need to list an image seq with missing image numbers in python
<p>I can list the image files in a directory and they look like this:</p>

<pre><code>['IMG_3842.CR2', 'IMG_3843.CR2', 'IMG_3844.CR2', 'IMG_3846.CR2', 'IMG_3847.CR2', 
'IMG_3848.CR2', 'IMG_3849.CR2', 'IMG_3850.CR2', 'IMG_3851.CR2', 'IMG_3852.CR2', 
'IMG_3855.CR2', 'IMG_3856.CR2', 'IMG_3857.CR2', 'IMG_3858.CR2', 'IMG_3859.CR2']
</code></pre>

<p>The file numbers range from 3842 to 3859. However, there are gaps in the numbers.
What I need to do is get a list like this as an output:</p>

<p><code>3842-3844, 3846-3852,  3855-3859</code></p>
