Python virtualenv broken after upgrading Homebrew or system Python: 'bad interpreter: No such file or directory', 'dyld: Library not loaded' or 'python: command not found' inside the venv

Cause: A venv does not contain a full Python; its bin/python is a symlink or copy tied to the base installation recorded as home in pyvenv.cfg. When brew upgrade (or an OS update) replaces or removes that Python, the venv points at something that no longer exists, so activating it fails with errors such as bad interpreter or dyld: Library not loaded for the old Python library. Homebrew automatically uninstalls old versions of a formula when it upgrades it, which is why this often happens right after a routine upgrade. Scripts in venv/bin also have the old interpreter path hard-coded in their shebang lines. Fix (best first): 1. Recreate the venv; the Python docs describe venvs as disposable. Save dependencies first if the old one still works: pip freeze > requirements.txt (or use your lock file). Then: deactivate; rm -rf .venv; python3 -m venv .venv; source .venv/bin/activate; python -m pip install -r requirements.txt. 2. If the old venv cannot run, rebuild from the project's requirements.txt, pyproject.toml, or lock file instead. 3. If the base Python was upgraded in place within the same minor version, python3 -m venv --upgrade .venv can rebind the environment; for a new minor version (for example 3.12 to 3.13) compiled packages must be reinstalled anyway, so recreate. 4. To avoid repeats, create venvs from a version-specific interpreter (python3.12 -m venv .venv) or use a version manager (pyenv, uv) so a Homebrew upgrade does not remove your interpreter. Homebrew can hold a formula back with brew pin, but this blocks updates, so prefer the version manager. 5. Do not copy or move venv directories; they are not portable. Re-create them on the new path. 6. Rebuild tools installed with pipx or global pip too, since they have the same dependency on the old Python. Verify: .venv/bin/python --version runs, which python points inside .venv after activation, and pip list matches your requirements. Sources: https://docs.python.org/3/library/venv.html https://docs.brew.sh/FAQ