What is a "slug" in Django?
A "slug" is a way of generating a valid URL, generally using data already obtained. For instance, a slug uses the title of an article to generate a URL. I advise to generate the slug by means of a function, given the title (or another piece of data), rather than setting it manually.
An example:
<title> The 46 Year Old Virgin </title>
<content> A silly comedy movie </content>
<slug> the-46-year-old-virgin </slug>
Now let's pretend that we have a Django model such as:
class Article(models.Model):
title = models.CharField(max_length=100)
content = models.TextField(max_length=1000)
slug = models.SlugField(max_length=40)
How would you reference this object with a URL and with a meaningful name? You could for instance use Article.id so the URL would look like this:
www.example.com/article/23
Or, you might want to reference the title like this:
www.example.com/article/The 46 Year Old Virgin
Since spaces aren't valid in URLs, they must be replaced by %20, which results in:
www.example.com/article/The%2046%20Year%20Old%20Virgin
Both attempts are not resulting in very meaningful, easy-to-read URL. This is better:
www.example.com/article/the-46-year-old-virgin
In this example, the-46-year-old-virgin is a slug: it is created from the title by down-casing all letters, and replacing spaces by hyphens -.
Also see the URL of this very web page for another example.
If I may provide some historical context :
The term "slug" has to do with casting metal—lead, in this case—out of which the press fonts were made. Every paper then had its fonts factory regularly re-melted and recast in fresh molds, since after many prints they became worn out. Apprentices like me started their career there, and went all the way to the top (not anymore).
Typographs had to compose the text of an article in a backward manner with lead characters stacked in a wise. So at printing time the letters would be straight on the paper. All typographs could read the newspaper mirrored as fast as the printed one. Therefore the slugs, (like snails) also the slow stories (the last to be fixed) were many on the bench waiting, solely identified by their fist letters, mostly the whole title generally more readable. Some "hot" news were waiting there on the bench, for possible last minute correction, (Evening paper) before last assembly and definitive printing.
Django emerged from the offices of the Lawrence journal in Kansas. Where probably some printing jargon still lingers. A-django-enthusiast-&-friendly-old-slug-boy-from-France.
The term 'slug' comes from the world of newspaper production.
It's an informal name given to a story during the production process. As the story winds its path from the beat reporter (assuming these even exist any more?) through to editor through to the "printing presses", this is the name it is referenced by, e.g., "Have you fixed those errors in the 'kate-and-william' story?".
Some systems (such as Django) use the slug as part of the URL to locate the story, an example being www.mysite.com/archives/kate-and-william.
Even Stack Overflow itself does this, with the GEB-ish(a) self-referential https://stackoverflow.com/questions/427102/what-is-a-slug-in-django/427201#427201, although you can replace the slug with blahblah and it will still find it okay.
It may even date back earlier than that, since screenplays had "slug lines" at the start of each scene, which basically sets the background for that scene (where, when, and so on). It's very similar in that it's a precis or preamble of what follows.
On a Linotype machine, a slug was a single line piece of metal which was created from the individual letter forms. By making a single slug for the whole line, this greatly improved on the old character-by-character compositing.
Although the following is pure conjecture, an early meaning of slug was for a counterfeit coin (which would have to be pressed somehow). I could envisage that usage being transformed to the printing term (since the slug had to be pressed using the original characters) and from there, changing from the 'piece of metal' definition to the 'story summary' definition. From there, it's a short step from proper printing to the online world.
(a) "Godel Escher, Bach", by one Douglas Hofstadter, which I (at least) consider one of the great modern intellectual works. You should also check out his other work, "Metamagical Themas".