Author Dusty Phillips Reviewers

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 119 
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raise PermissionError("Permission Exists")
def permit_user(self, perm_name, username):
'''Grant the given permission to the user'''
try:
perm_set = self.permissions[perm_name]
except KeyError:
raise PermissionError("Permission does not exist")
else:
if username not in self.authenticator.users:
raise InvalidUsername(username)
perm_set.add(username)
The first 
method allows us to create a new permission, unless it already exists, in 
which case an exception is raised. The second allows us to add a username to a 
permission, unless either the permission or the username doesn't yet exist.
We use 
set
instead of 
list
for usernames, so that even if you grant a user 
permission more than once, the nature of sets means the user is only in the set
once. We'll discuss sets further in a later chapter.
A 
PermissionError
is raised in both methods. This new error doesn't require 
a username, so we'll make it extend 
Exception
directly, instead of our custom 
AuthException
:
class PermissionError(Exception):
pass
Finally, we can add a method to check whether a user has a specific 
permission
or not. In order for them to be granted access, they have to be both logged into the 
authenticator and in the set of people who have been granted access to that privilege. 
If either of these conditions is unsatisfied, an exception is raised:
def check_permission(self, perm_name, username):
if not self.authenticator.is_logged_in(username):
raise NotLoggedInError(username)
try:
perm_set = self.permissions[perm_name]
except KeyError:
raise PermissionError("Permission does not exist")
else:
if username not in perm_set:
raise NotPermittedError(username)
else:
return True
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Expecting the Unexpected