Introduction

I checking how Label Studio was initializing the database used to store the data generated by itself and found an interesting function to get the path to the user data folder, i.e., the folder used by the user to store application data. The function checks the operating system being used and then gets the appropriated path.

The module where the function is defined keeps track of the operating system name through the global variable system.

import os
import sys

if sys.platform.startswith('java'):
    import platform
    os_name = platform.java_ver()[3][0]
    if os_name.startswith('Windows'): # "Windows XP", "Windows 7", etc.
        system = 'win32'
    elif os_name.startswith('Mac'): # "Mac OS X", etc.
        system = 'darwin'
    else: # "Linux", "SunOS", "FreeBSD", etc.
        # Setting this to "linux2" is not ideal, but only Windows or Mac
        # are actually checked for and the rest of the module expects
        # *sys.platform* style strings.
        system = 'linux2'
else:
    system = sys.platform

Then, according to the value in system, the function returns the path the application folder inside the user data folder.

def user_data_dir(appname=None, appauthor=None, version=None, roaming=False):
    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.
        "appauthor" (only used on Windows) is the name of the
            appauthor or distributing body for this application. Typically
            it is the owning company name. This falls back to appname. You may
            pass False to disable it.
        "version" is an optional version path element to append to the
            path. You might want to use this if you want multiple versions
            of your app to be able to run independently. If used, this
            would typically be "<major>.<minor>".
            Only applied when appname is present.
        "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:
        Mac OS X:               ~/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\<AppAuthor>\<AppName>
        Win XP (roaming):       C:\Documents and Settings\<username>\Local Settings\Application Data\<AppAuthor>\<AppName>
        Win 7  (not roaming):   C:\Users\<username>\AppData\Local\<AppAuthor>\<AppName>
        Win 7  (roaming):       C:\Users\<username>\AppData\Roaming\<AppAuthor>\<AppName>

    For Unix, we follow the XDG spec and support $XDG_DATA_HOME.
    That means, by default "~/.local/share/<AppName>".
    """
    if system == "win32":
        if appauthor is None:
            appauthor = appname
        const = roaming and "CSIDL_APPDATA" or "CSIDL_LOCAL_APPDATA"
        path = os.path.normpath(_get_win_folder(const))
        if appname:
            if appauthor is not False:
                path = os.path.join(path, appauthor, appname)
            else:
                path = os.path.join(path, appname)
    elif system == 'darwin':
        path = os.path.expanduser('~/Library/Application Support/')
        if appname:
            path = os.path.join(path, appname)
    else:
        path = os.getenv('XDG_DATA_HOME', os.path.expanduser("~/.local/share"))
        if appname:
            path = os.path.join(path, appname)
    if appname and version:
        path = os.path.join(path, version)
    return path