Pass user as parameter

In short: you can not pass a User object as parameter, you can pass an encoding of (a reference to) a User, but this is typically a security risk, and will probably not make life easier than just passing the id.

It is possible that the path converters [Django-doc] give a false impression, but you can not simply parse all sorts of objects. By default (on the moment of writing), there are only five path converters: str, int, slug, uuid and path.

So you will have to find an "encoding" to pass a reference to the object. The most straightforward way is probably the primary key, so you can implement it like:

path('<int:user_id>',views.sunscribe, name="subscribe")

and in the template use it like:

<p>
  <a href="{% url 'subscribe' user=article.author.id %}">
    Подписаться на пользователя {{article.author}}
  </a>
</p>

In that case you thus obtain the id of the user, and you will need to fetch the user object.

The documentation also mentions how you can write path conversions yourself [Django-doc]. But still you will pass a representation of the object (you could, as really bad solution generate for example a pickle stream, but that would be very unsafe, since it would probably impose a severe security risk).

An URL can only contain text, not objects. So even if you write a custom path conversion, you still will need some sort of encoding and decoding.

You can of course ease the process a bit, by making a path converter, that makes the process of generating an id and retrieving the id transparent, by writing a custom path converter, like:

from django.urls.converters import IntConverter

class UserConverter:

    def to_python(self, value):
        return User.object.get(pk=value)

    def to_url(self, value):
        return value.pk

register_converter(converters.UserConverter, 'user')

This will then make the process more transparent, then you can use indeed {% url 'subscribe' user=article.author %}, and <user:user> in the path(..) definition. But I do not know if this improves the situation a lot.