Python - why it doesn't create a new instance of object?

Question

I have a little problem that I do not understand.

I have a method:

def appendMethod(self, newInstance = someObject()):
    self.someList.append(newInstace)

I call this method without attributes:

object.appendMethod()

And actually I append the list with the same instance of someObject.

But if I change it to:

def appendMethod(self):
    newInstace = someObject()
    self.someList.append(newInstance)

I get new instance of that object every time, what's the difference?

Here's an example:

class someClass():
    myVal = 0

class otherClass1():

    someList = []

    def appendList(self):
        new = someClass()
        self.someList.append(new)

class otherClass2():

    someList = []

    def appendList(self, new = someClass()):
        self.someList.append(new)

newObject = otherClass1()
newObject.appendList()
newObject.appendList()
print newObject.someList[0] is newObject.someList[1]
>>>False

anotherObject = otherClass2()
anotherObject.appendList()
anotherObject.appendList()
print anotherObject.someList[0] is anotherObject.someList[1]
>>>True

this question isn't *strictly* related to the mutable default value, but is about *when* default values are created. @tomek keep in mind that every function keeps a **`tuple`** of its default values in the `__defaults__` attribute. But, what does this mean? Well, surely since `tuple`s are immutable functions *cannot* create a default value every time they are called, hence the default value is created only *once* at function *definition*. Try to change `someObject` with a function like `def func(): print("called")` and see when this function gets called. — Bakuriu, Sep 05 '13 at 17:56
This is a good question. It certainly confused me when I came from C++-land where functions are second-class objects evaluated at function execution rather than function definition. — Shashank, Sep 05 '13 at 18:07

score 3 · Accepted Answer · edited May 23 '17 at 12:24

3

This is because you are assigning your default argument as a mutable object.

In python, a function is an object that gets evaluated when it is defined, So when you type def appendList(self, new = someClass()) you are defining new as a member object of the function, and it does NOT get re-evaluated at execution time.

see “Least Astonishment” in Python: The Mutable Default Argument

edited May 23 '17 at 12:24

Community

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answered Sep 05 '13 at 17:47

Cameron Sparr

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thank you, now I understand what happens. It's counter intuitive to me, but I get it. I forgot to think of functions as objects. – Tomek Urbańczyk Sep 05 '13 at 18:49

Python - why it doesn't create a new instance of object?

1 Answers1