Great question, Benjamin! The pip flag --system-site-packages is used within a virtual environment to give it access to the globally installed Python packages (i.e., those installed in the system-wide Python environment). This means the virtual environment can see and use packages installed outside of it.

Equivalent Without a Virtual Environment

If you are not using a virtual environment, your Python environment already has access to all system-wide installed packages by default. There is no need for a special flag—your Python interpreter will automatically look in the system site-packages directory for installed packages.

How Python Finds Packages

• When you run Python or install packages with pip outside of a virtual environment, it uses the system-wide Python installation and its site-packages directory.
• The paths where Python looks for packages are listed in sys.path. You can check this by running:
  python

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  import sys
  print(sys.path)

  This will show you all the directories Python searches for packages, including the system site-packages.

Example

• If you install a package globally (e.g., pip install numpy), it will be available to all Python scripts and environments on your system (unless you are in a virtual environment with --no-site-packages or the default isolation).

Key Point

• No flag is needed outside a virtual environment—your Python environment already includes the system site-packages.

Why Use Virtual Environments?

• Virtual environments are used to isolate project dependencies. By default, they exclude system packages to avoid conflicts. The --system-site-packages flag is only relevant when creating a virtual environment and you want to include system packages.

Would you like to know more about how Python manages package paths or how to inspect your current environment?