pip & Virtual Environments

Python's standard library is large, but sooner or later you will want a package written by someone else: requests for calling web APIs, pandas for data, Django or Flask for websites. You install these with pip, Python's package installer. You should install them inside a virtual environment, a private folder of packages that belongs to one project.

This lesson shows the everyday commands. None of them run in the browser editor, so open a terminal on your own computer (see Install Python & Set Up).

What pip does

pip downloads packages from the Python Package Index (PyPI), the public online store of free Python packages, and installs them so that import can find them. The safest way to call pip is through the Python you want to use, with python -m pip:

python -m pip install requests
python -m pip list
python -m pip show requests
python -m pip install --upgrade requests
python -m pip uninstall requests

install adds a package and anything it depends on, list shows what is installed, show prints details about one package, --upgrade moves to the newest version and uninstall removes it. On Windows with the Python install manager you can write py -m pip instead of python -m pip.

Why you need a virtual environment

Imagine Priya has two projects. An old college project needs version 4 of a package, and her new internship project needs version 5. If both projects share one global set of packages, installing one version breaks the other project.

A virtual environment (often called a "venv") solves this. It is a folder inside your project that holds its own copy of pip and its own installed packages. Each project gets its own environment, so their packages never clash. It also keeps your main Python installation clean, and makes it easy to tell exactly which packages a project needs.

Creating and activating a venv

Python includes the venv module for this. In your project folder, run:

python -m venv .venv

This creates a folder called .venv (the leading dot is a common convention that keeps it out of the way). Next you activate it, which tells your terminal to use the Python and pip from inside that folder. The command depends on your system:

Once it is active, your prompt starts with (.venv). Now python and pip refer to the environment, and anything you install goes into .venv only:

(.venv) PS C:\projects\weather> python -m pip install requests
(.venv) PS C:\projects\weather> python main.py
(.venv) PS C:\projects\weather> deactivate

deactivate switches the terminal back to your normal Python. Code editors such as VS Code detect a .venv folder in your project and offer to use it automatically.

You can check from Python whether you are inside a virtual environment. The first line of the output shows your own project path:

import sys

print(sys.prefix)
print("In a venv:", sys.prefix != sys.base_prefix)

C:\projects\weather\.venv
In a venv: True

Saving dependencies in requirements.txt

Your project's dependencies are the packages it needs. To share the project, you record them so others can install the same versions. pip freeze prints every installed package with its exact version, and > saves that output to a file:

python -m pip freeze > requirements.txt

After installing only requests, the file looks something like this (your version numbers will be different, because packages keep releasing updates):

certifi==2026.10.23
charset-normalizer==3.5.2
idna==3.20
requests==2.34.2
urllib3==2.8.0

requests brought four other packages with it. Kabir, who clones the project on his laptop in Lucknow, creates and activates his own .venv, then installs everything in one step:

python -m pip install -r requirements.txt

pyproject.toml: the modern project file

Larger projects describe themselves in a file called pyproject.toml. It is the standard place for the project name, version, required Python version and dependencies, and most modern tools read it. A small one looks like this:

[project]
name = "weather-report"
version = "0.1.0"
requires-python = ">=3.13"
dependencies = [
    "requests>=2.32",
]

Here >=2.32 means "version 2.32 or newer". You do not have to write this file by hand: tools like uv create and update it for you.

uv: a fast alternative

uv is a popular, very fast tool that handles Python versions, virtual environments and packages in one program. Many developers now use it instead of calling venv and pip separately. You can install it with python -m pip install uv or with the installer described in the official uv documentation.

uv init weather-report
cd weather-report
uv add requests
uv run main.py

uv init creates a project with a pyproject.toml. uv add installs a package into the project's .venv (creating it if needed) and records it in pyproject.toml and a lock file, uv.lock, which pins exact versions. uv run runs your script inside that environment, so you do not even need to activate it. If you prefer the classic workflow, uv venv and uv pip install requests work like their slower cousins.

Common mistakes

• Installing without activating. The package goes into your global Python, and your project's python says ModuleNotFoundError. Check for (.venv) in the prompt first.
• PowerShell refuses to activate. If you see "running scripts is disabled on this system", run Set-ExecutionPolicy -Scope CurrentUser RemoteSigned once in PowerShell, then try again.
• Using pip and python from different installations. If you have several Pythons, a bare pip may install into the wrong one. python -m pip always matches the python you run.
• Putting your own files inside .venv. That folder belongs to the tools and may be deleted and rebuilt at any time. Keep your code next to it, not in it.

Key points

• pip installs packages from PyPI; call it as python -m pip so it uses the right Python.
• A virtual environment gives each project its own private set of packages.
• Create one with python -m venv .venv, activate it with the command for your system, and leave it with deactivate.
• Save dependencies with pip freeze > requirements.txt and restore them with pip install -r requirements.txt.
• pyproject.toml is the modern file that describes a project and its dependencies.
• uv is a fast all-in-one alternative: uv init, uv add, uv run.

mkdir quotes-app
cd quotes-app
python -m venv .venv
.venv\Scripts\Activate.ps1
python -m pip install requests
python -m pip freeze > requirements.txt
deactivate
Remove-Item -Recurse -Force .venv
python -m venv .venv
.venv\Scripts\Activate.ps1
python -m pip install -r requirements.txt