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Python Virtual Environments

A virtual environment is a private workspace for one project, like giving each project its own toolbox. It stops the tools of one project from clashing with another.

In this page:

  1. What is a Virtual Environment?
  2. Detecting Active Environment
  3. Simulating Package Isolation
  4. Site-Packages Path Inspection
  5. Environment Configuration Check
Syntax
python 
python -m venv env_name
source env_name/bin/activate  # Windows: env_name\Scripts\activate
deactivate

What is a Virtual Environment?

A virtual environment is a self-contained copy of the Python interpreter and its own package directory, letting one project depend on Django 3 while another on the same machine depends on Django 5 without either installation interfering with the other. sys.prefix points at the currently active environment's root directory.

Example: What is a Virtual Environment?

python 
import sys
print(sys.prefix)

Detecting Active Environment

When a virtual environment is active, your shell's PATH is temporarily modified so python and pip resolve to the environment's own copies rather than the system-wide install -- you can confirm which one is active by checking sys.executable's file path.

Example: Detecting Active Environment

python 
import sys
print(sys.executable)

Simulating Package Isolation

Each virtual environment maintains its own independent package list, isolated from the system Python and every other environment -- inspecting sys.path from within an active environment shows it searching that environment's own site-packages directory first.

Example: Simulating Package Isolation

python 
import sys
print(sys.path[0])

Site-Packages Path Inspection

Every package installed inside an environment physically lives in a folder called site-packages, and the built-in site module can report exactly where that folder sits on disk -- useful when you need to manually inspect or clean up installed files.

Example: Site-Packages Path Inspection

python 
import site
print(site.getsitepackages())

Environment Configuration Check

Every virtual environment contains a small pyvenv.cfg file recording which Python version created it and where; checking for this file's existence is a reliable way to programmatically detect whether a given directory is actually a valid virtual environment.

Example: Environment Configuration Check

python 
import os
print(os.path.exists("pyvenv.cfg"))
Related Topics

Python pip & Packages · Python name == 'main' · Python Standard Library · Python import Statement

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Common Mistakes
  1. Creating a virtual environment but forgetting to activate it, so pip install puts packages in the global Python.
  2. Committing the venv folder to version control, when it should be ignored and rebuilt from requirements.txt.
  3. Moving or renaming the environment folder, which breaks it because its scripts contain absolute paths.
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