Activating a Python virtual environment from a script file

This guide explains how to activate a Python virtual environment directly from a script, allowing you to use modules installed in that environment. This is useful for scenarios like Python CGI scripts, which are executed directly by the web server.

Important Notes

• You must have already created a Python virtual environment and installed the required modules.
• For applications that require Python 3, the Python Selector should be used if available.

Activating the virtual environment in a script

To activate a virtual environment from a script, use the activate_this.py file located inside the virtual environment. This allows the script to access all installed modules without manually activating the environment in the shell.

Here’s an example Python CGI script:

#!/home/username/virtualenv/application/x.y/bin/python

import os
import sys
import pkg_resources

# Activate the virtual environment
activate_this = str(os.path.dirname(sys.executable)) + '/activate_this.py'
with open(activate_this) as f:
    code = compile(f.read(), activate_this, 'exec')
    exec(code, dict(__file__=activate_this))

# Import a module from the virtual environment
from module import variable

# Output HTML
print("Content-type:text/html\r\n\r\n")
print('<html>')
print('<head>')
print('<title>Virtualenv test</title>')
print('</head>')
print('<body>')
print('<h3>If you see this, the module import was successful</h3>')
print('Python version: ' + sys.version)
print('<br/>')
print('Python executable: ' + str(sys.executable))
print('<br/>')
print('Installed modules: ')
print([p.project_name for p in pkg_resources.working_set])
print('<br/>')
print('</body>')
print('</html>')

How to use this script

1. Replace the following placeholders in the script:

   • username → your Ruachost account username
   • application → the name of your Python application
   • x.y → Python version of the virtual environment (e.g., 2.7, 3.8)
   • module → a module installed in your virtual environment
   • variable → a variable or class from that module

2. Save the script in your CGI-enabled directory (e.g., ~/public_html/script.cgi).

3. Run the script from the command line or load it in a web browser:

python ~/public_html/script.cgi

Expected Output

If the script runs successfully, you should see a message like:

“If you see this, the module import was successful”

It also displays:

• Python version
• Path to the Python executable
• List of installed modules

If you do not see this message, check for syntax errors or missing modules and ensure the virtual environment path is correct.