Why pip3 install in python2 sitepackages

It seems that pip3 refers to Python-2.7's pip module or any other version of Python-3 that you have installed on your machine. However, you can install packages directly using the intended Python version. You'd need to just use -m option.

python3.6 -m pip install numpy

Another option is to changing the source path that pip3 refers to. You can do this by finding the path of Python-3.6's pip and just bind it to pip3 alias.

This works for me:

python3 -m pip install -U --force-reinstall pip

Find the absolute path of the python3 interpreter with a command like this:

$ which python3
/Library/Frameworks/Python.framework/Versions/3.6/bin/python

Your path may be something different, of course. Copy that line to your clipboard.

Edit the pip3 script, which was installed using incorrect interpreter. Something like this:

vi $(which pip3)

You might need to use sudo here, but try it first without. The first line will be something like:

#!/Library/Frameworks/Python.framework/Versions/2.7/bin/python

Change it to the path found in the previous step, e.g.

#!/Library/Frameworks/Python.framework/Versions/3.6/bin/python

Save the pip3 file and exit. This should be sufficient to associate pip3 with the correct environment. Check and verify the result with pip3 --version. Now pip3 install numpy should work as expected.