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1. Creating and Activating a Virtual Environment (venv)

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What is a Virtual Environment?

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Why Use a Virtual Environment?

• Prevents package conflicts between projects.
• Keeps project dependencies separate.
• Makes projects easier to share.
• Ensures consistent development environments.

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Creating a Virtual Environment

python -m venv venv

python3 -m venv venv

• python -m venv → Runs Python’s built-in virtual environment module.
• venv → Name of the virtual environment folder (can be any name).

MyProject/
│
├── venv/
├── main.py
└── requirements.txt

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Activating the Virtual Environment

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Windows (Command Prompt)

venv\Scripts\activate

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Windows (PowerShell)

venv\Scripts\Activate.ps1

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macOS/Linux

source venv/bin/activate

(venv) C:\Projects\MyProject>

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Deactivating the Virtual Environment

deactivate

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Checking Installed Packages

pip list

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2. Managing requirements.txt

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What is requirements.txt?

Flask==3.1.0
requests==2.32.3
numpy==2.1.1

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Creating requirements.txt

pip freeze > requirements.txt

Flask==3.1.0
requests==2.32.3
numpy==2.1.1

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Installing Packages from requirements.txt

pip install -r requirements.txt

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Updating requirements.txt

pip freeze > requirements.txt

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Viewing Installed Packages

pip list

pip freeze

• pip list → Shows packages in a readable table.
• pip freeze → Outputs packages in requirements.txt format.

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3. Pinning Package Versions

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What is Version Pinning?

requests==2.32.3

requests

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Why Pin Versions?

• Different developers may install different versions.
• Updates can introduce bugs.
• The application may behave differently across systems.

• Consistent behavior.
• Easier debugging.
• Reliable deployments.

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Installing a Specific Version

pip install requests==2.32.3

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Version Specifiers

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Example requirements.txt

Flask==3.1.0
requests==2.32.3
numpy==2.1.1
pandas==2.2.2

pip install -r requirements.txt

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Complete Workflow

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Step 1: Create a project

mkdir MyProject
cd MyProject

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Step 2: Create a virtual environment

python -m venv venv

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Step 3: Activate it

venv\Scripts\activate

source venv/bin/activate

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Step 4: Install packages

pip install requests flask

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Step 5: Save dependencies

pip freeze > requirements.txt

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Step 6: Share the project

MyProject/
│
├── main.py
├── requirements.txt
└── .gitignore

Typically, do not include the venv/ folder in version control. Instead, include requirements.txt so others can recreate the environment.

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Step 7: Another developer installs dependencies

git clone <repository>
cd MyProject
python -m venv venv

# Windows
venv\Scripts\activate

# Linux/macOS
source venv/bin/activate

pip install -r requirements.txt

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Summary

• python -m venv venv → Create a virtual environment.
• Activate: venv\Scripts\activate (Windows) or source venv/bin/activate (Linux/macOS).
• deactivate → Exit the virtual environment.
• pip install package_name → Install a package.
• pip freeze > requirements.txt → Save installed packages.
• pip install -r requirements.txt → Install dependencies from the file.
• Version pinning (==) → Ensures everyone uses the same package version for consistent and reproducible environments.