overwritten. For instance, an old spectral.so file leftover hides the spectral.py file from the new installation causing broken behavior.
Similarly trying
 pip uninstall scipy
 pip install scipy
does the same, because the distutils uninstall does not uninistall anything, leaves the scipy directory there and all cheerful ends with a 'Successfully uninstalled scipy'.
This is *dangerous*. Mixing old and new code could lead to the weirdest behavior. In principle, one could even use this property to craft packages such that when two subsequent versions are installed one on top of the other malicious things happen even if individually each version is innoquous.