Fix inaccessible python packages after Python update w/ Conda (#1052)

Author: RobertLucian

Diff for: docs/deployments/python-packages.md

@@ -78,4 +78,6 @@ conda-forge::pygpu
 
 In situations where both `requirements.txt` and `conda-packages.txt` are provided, Cortex installs Conda packages in `conda-packages.txt` followed by PyPI packages in `requirements.txt`. Conda and Pip package managers install packages and dependencies independently. You may run into situations where Conda and pip package managers install different versions of the same package because they install and resolve dependencies independently from one another. To resolve package version conflicts, it may be in your best interest to specify their exact versions in `conda-packages.txt`.
 
+The current version of Python is `3.6.9`. Updating Python to a different version is possible with Conda, but there are no guarantees that Cortex's web server will continue functioning correctly. If there's a change in Python's version, the necessary core packages for the web server will be reinstalled. If you are using a custom base image, any other Python packages that are built in to the image won't be accessible at runtime.
+
 Check the [best practices](https://www.anaconda.com/using-pip-in-a-conda-environment/) on using `pip` inside `conda`.

Diff for: pkg/workloads/cortex/serve/run.sh

@@ -42,7 +42,19 @@ fi
 
 # install from conda-packages.txt
 if [ -f "/mnt/project/conda-packages.txt" ]; then
+    py_version_cmd='echo $(python -c "import sys; v=sys.version_info[:2]; print(\"{}.{}\".format(*v));")'
+    old_py_version=$(eval $py_version_cmd)
+
     conda install --file /mnt/project/conda-packages.txt
+    new_py_version=$(eval $py_version_cmd)
+
+    # reinstall core packages if Python version has changed
+    if [ $old_py_version != $new_py_version ]; then
+        echo "warning: you have changed the Python version from $old_py_version to $new_py_version; this may break Cortex's web server"
+        echo "reinstalling core packages ..."
+        pip --no-cache-dir install -r /src/cortex/serve/requirements.txt
+        rm -rf $CONDA_PREFIX/lib/python${old_py_version}  # previous python is no longer needed
+    fi
 fi
 
 # install pip packages