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Python pip & Packages

pip is like an app store for Python: it downloads extra code packages other people wrote so you can use them. Packages are ready-made toolkits for particular jobs.

In this page:

  1. What is pip?
  2. The requirements.txt File
  3. Virtual Environments
  4. Querying Installed Packages
  5. Best Practices
Syntax
python 
pip install package_name
pip uninstall package_name
pip list

What is pip?

pip is Python's package installer, letting you pull third-party code from the Python Package Index (PyPI) with a single command like 'pip install requests' instead of manually downloading and configuring a library yourself.

Example: What is pip?

python 
# pip install requests
print("pip installs packages from PyPI")

The requirements.txt File

A requirements.txt file pins the exact packages (and often exact versions) a project depends on, so anyone else can recreate your environment with 'pip install -r requirements.txt' instead of guessing which libraries and versions you used.

Example: The requirements.txt File

python 
# requirements.txt would contain:
# requests==2.31.0
# Install with: pip install -r requirements.txt
print("requirements.txt pins project dependencies")

Virtual Environments

A virtual environment creates an isolated Python installation just for one project, so installing a package there doesn't affect (or conflict with) packages installed for other projects on the same machine -- this is what makes reproducible, per-project dependencies possible.

Example: Virtual Environments

python 
# python -m venv venv
# source venv/bin/activate
print("Virtual environments isolate per-project dependencies")

Querying Installed Packages

You can inspect where pip actually installs packages by checking a project's site-packages directory, which is useful for debugging 'why isn't Python finding my installed library' issues -- often the answer is that pip installed into a different Python environment than the one running your script.

Example: Querying Installed Packages

python 
import sys
print(sys.prefix)  # shows which Python environment is active

Best Practices

Wrapping a third-party import in try/except ImportError lets your program fail with a clear, actionable message ('please install requests') instead of a confusing traceback, which matters most for optional dependencies that not every user of your script will need.

Example: Best Practices

python 
try:
    import requests  # optional third-party dependency
except ImportError:
    print("please install requests")  # clear message instead of a raw traceback
Related Topics

Python Standard Library · Python Virtual Environments · Python import Statement · Python name == 'main'

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Common Mistakes
  1. Installing packages with a pip that belongs to a different Python than the one you run; python -m pip install avoids the mismatch.
  2. Installing everything globally instead of in a virtual environment, so projects conflict over package versions.
  3. Forgetting to record dependencies with pip freeze > requirements.txt, so nobody else can recreate your setup.
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