Explicitly creating a new object in Python

Question

I am trying to create new objects and store them in a dictionary. But it doesn't seem to be working the way I expect.

class Fruit:
   name = ''
   facts = []
   def __init__(self, FruitName):
       self.name = FruitName
       facts = []
   def addfact(self, FruitName):
       if FruitName == 'banana':
           self.facts.append('bananas contain potassium')
       elif FruitName == 'orange':
           self.facts.append('These are some facts about oranges')
       else:
           self.facts.append('Unrecognized fruit submitted')

Files = ['banana', 'orange', 'apple']

ObjLibrary = {}

for File in Files:
    if not File in ObjLibrary:
        ObjLibrary[File] = Fruit(File)
    ObjLibrary[File].addfact(File)

print ObjLibrary['banana'].facts
print ObjLibrary['orange'].facts
print ObjLibrary['apple'].facts

I get the following output:

['bananas contain potassium', 'These are some facts about oranges', 'Unrecognized fruit submitted']
['bananas contain potassium', 'These are some facts about oranges', 'Unrecognized fruit submitted']
['bananas contain potassium', 'These are some facts about oranges', 'Unrecognized fruit submitted']

Now I suspect what's going on is that I'm not explicitly creating three objects the way I want, but just pointing all three items in the dictionary to the same object. My question is: why? On each pass of the loop, File should have a different value. More importantly, how to I get around it? In my "real" code, it's not practical to create a totally new variable to store each object.

Thanks for your help.

Casey

Daniel Roseman · Accepted Answer · 2013-12-04 14:56:50Z

4

The problem is that facts is a class variable, not an instance one. You should define it inside __init__ (and remove the useless local variable declaration there):

class Fruit:

   def __init__(self, FruitName):
       self.name = FruitName
       self.facts = []

   def addfact(self, FruitName):
       ...

Note Python is not Java, there is no need to "declare" attributes at the class level.

answered Dec 4, 2013 at 14:56

Daniel Roseman

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1 Comment

Eric Over a year ago

Not only is there no need, but if you try, what you're really doing is declaring static/class attributes. Note you can do __slots__ = ['name', 'facts'] to declare them, but this is not normally a good idea.

jonrsharpe · Accepted Answer · 2013-12-04 14:59:23Z

0

facts is defined as a class attribute, i.e. shared by all instances of the class. Move the assignment inside __init__:

def __init__(self, name):
    self.name = name
    self.facts = []

Also, the class definition doesn't seem like the right place to store the facts, surely they should be arguments to addfact, where you can already access the name using self.name anyway:

def addfact(self, fact):
    self.facts.append(fact)

answered Dec 4, 2013 at 14:59

jonrsharpe

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Comments

Michael Kruglos · Accepted Answer · 2013-12-04 15:10:14Z

There's a difference between class variables and instance variables. Consider the following:

>>> class Test:
...     facts = [1]
...     def __init__(self,v=None):
...             if v:
...                     self.facts = v
... 
>>> t = Test()
>>> t.facts
[1]
>>> Test.facts
[1]
>>> Test.facts = [2]
>>> t.facts
[2]
>>> t2 = Test([3])
>>> t.facts
[2]
>>> t2.facts
[3]
>>> Test.facts
[2]
>>> Test.facts = [4]
>>> t.facts
[4]
>>> t2.facts
[3]

Instances of the class will inherit the class variable. If you change it for the instance, it will keep it as instance variable and changes to class variable won't matter. If you change it in the class, and never changed it in the instance. Instance will see the updated value of the class variable.

Andrei Kaigorodov · Accepted Answer · 2014-02-28 23:08:37Z

0

try this:

class Fruit:
   name = ''
   # remove this string facts = [] 
   def __init__(self, FruitName):
       self.name = FruitName
       self.facts = [] # add "self."
   ...

In your example facts is a class member (not an instance member)

edited Feb 28, 2014 at 23:08

answered Dec 4, 2013 at 14:56

Andrei Kaigorodov

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