Released under CC0 1.0

reg_win_fileext.py

import os

def register_file_ext(filetypes, docformat_name, command, icon_path):
    """
    
    Parameters:
    -----------
    filetypes: sequence of strings
        Sequence of string of file extensions with a leading dot (e.g. '.txt')
    docformat_name: string
        Typically of the format ApplicationName.Document, but other examples
        include ..., ... and ...
    command: string
        The command to execute when a file with one of the given extensions
        is executed. You typically want your command to end with '"%1" %*' to
        ensure it gets all the passed command line arguments.
    icon_path:
        Path to an .ico file to use for files associated with your command.
    """

    # Setup default icon
    cmd = r'1>nul 2>nul REG ADD "HKCR\%s\DefaultIcon" ' % docformat_name
    cmd += '/t REG_SZ /f /d "{0}"'.format(
        os.path.abspath(icon_path))

    # Try to register for all users (requires admin)
    r = subprocess.call(cmd, shell=True)
    cmds = []
    if r == 0:  # Everything OK, we're admin. Use ASSOC and FTYPE
        for ft in filetypes:
            # Call ASSOC .ext=Document.Format
            cmds.append(r'1>nul 2>nul ASSOC %s=%s' % (ft, docformat_name))
        # Call FTYPE Document.Format=command
        cmds.append(r'1>nul 2>nul FTYPE ' +
                    r'{0}="{1}" "'.format(docformat_name, command))
    else:
        # Not admin. We have to add everything to HKCU manually
        # Add icon:
        cmd = (r'1>nul 2>nul REG ADD ' +
               r'"HKCU\Software\Classes\%s\DefaultIcon"' % docformat_name) +
               r' /t REG_SZ /f /d "'
        cmd += os.path.abspath(icon_path)
        cmds.append(cmd)
        for ft in filetypes:
            # ASSOC
            cmds.append(
                (r'1>nul 2>nul REG ADD "HKCU\Software\Classes\%s" ' % ft) +
                (r'/v "" /t REG_SZ /d "%s" /f' % docformat_name))
        # FTYPE
        cmds.append(
            r'1>nul 2>nul REG ADD ' +
            (r'"HKCU\Software\Classes\%s\shell\open\command"' %
             docformat_name) +
            r' /v "" /t REG_EXPAND_SZ /d ' +
            (r'"{0}" %*" /f'.format(command))

    for cmd in cmds:
        r = subprocess.call(cmd, shell=True)