Here's a simple example:
def app($prop):
    tr:
        td:
            $prop
app('year,')
Compiling this with RapydML produces this error:
Error in mltest.pyml: line 6: Method 'app' expects 1 attributes, 0 given.
When the comma after year is removed, the file correctly compiles to
<tr>
    <td>
        year
    </td>
</tr>