Parsing this markdown block:

```python
# Line 1
# Line 2
``` 
{: .no_line_numbers}

(space after ``` only to allow github markdown parsing)

with fenced_code and attr_list extensions enabled with default settings

renders this html:

<pre><code class="python"># Line 1
# Line 2
</code></pre>
<p>{:&nbsp;.no_line_numbers}</p>

There doesn't seem to be a way to manually specify attributes for code blocks in the current implementation.