<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_existing_path(path, topmost_path=None):
 """Get the longest parent path in `path` that exists. If `path` exists, it is returned. Args: path (str):
 Path to test topmost_path (str):
 Do not test this path or above Returns: str: Existing path, or None if no path was found. """

    prev_path = None

    if topmost_path:
        topmost_path = os.path.normpath(topmost_path)

    while True:
        if os.path.exists(path):
            return path

        path = os.path.dirname(path)
        if path == prev_path:
            return None

        if topmost_path and os.path.normpath(path) == topmost_path:
            return None

        prev_path = path