CODEWITHSUNDEEP
python
Quinma
Quinma

Reputation: 1476

Creating new object in python is not returning a new object

I am new to python.

Why am I not getting a new object when I call tempMyObject = myObject()?

class myObject(object):
  x = []

def getMyObject():
  tempMyObject = myObject()
  print "debug: %s"%str(tempMyObject.x)
  tempMyObject.x.append("a")
  return tempMyObject
#run
a = getMyObject()
b = getMyObject()

My debug prints out: 

debug: []
debug: ["a"]

I don't understand why both of these debug arrays are not null, can someone please enlighten me?

EDIT: I found the mistake i put in python code on my post. I am using the .append("a") in my function

Upvotes: 2

Views: 287

Answers (2)

Henry Keiter
Henry Keiter

Reputation: 17168

You have created x as a class variable rather than an instance variable. To associate the variable with a particular instance of a class, do something like this:

class myObject(object):
    def __init__(self): # The "constructor"
        self.x = [] # Assign x to this particular instance of myObject

>>> debug: []
>>> debug: []

For a little better explanation of what's going on, have a look at this little mockup that demonstrates the same thing, a little more explicitly (if also more verbosely).

class A(object):
    class_var = [] # make a list attached to the A *class*
    def __init__(self):
        self.instance_var = [] # make a list attached to any *instance* of A

print 'class var:', A.class_var # prints []
# print 'instance var:', A.instance_var # This would raise an AttributeError!

print

a = A() # Make an instance of the A class
print 'class var:', a.class_var # prints []
print 'instance var:', a.instance_var # prints []

print

# Now let's modify both variables
a.class_var.append(1)
a.instance_var.append(1)
print 'appended 1 to each list'
print 'class var:', a.class_var # prints [1]
print 'instance var:', a.instance_var # prints [1]

print

# So far so good. Let's make a new object...
b = A()
print 'made new object'
print 'class var:', b.class_var # prints [1], because this is the list bound to the class itself
print 'instance var:', b.instance_var # prints [], because this is the new list bound to the new object, b

print

b.class_var.append(1)
b.instance_var.append(1)
print 'class var:', b.class_var # prints [1, 1]
print 'instance var:', b.instance_var # prints [1]

Upvotes: 6

Aleksander Lidtke
Aleksander Lidtke

Reputation: 2926

There a few bits missing in your code, like the class initialiser first and foremost. The correct code is as follows:

class myObject(object):
    def __init__(self):
         self.x=[] #Set x as an attribute of this object.

def getMyObject():
  tempMyObject = myObject()
  print "debug: %s"%str(tempMyObject.x) #Just after object initialisation this is an     empty list.
  tempMyObject.x = ["a"]
  print "debug2: %s"%str(tempMyObject.x) #Now we set a value to it.
  return tempMyObject
#run
a = getMyObject()
b = getMyObject()

Now the debug will first print out an empty list and then, once it was set, "a". Hope this helps. I recommend looking at basic python classes tutorial.

Upvotes: 1

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