Why the "fullname" not update ?

Question

Here is my code:

class Person():

def __init__(self, firstname, lastname):
self.first = firstname
self.last = lastname
self.fullname = self.first + ' '+ self.last

def email(self):
return '{}.{}@email.com'.format(self.first, self.last)

person = Person('joe', 'lee') # 1. Here is the define of name original
person.last = 'lin' # 2. I change the the last name from "lee" to "lin".
print(person.first) # 3. Now I print: first name not change, output: "joe"
print(person.last) # 4. Now I print: last name has changed: output: "lin"
print(person.fullname) # Q: Why ??!! Why the fullname still "joe lee" ??? It
was supposed to be "joe lin"... It's strange !!!

Answer 1

You can (and should) make fullname a read-only property that will always return the current values of first name and last name:

class Person():
def __init__(self, firstname, lastname):
self.first = firstname
self.last = lastname

@property
def fullname(self):
return self.first + ' ' + self.last

person = Person('Joe', 'Lee')
person.last = 'Lin'
print(person.fullname) # Joe Lin

(same with e-mail)