Support POETRY_HOME for regular operations and not only installation

• [ x] I have searched the issues of this repo and believe that this is not a duplicate.
• [ x] I have searched the documentation and believe that my question is not covered.

Feature Request

One can install poetry (for example with the latest install_poetry.py) and control the deployment directory with the environment variable POETRY_HOME

But the same environment variable is not respected (used) during regular operations.

Example after having installed poetry

$ poetry.exe add wxpython
Creating virtualenv wxpython-test-YATQ2KFZ-py3.9 in C:\Users\poetry_user\AppData\Local\pypoetry\Cache\virtualenvs
Using version ^4.1.1 for wxPython
...

That directory (and other user dependent directories) are computed here:

• https://github.com/python-poetry/poetry/blob/master/src/poetry/utils/appdirs.py

Although it may seem cool to follow Windows conventions, it may be really not the choice for some users to have the virtual environments buried inside the dreaded Microsoft conventions.

Using the same paradigm already implemented in install_poetry.py one could allow the user fine control of where things are going to go.

Below is a sample modified app_dirs.py. Rather than sending a pull-request, I would first like to know what the developers think.

Best regards

"""
This code was taken from https://github.com/ActiveState/appdirs and modified
to suit our purposes.
"""
from __future__ import annotations

import os
import sys

from typing import TYPE_CHECKING


if TYPE_CHECKING:
    from pathlib import Path


WINDOWS = sys.platform.startswith("win") or (sys.platform == "cli" and os.name == "nt")


def expanduser(path: str | Path) -> str:
    """
    Expand ~ and ~user constructions.

    Includes a workaround for http://bugs.python.org/issue14768
    """
    expanded = os.path.expanduser(path)
    if path.startswith("~/") and expanded.startswith("//"):
        expanded = expanded[1:]
    return expanded


def user_cache_dir(appname: str) -> str:
    r"""
    Return full path to the user-specific cache dir for this application.

        "appname" is the name of application.

    Typical user cache directories are:
        macOS:      ~/Library/Caches/<AppName>
        Unix:       ~/.cache/<AppName> (XDG default)
        Windows:    C:\Users\<username>\AppData\Local\<AppName>\Cache

    On Windows the only suggestion in the MSDN docs is that local settings go
    in the `CSIDL_LOCAL_APPDATA` directory. This is identical to the
    non-roaming app data dir (the default returned by `user_data_dir`). Apps
    typically put cache data somewhere *under* the given dir here. Some
    examples:
        ...\Mozilla\Firefox\Profiles\<ProfileName>\Cache
        ...\Acme\SuperApp\Cache\1.0

    OPINION: This function appends "Cache" to the `CSIDL_LOCAL_APPDATA` value.
    """
    if os.getenv("POETRY_HOME"):
        path = os.path.expanduser(os.getenv("POETRY_HOME"))
        return os.path.join(path, ".cache")

    if WINDOWS:
        # Get the base path
        path = os.path.normpath(_get_win_folder("CSIDL_LOCAL_APPDATA"))

        # Add our app name and Cache directory to it
        path = os.path.join(path, appname, "Cache")
    elif sys.platform == "darwin":
        # Get the base path
        path = expanduser("~/Library/Caches")

        # Add our app name to it
        path = os.path.join(path, appname)
    else:
        # Get the base path
        path = os.getenv("XDG_CACHE_HOME", expanduser("~/.cache"))

        # Add our app name to it
        path = os.path.join(path, appname)

    return path


def user_data_dir(appname: str, roaming: bool = False) -> str:
    r"""
    Return full path to the user-specific data dir for this application.

        "appname" is the name of application.
            If None, just the system directory is returned.
        "roaming" (boolean, default False) can be set True to use the Windows
            roaming appdata directory. That means that for users on a Windows
            network setup for roaming profiles, this user data will be
            sync'd on login. See
            <http://technet.microsoft.com/en-us/library/cc766489(WS.10).aspx>
            for a discussion of issues.

    Typical user data directories are:
        macOS:                  ~/Library/Application Support/<AppName>
        Unix:                   ~/.local/share/<AppName>    # or in
                                $XDG_DATA_HOME, if defined
        Win XP (not roaming):   C:\Documents and Settings\<username>\ ...
                                ...Application Data\<AppName>
        Win XP (roaming):       C:\Documents and Settings\<username>\Local ...
                                ...Settings\Application Data\<AppName>
        Win 7  (not roaming):   C:\Users\<username>\AppData\Local\<AppName>
        Win 7  (roaming):       C:\Users\<username>\AppData\Roaming\<AppName>

    For Unix, we follow the XDG spec and support $XDG_DATA_HOME.
    That means, by default "~/.local/share/<AppName>".
    """
    if os.getenv("POETRY_HOME"):
        path = os.path.expanduser(os.getenv("POETRY_HOME"))
        return os.path.join(path, ".share")

    if WINDOWS:
        const = "CSIDL_APPDATA" if roaming else "CSIDL_LOCAL_APPDATA"
        return os.path.join(os.path.normpath(_get_win_folder(const)), appname)
    elif sys.platform == "darwin":
        return os.path.join(expanduser("~/Library/Application Support/"), appname)
    else:
        return os.path.join(
            os.getenv("XDG_DATA_HOME", expanduser("~/.local/share")), appname
        )


def user_config_dir(appname: str, roaming: bool = True) -> str:
    """Return full path to the user-specific config dir for this application.

        "appname" is the name of application.
            If None, just the system directory is returned.
        "roaming" (boolean, default True) can be set False to not use the
            Windows roaming appdata directory. That means that for users on a
            Windows network setup for roaming profiles, this user data will be
            sync'd on login. See
            <http://technet.microsoft.com/en-us/library/cc766489(WS.10).aspx>
            for a discussion of issues.

    Typical user data directories are:
        macOS:                  same as user_data_dir
        Unix:                   ~/.config/<AppName>
        Win *:                  same as user_data_dir

    For Unix, we follow the XDG spec and support $XDG_CONFIG_HOME.
    That means, by default "~/.config/<AppName>".
    """
    if os.getenv("POETRY_HOME"):
        path = os.path.expanduser(os.getenv("POETRY_HOME"))
        return os.path.join(path, ".config", appname)

    if WINDOWS:
        path = user_data_dir(appname, roaming=roaming)
    elif sys.platform == "darwin":
        path = user_data_dir(appname)
    else:
        path = os.getenv("XDG_CONFIG_HOME", expanduser("~/.config"))
        path = os.path.join(path, appname)

    return path


# for the discussion regarding site_config_dirs locations
# see <https://github.com/pypa/pip/issues/1733>
def site_config_dirs(appname: str) -> list[str]:
    r"""Return a list of potential user-shared config dirs for this application.

        "appname" is the name of application.

    Typical user config directories are:
        macOS:      /Library/Application Support/<AppName>/
        Unix:       /etc or $XDG_CONFIG_DIRS[i]/<AppName>/ for each value in
                    $XDG_CONFIG_DIRS
        Win XP:     C:\Documents and Settings\All Users\Application ...
                    ...Data\<AppName>\
        Vista:      (Fail! "C:\ProgramData" is a hidden *system* directory
                    on Vista.)
        Win 7:      Hidden, but writeable on Win 7:
                    C:\ProgramData\<AppName>\
    """
    if os.getenv("POETRY_HOME"):
        path = os.path.expanduser(os.getenv("POETRY_HOME"))
        return os.path.join(path, "etc", appname)

    if WINDOWS:
        path = os.path.normpath(_get_win_folder("CSIDL_COMMON_APPDATA"))
        pathlist = [os.path.join(path, appname)]
    elif sys.platform == "darwin":
        pathlist = [os.path.join("/Library/Application Support", appname)]
    else:
        # try looking in $XDG_CONFIG_DIRS
        xdg_config_dirs = os.getenv("XDG_CONFIG_DIRS", "/etc/xdg")
        if xdg_config_dirs:
            pathlist = [
                os.path.join(expanduser(x), appname)
                for x in xdg_config_dirs.split(os.pathsep)
            ]
        else:
            pathlist = []

        # always look in /etc directly as well
        pathlist.append("/etc")

    return pathlist


# -- Windows support functions --


def _get_win_folder_from_registry(csidl_name: str) -> str:
    """
    This is a fallback technique at best. I'm not sure if using the
    registry for this guarantees us the correct answer for all CSIDL_*
    names.
    """
    import _winreg

    shell_folder_name = {
        "CSIDL_APPDATA": "AppData",
        "CSIDL_COMMON_APPDATA": "Common AppData",
        "CSIDL_LOCAL_APPDATA": "Local AppData",
    }[csidl_name]

    key = _winreg.OpenKey(
        _winreg.HKEY_CURRENT_USER,
        r"Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders",
    )
    directory, _type = _winreg.QueryValueEx(key, shell_folder_name)
    return directory


def _get_win_folder_with_ctypes(csidl_name: str) -> str:
    csidl_const = {
        "CSIDL_APPDATA": 26,
        "CSIDL_COMMON_APPDATA": 35,
        "CSIDL_LOCAL_APPDATA": 28,
    }[csidl_name]

    buf = ctypes.create_unicode_buffer(1024)
    ctypes.windll.shell32.SHGetFolderPathW(None, csidl_const, None, 0, buf)

    # Downgrade to short path name if have highbit chars. See
    # <http://bugs.activestate.com/show_bug.cgi?id=85099>.
    has_high_char = any(ord(c) > 255 for c in buf)
    if has_high_char:
        buf2 = ctypes.create_unicode_buffer(1024)
        if ctypes.windll.kernel32.GetShortPathNameW(buf.value, buf2, 1024):
            buf = buf2

    return buf.value


if WINDOWS:
    try:
        import ctypes

        _get_win_folder = _get_win_folder_with_ctypes
    except ImportError:
        _get_win_folder = _get_win_folder_from_registry

Start by reading src/poetry/utils/appdirs.py and the POETRY_HOME handling in install_poetry.py. Trace how regular Poetry operations obtain cache, data, config, and site-config paths, then verify that setting POETRY_HOME consistently redirects those locations without breaking platform defaults.