Question

I have my class template here:

 import sqlite3

class Patron(object):
    #Let's set some basic attributes
    attributes = { "patron_id" : None,
    "name" : None,
    "address" : None,
    "phone" : None,
    "email" : None,
    "fee_balance" : None,
    "fees_per_day" : None,
    "books_checked_out" : [],
    "books_overdue" : []}

    def __init__(self):
        #Create a empty instance
        pass

    def new(self, patron_id, name, address, phone, email):
        #Create an instance with new values
        self.attributes["patron_id"] = patron_id
        self.attributes["name"] = name
        self.attributes["address"] = address
        self.attributes["phone"] = phone
        self.attributes["email"] = email

    def retrieve(self, patron_id):
        #Connect to database and prepare patron id
        self.attributes["patron_id"] = patron_id
        patron_database = sqlite3.connect('patrons.db')
        cursor = patron_database.cursor()
        t = (str(patron_id),)

        #Get data from database
        cursor.execute("SELECT * FROM patrons WHERE id =?", t)
        data = cursor.fetchone()

        #Now close your database connection
        patron_database.close()

        #Parse tuple into attributes
        self.attributes["name"] = data[1]
        self.attributes["address"] = data[2]
        self.attributes["phone"] = data[3]
        self.attributes["email"] = data[4]
        self.attributes["fee_balance"] = data[5]
        self.attributes["fees_per_day"] = data[6]
        self.attributes["books_checked_out"] = data[7]
        self.attributes["books_overdue"] = data[8]

    def save(self):
        #Connect to the database
        patron_database = sqlite3.connect('patrons.db')
        cursor = patron_database.cursor()

        #Compile the data into a list
        attributes = []
        for value in self.attributes.itervalues():
            attributes.append(value)

        #Insert the values and save them
        cursor.execute("INSERT INTO patrons VALUES(?,?,?,?,?,?,?,?,?)", attributes)
        patron_database.commit()

        #Close the connection
        patron_database.close()

and then I have my test code here:

'''
Created on Feb 2, 2013

@author: Zach
'''
from Patron import Patron


zach = Patron()
braden = Patron()

zach.retrieve(1187277)

print zach.attributes
print braden.attributes

My console says that both the “zach” and “braden” instance have the exact same attributes, even though I have not set anything to the “braden” instance. If I assign something to the “braden” instance, then they both share the properties of that instance.

I think it’s a problem related to mutable default argument behavior, but I can’t figure out my problem.

Answer 1

You have made attributes a class level variable and because dicts are mutable. Move it’s definition inside of __init__ and it should work.

class demo(object):
    class_level = {'a': 0}
    class_level_nm = 0
    class_level2 = 0
    def __init__(self, v):
        self.instance_level = v 
        self.class_level['a'] += 1
        self.class_level_nm += 1
        demo.class_level2 += 1
    def __str__(self):
        return 'class level (mut): %d  class level (unmut): %d  instance level: %s  class level2: %d' % (self.class_level['a'],
                                                                                     self.class_level_nm,
                                                                                     self.instance_level,
    self.class_level2)

a = demo('a')
b = demo('b')

print a
print b

c = demo('c')

print a
print b
print c

Gives:

class level (mut): 2  class level (unmut): 1  instance level: a  class level2: 2
class level (mut): 2  class level (unmut): 1  instance level: b  class level2: 2
class level (mut): 3  class level (unmut): 1  instance level: a  class level2: 3
class level (mut): 3  class level (unmut): 1  instance level: b  class level2: 3
class level (mut): 3  class level (unmut): 1  instance level: c  class level2: 3