What is the scope of a defaulted parameter in Python?

Question

When you define a function in Python with an array parameter, what is the scope of that parameter?

This example is taken from the Python tutorial:

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Answer 1

The explaination is given in answers to this question. To sum it up here:

Functions in Python are a kind of object. Because they are a kind of object, they act like objects when instantiated. A function, if defined with a mutable attribute as a default argument, is exactly the same as a class with a static attribute that is a mutable list.

Lennart Regebro has a good explanation and the answer to the question by Roberto Liffredo is excellent.

To adapt Lennart's answer ... if I have a BananaBunch class:

class BananaBunch:
    bananas = []

    def addBanana(self, banana):
        self.bananas.append(banana)


bunch = BananaBunch()
>>> bunch
<__main__.BananaBunch instance at 0x011A7FA8>
>>> bunch.addBanana(1)
>>> bunch.bananas
[1]
>>> for i in range(6):
    bunch.addBanana("Banana #" + i)
>>> for i in range(6):
    bunch.addBanana("Banana #" + str(i))

>>> bunch.bananas
[1, 'Banana #0', 'Banana #1', 'Banana #2', 'Banana #3', 'Banana #4', 'Banana #5']

// And for review ... 
//If I then add something to the BananaBunch class ...
>>> BananaBunch.bananas.append("A mutated banana")

//My own bunch is suddenly corrupted. :-)
>>> bunch.bananas
[1, 'Banana #0', 'Banana #1', 'Banana #2', 'Banana #3', 'Banana #4', 'Banana #5', 'A mutated banana']

How does this apply to functions? Functions in Python are objects. This bears repeating. Functions in Python are objects.

So when you create a function, you are creating an object. When you give a function a mutable default value, you are populating that object's attribute with a mutable value, and every time you call that function you are operating on the same attribute. So if you are using a mutable call (like append), then you are modifying the same object, just as if you were adding bananas to the bunch object.