human
I wrote the below code

    if (('$' not in path) and ('%' not in path)):
        return path
    import string
    varchars = ((string.ascii_letters + string.digits) + '_-')
    if isinstance(path, unicode):
        encoding = sys.getfilesystemencoding()
        def getenv(var):
            return os.environ[var.encode(encoding)].decode(encoding)
    else:
        def getenv(var):
            return os.environ[var]
    res = ''
    index = 0
    pathlen = len(path)
    while (index < pathlen):
        c = path[index]
        if (c == "'"):
            path = path[(index + 1):]
            pathlen = len(path)
            try:
                index = path.index("'")
                res = ((res + "'") + path[:(index + 1)])
            except ValueError:
                res = (res + path)
                index = (pathlen - 1)
        elif (c == '%'):
            if (path[(index + 1):(index + 2)] == '%'):
                res = (res + c)
                index = (index + 1)
            else:
                path = path[(index + 1):]
                pathlen = len(path)
                try:
                    index = path.index('%')
                except ValueError:
                    res = ((res + '%') + path)
                    index = (pathlen - 1)
                else:
                    var = path[:index]
                    try:
                        res = (res + getenv(var))
                    except KeyError:
                        res = (((res + '%') + var) + '%')
        elif (c == '$'):
            if (path[(index + 1):(index + 2)] == '$'):
                res = (res + c)
                index = (index + 1)
            elif (path[(index + 1):(index + 2)] == '{'):
                path = path[(index + 2):]
                pathlen = len(path)
                try:
                    index = path.index('}')
                    var = path[:index]
                    try:
                        res = (res + getenv(var))
                    except KeyError:
                        res = (((res + '${') + var) + '}')
           ...

What's a good function header?

=====
Here's a good function header?
assistant
#Here's a good function header.

def expandvars(path):