create venv within venv (with_pip=True)

v(venv)env

"""
Normally to create a virtual environment, we can do so from the command prompt with::

    C:\Users\me>python -m venv venv

This doesn't work when we are within a virtual environment though::

    (venv) C:Users\me>python -m venv venv2
    Error: Command '['C:\\Users\\me\\venv2\\Scripts\\python.exe', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 3221226505.

The issue seems to stem from the home variable in the pyvenv.cfg created from creating the virtual environment within the virtual environment::

    home = C:\Users\me\venv\Scripts
    include-system-site-packages = false
    version = 3.7.0

We need to override this home variable so that it points to a non-virtual enviroment Python.

Below is a shows how to create a virtual environment even if you have a virtual environment activated.

"""
import os
import re
import subprocess
import venv


class EnvB(venv.EnvBuilder):
    """ Subclassing venv.EnvBuilder so we can customize the pyvenv.cfg file """

    def create_configuration(self, context: SimpleNamespace) -> None:
        """
        Create a configuration file indicating where the environment's Python
        was copied from, and whether the system site-packages should be made
        available in the environment.

        :param context: The information for the environment creation request
                        being processed.
        """
        context.cfg_path = path = os.path.join(context.env_dir, 'pyvenv.cfg')

        try:  # this part is different from the superclass
            # we want the correct home python rather than the one from within a potential virtual environment
            with open(os.path.join(os.path.dirname(context.python_dir), 'pyvenv.cfg'), 'r', encoding='utf-8') as f:
                context.python_dir = re.search(r'home\s=\s(.+)', f.read().split('\n')[0]).group(1)
        except FileNotFoundError:
            pass  # the rest is the same

        with open(path, 'w', encoding='utf-8') as f:
            f.write('home = %s\n' % context.python_dir)
            if self.system_site_packages:
                incl = 'true'
            else:
                incl = 'false'
            f.write('include-system-site-packages = %s\n' % incl)
            f.write('version = %d.%d.%d\n' % sys.version_info[:3])


def create_env(venv_dir_):
    venv_py = os.path.join(venv_dir_, 'Scripts', 'python.exe')

    v = EnvB()
    v.create(venv_dir_)
    r = subprocess.run([venv_py, '-c', 'import ensurepip;ensurepip.bootstrap(upgrade=True,default_pip=True)'],
                       capture_output=True)
    if r.stderr and not r.stdout:
        raise Exception(r.stderr.decode('utf-8'))


if __name__ == '__main__':
    # activate your virtual environment before running
    create_env(r'C:\Users\me\venv2')