Upgrade python in a virtualenv

If you happen to be using the venv module that comes with Python 3.3+, it supports an --upgrade option. Per the docs:

Upgrade the environment directory to use this version of Python, assuming Python has been upgraded in-place

python3 -m venv --upgrade ENV_DIR

Did you see this? If I haven't misunderstand that answer, you may try to create a new virtualenv on top of the old one. You just need to know which python is going to use your virtualenv (you will need to see your virtualenv version).

If your virtualenv is installed with the same python version of the old one and upgrading your virtualenv package is not an option, you may want to read this in order to install a virtualenv with the python version you want.

EDIT

I've tested this approach (the one that create a new virtualenv on top of the old one) and it worked fine for me. I think you may have some problems if you change from python 2.6 to 2.7 or 2.7 to 3.x but if you just upgrade inside the same version (staying at 2.7 as you want) you shouldn't have any problem, as all the packages are held in the same folders for both python versions (2.7.x and 2.7.y packages are inside your_env/lib/python2.7/).

If you change your virtualenv python version, you will need to install all your packages again for that version (or just link the packages you need into the new version packages folder, i.e: your_env/lib/python_newversion/site-packages)

Updated again: The following method might not work in newer versions of virtualenv. Before you try to make modifications to the old virtualenv, you should save the dependencies in a requirement file (pip freeze > requirements.txt) and make a backup of it somewhere else. If anything goes wrong, you can still create a new virtualenv and install the old dependencies in it (pip install -r requirements.txt).

Updated: I changed the answer 5 months after I originally answered. The following method is more convenient and robust.

Side effect: it also fixes the Symbol not found: _SSLv2_method exception when you do import ssl in a virtual environment after upgrading Python to v2.7.8.

Notice: Currently, this is for Python 2.7.x only.

If you're using Homebrew Python on OS X, first deactivate all virtualenv, then upgrade Python:

brew update && brew upgrade python

Run the following commands (<EXISTING_ENV_PATH> is path of your virtual environment):

cd <EXISTING_ENV_PATH>
rm .Python
rm bin/pip{,2,2.7}
rm bin/python{,2,2.7}
rm -r include/python2.7
rm lib/python2.7/*
rm -r lib/python2.7/distutils
rm lib/python2.7/site-packages/easy_install.*
rm -r lib/python2.7/site-packages/pip
rm -r lib/python2.7/site-packages/pip-*.dist-info
rm -r lib/python2.7/site-packages/setuptools
rm -r lib/python2.7/site-packages/setuptools-*.dist-info

Finally, re-create your virtual environment:

virtualenv <EXISTING_ENV_PATH>

By doing so, old Python core files and standard libraries (plus setuptools and pip) are removed, while the custom libraries installed in site-packages are preserved and working, as soon as they are in pure Python. Binary libraries may or may not need to be reinstalled to function properly.

This worked for me on 5 virtual environments with Django installed.

BTW, if ./manage.py compilemessages is not working afterwards, try this:

brew install gettext && brew link gettext --force

Step 1: Freeze requirement & take a back-up of existing env

pip freeze > requirements.txt
deactivate
mv env env_old

Step 2: Install Python 3.7 & activate virutal environment

sudo apt-get install python3.7-venv
python3.7 -m venv env
source env/bin/activate
python --version

Step 3: Install requirements

sudo apt-get install python3.7-dev
pip3 install -r requirements.txt

How to upgrade the Python version for an existing virtualenvwrapper project and keep the same name

I'm adding an answer for anyone using Doug Hellmann's excellent virtualenvwrapper specifically since the existing answers didn't do it for me.

Some context:

• I work on some projects that are Python 2 and some that are Python 3; while I'd love to use python3 -m venv, it doesn't support Python 2 environments
• When I start a new project, I use mkproject which creates the virtual environment, creates an empty project directory, and cds into it
• I want to continue using virtualenvwrapper's workon command to activate any project irrespective of Python version

Directions:

Let's say your existing project is named foo and is currently running Python 2 (mkproject -p python2 foo), though the commands are the same whether upgrading from 2.x to 3.x, 3.6.0 to 3.6.1, etc. I'm also assuming you're currently inside the activated virtual environment.

1. Deactivate and remove the old virtual environment:

$ deactivate
$ rmvirtualenv foo

Note that if you've added any custom commands to the hooks (e.g., bin/postactivate) you'd need to save those before removing the environment.

2. Stash the real project in a temp directory:

$ cd ..
$ mv foo foo-tmp

3. Create the new virtual environment (and project dir) and activate:

$ mkproject -p python3 foo

4. Replace the empty generated project dir with real project, change back into project dir:

$ cd ..
$ mv -f foo-tmp foo
$ cdproject

5. Re-install dependencies, confirm new Python version, etc:

$ pip install -r requirements.txt
$ python --version

If this is a common use case, I'll consider opening a PR to add something like $ upgradevirtualenv / $ upgradeproject to virtualenvwrapper.