Determine if Python is running inside virtualenv

Question

Is it possible to determine if the current script is running inside a virtualenv environment?

Answer 1

The most reliable way to check for this is to check whether sys.prefix == sys.base_prefix. If they are equal, you are not in a virtual environment; if they are unequal, you are. Inside a virtual environment, sys.prefix points to the virtual environment, and sys.base_prefix is the prefix of the system Python the virtualenv was created from.

The above always works for Python 3 stdlib venv and for recent virtualenv (since version 20). Older versions of virtualenv used sys.real_prefix instead of sys.base_prefix (and sys.real_prefix did not exist outside a virtual environment), and in Python 3.3 and earlier sys.base_prefix did not ever exist. So a fully robust check that handles all of these cases could look like this:

import sys

def get_base_prefix_compat():
    """Get base/real prefix, or sys.prefix if there is none."""
    return getattr(sys, "base_prefix", None) or getattr(sys, "real_prefix", None) or sys.prefix

def in_virtualenv():
    return get_base_prefix_compat() != sys.prefix

If you only care about supported Python versions and latest virtualenv, you can replace get_base_prefix_compat() with simply sys.base_prefix.

Using the VIRTUAL_ENV environment variable is not reliable. It is set by the virtualenv activate shell script, but a virtualenv can be used without activation by directly running an executable from the virtualenv's bin/ (or Scripts) directory, in which case $VIRTUAL_ENV will not be set. Or a non-virtualenv Python binary can be executed directly while a virtualenv is activated in the shell, in which case $VIRTUAL_ENV may be set in a Python process that is not actually running in that virtualenv.