[python] Determine if Python is running inside virtualenv

There are multiple good answers here, and some less robust ones. Here's an overview.

How not to do it

Do not rely on on the location of Python or the site-packages folder.

If these are set to non-standard locations, that does not mean you're actually in a virtual environment. Users can have more than one Python version installed, and those are not always where you expect them to be.

Avoid looking at:

• sys.executable
• sys.prefix
• pip -V
• which python

Also, do not check for the presence of venv, .venv or envs in any of these paths. This will break for environments with a more unique location. For example, Pipenv uses hash values as the name for its environments.

VIRTUAL_ENV environment variable

Both virtualenv and venv set the environment variable $VIRTUAL_ENV when activating an environment. See PEP 405.

You can read out this variable in shell scripts, or use this Python code to determine if it's set.

import os
running_in_virtualenv = "VIRTUAL_ENV" in os.environ

# alternative ways to write this, also supporting the case where
# the variable is set but contains an empty string to indicate
# 'not in a virtual environment':
running_in_virtualenv = bool(os.environ.get("VIRTUAL_ENV"))
running_in_virtualenv = bool(os.getenv("VIRTUAL_ENV"))

The problem is, this only works when the environment is activated by the activate shell script.

You can start the environment's scripts without activating the environment, so if that is a concern, you have to use a different method.

sys.base_prefix

virtualenv, venv and pyvenv point sys.prefix to the Python installed inside of the virtualenv as you would expect.

At the same time, the original value of sys.prefix is also made available as sys.base_prefix.

We can use that to detect if we're in a virtualenv.

import sys
# note: Python versions before 3.3 don't have sys.base_prefix
# if you're not in virtual environment
running_in_virtualenv = sys.prefix != sys.base_prefix

Fallback: sys.real_prefix

Now watch out, virtualenv before version 20 did not set sys.base_prefix but it set sys.real_prefix instead.

So to be safe, check both as suggested in hroncok's answer:

import sys

real_prefix = getattr(sys, "real_prefix", None)
base_prefix = getattr(sys, "base_prefix", sys.prefix)

running_in_virtualenv = (base_prefix or real_prefix) != sys.prefix

Anaconda

If you're using Anaconda virtual environments, check Victoria Stuart's answer.