Use the PY_PRELOAD environment variable to execute a snippet of Python code
early in interpreter startup, e.g.:
$ PY_PRELOAD='print("👀")' python -c 'print("hello world")'
👀
hello world
Before you can use PY_PRELOAD, you must first copy usercustomize.py into
your user site directory, e.g.:
cp -v usercustomize.py "$(python -c 'import site; print(site.USER_SITE)')"
Alternatively, you can install it as sitecustomize.py in a user or system
site directory.
Use the pypreload program to exec-chain into another program (presumably
written in Python) after arranging for a snippet of Python code to be executed
early in interpreter startup, e.g.:
pypreload 'print("👀")' python -c 'print("hello world")'
pypreload accomplishes this by creating a new temporary directory, writing a
sitecustomize.py file to it with the given code snippet (plus some
bookkeeping), and prepending the directory to PYTHONPATH. After these
arrangements are made, pypreload exec-chains into the other program (given as
the remaining arguments).