Allow more IANA schemes in URLValidator

[Sascha Peilicke]

Currently, URLValidator only allows the following schemes: http(s) and ftp(s). Since the formal definition of URL (being a subset of URI) includes many more valid schemes [1], URLValidator should be expanded to allow those. Of course Django users could create their own validator but this seems to be a rather common task it's probably better to have that in core Django.

An alternative solution would be to provide an URIValidator which allows more schemes than what we currently have in URLValidator. However, since sth. like ​ssh://example.com is not only a valid URI but also a valid URL [2] it doesn't seem to make sense to differentiate URI and URL.

Related to that, #15229 already added support for ftp(s)

[1] ​https://en.wikipedia.org/wiki/URI_scheme#Official_IANA-registered_schemes
[2] ​https://tools.ietf.org/html/rfc3986

[Sascha Peilicke]

So ​https://github.com/django/django/pull/1717 addresses at least the ssh example, but I'd rather want to generalize URLValidator further.

[Claude Paroz]

Yes, this might be interesting, but only if we generalize accepted schemes. We should probably add a new scheme init parameter allowing to specify a scheme list (defaulting to ['http', 'https', 'ftp', 'ftps']), and then check for valid schemes in __call__.

[Sascha Peilicke]

It's now ​https://github.com/django/django/pull/1759

[Claude Paroz]

I've provided an alternative patch, based on Sascha's patch: ​https://github.com/django/django/pull/2099

[Claude Paroz <claude@…>]

In 6d66ba59488bbd0d4f0c2f32b2921f1512301143:

Fixed #21242 -- Allowed more IANA schemes in URLValidator

Thanks Sascha Peilicke for the report and initial patch, and
Tim Graham for the review.