__setattr__
here’s a trivial person class
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
notice that you can add any attribute to an instance of person
terry = Person('Terry Gilliam', 78)
terry.new_attribute = 'lets make sure this fails'
print(terry.new_attribute) # 'lets make sure this fails'
can you modify class Person so that it disallows adding any new attributes beyond .name and .age?
hint: add a
__setattr__(self, name, value)function to Person
the following code should succeed:
terry = Person('Terry Gilliam', 78)
terry.age = terry.age + 1
print(terry.age) # 79
ok = False
try:
terry.new_attribute = 'lets make sure this fails'
except:
ok = True
print('good')
assert ok
class Person:
BLOCK_MORE_ATTRIBUTES = '__block_more_attributes'
def __setattr__(self, name, value):
if getattr(self, Person.BLOCK_MORE_ATTRIBUTES, False):
# check the attribute already exists, raise otherwise
getattr(self, name)
# if we got here, the attribute already exists, or we're not blocking
super().__setattr__(name, value)
def __init__(self, name, age):
self.name = name
self.age = age
# tell our instance's __setattr_ attribute to start blocking
setattr(self, Person.BLOCK_MORE_ATTRIBUTES, True)
### useful test - the following code should succeed
terry = Person('Terry Gilliam', 78)
terry.age = terry.age + 1
print(terry.age) # 79
ok = False
try:
terry.new_attribute = 'lets make sure this fails'
except:
ok = True
print('good')
assert ok