what shebang to use for python scripts run under a pyenv virtualenv

I don't really know why calling the interpreter with the full path wouldn't work for you, I use it all the time, but if you want to use the python interpreter that is in your environment you should do:

#!/usr/bin/env python

That way you search your environment for the python interpreter to use.

As you expected, you should be able to use the full path to the virtual environment's python in the shebang to choose/control the environment the script runs in regardless of the environment of the controlling script.

In the comments on your question, VPfB & you find that the /Users/username/.pyenv/shims/python is a shell script that does an exec $pyenv_python. You should be able to echo $pyenv_python to determine the real python and use that as your shebang.

See also: https://unix.stackexchange.com/questions/209646/how-to-activate-virtualenv-when-a-python-script-starts

Try pyenv virtualenvs to find a list of virtual environment directories.

And then you might find a using shebang something like this:

#!/Users/username/.pyenv/python/versions/venv_name/bin/python
import pandas as pd
print 'success'

... will enable the script to work using the chosen virtual environment in other (virtual or not) environments:

(venv_name) $ ./script.py 
success
(venv_name) $ pyenv activate non_pandas_venv 
(non_pandas_venv) $ ./script.py
success
(non_pandas_venv) $ . deactivate
$ ./script.py
success
$

The trick is that if you call out the virtual environment's python binary specifically, python looks around that binary's path location for the supporting files and ends up using the surrounding virtual environment. (See per How does virtualenv work? )

If you need to use more shell than you can put in the #! shebang line, you can start the file with a simple shell script which launches Python on the same file.

#!/bin/bash
"exec" "pyenv" "exec" "python" "$0" "$@"
# the rest of your Python script can be written below

Because of the quoting, Python doesn't execute the first line, and instead joins the strings together for the module docstring... which effectively ignores it.

You can see more here.