The callback function object in Django URLconf is not called - django

Question

I am learing Django on Django at a glance | Django documentation | Django
When introducing URLconf It reads:
To design URLs for an app, you create a Python module called a URLconf. A table of contents for your app, it contains a simple mapping between URL patterns and Python callback functions.
Nevertheless, a callback function is not called.
mysite/news/urls.py
from django.conf.urls import url
from . import views
urlpatterns = [
url(r'^articles/([0-9]{4})/$', views.year_archive),
]
print('callback funtion:',views.year_archive)
output:
callback function: <function views.year_archive at 0x1039611e0>
So it a funtion object which is not called.
I suppose it should be views.year_archive() to enable it to call.
If not called,how does it work? I assume there a decorator to process it in parent class.
I check the source codes of urldjango.conf.urls | Django documentation | Django
The key statement is:
elif callable(view):
return RegexURLPattern(regex, view, kwargs, name)
I dig on to explore LocaleRegexProvider, RegexURLPattern django.urls.resolvers | Django documentation | Django.
No appopriate codes were found to call the callback function views.year_archive
What's the mechanism of its working?

Answer 1

Django automatically calls the callback view function whenever a request is made to the given url pattern.
To understand how it works, look at this example:
>>> def a():
print("Called function a")
>>> def b():
print("Called function b")
>>> def c(callback):
# call the callback function like this
callback()
>>> c(a)
Called function a
>>> c(b)
Called function b
Basically, this is how Django's url function works.