Default python on Raspbian is of version 2. Clearly, the symbolic link /usr/bin/python is linked to /usr/bin/python2.7. We need to manually change the symbolic link to /usr/bin/python3, which is another symbolic link to /usr/bin/python3.5. Please refer to:
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Here, we are going to install some python packages, including scipy, numpy, matplotlib, pandas, sklearn, skimage, etc., via Synaptic Package Manager.
NOTE: It seems numpy has alreadly been installed by default after Raspbian installation.
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For some other python packages, such as: statsmodels, there seems to be NO version for python3 yet. We are going to install such packages from source in the next subsection.