Author Dusty Phillips Reviewers

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We can also add a method to check whether a particular username is logged in. 
Deciding whether to use an exception here is trickier. Should we raise an exception if 
the username does not exist? Should we raise an exception if the user is not logged in?
To answer these questions, we need to think about how the method would be 
accessed. Most often, this method will be used to answer the yes/no question, 
"Should I allow them access to 

?" The answer will either be, "Yes, the 
username is valid and they are logged in", or "No, the username is not valid or they 
are not logged in". Therefore, a Boolean return value is sufficient. There is no need 
to use exceptions here, just for the sake of using an exception.
def is_logged_in(self, username):
if username in self.users:
return self.users[username].is_logged_in
return False
Finally, we can add a default authenticator instance to our module so that the client 
code can access it easily using 
auth.authenticator
:
authenticator = Authenticator()
This line goes 
at the module level, outside any class definition, so the authenticator 
variable can be accessed as 
auth.authenticator
. Now we can start on the 
Authorizor
class, which maps permissions to users. The 
Authorizor
class should 
not permit user access to a permission if they are not logged in, so they'll need 
a reference to a specific authenticator. We'll also need to set up the permission 
dictionary upon initialization:
class Authorizor:
def __init__(self, authenticator):
self.authenticator = authenticator
self.permissions = {}
Now we can write methods to add new permissions and to set up which users are 
associated with each permission:
def add_permission(self, perm_name):
'''Create a new permission that users
can be added to'''
try:
perm_set = self.permissions[perm_name]
except KeyError:
self.permissions[perm_name] = set()
else:
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Chapter 4