[Python] What does '@reify' do and when should it be used?

I saw it in the Pyramid tutorial for UX design. I couldn't make out much what this decorator is all about.

Sample code where I saw its usage.

def __init__(self, request):
self.request = request
renderer = get_renderer("templates/global_layout.pt")
self.global_template = renderer.implementation().macros['layout']

@reify
def company_name(self):
return COMPANY

@reify
def site_menu(self):
new_menu = SITE_MENU[:]
url = self.request.url
for menu in new_menu:
if menu['title'] == 'Home':
menu['current'] = url.endswith('/')
else:
menu['current'] = url.endswith(menu['href'])
return new_menu

@view_config(renderer="templates/index.pt")
def index_view(self):
return {"page_title": "Home"}

@view_config(renderer="templates/about.pt", name="about.html")
def about_view(self):
return {"page_title": "About"}

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