Question

Here is a python class:

class TxdTest(object):
    def __init__(self, name = '', atrributes = []):
        self.name = name
        self.attributes = atrributes

and then I use it like this:

def main():
    for i in range(3):
        test = TxdTest()
        print test.name
        print test.attributes
        test.name = 'a'
        test.attributes.append(1)

so, what’s the result? The result is:

[]

[1]

[1, 1]

Why the ‘self.attributes’ in class still obtain the value ?

Answer 1

When passing a mutable object (like a list) to a function or method in python, a single object is used across all invocations of the function or method. This means that ever time that function or method (in this case, your __init__ method) is called, the exact same list is used every time, holding on to modifications made previously.

If you want to have an empty list as a default, you should do the following:

class TxdTest(object):
    def __init__(self, name = '', atrributes = None):
        self.name = name
        if attributes is None
             self.attributes = []
        else:
             self.attributes = attributes

For a detailed explanation of how this works see: “Least Astonishment” in Python: The Mutable Default Argument, as mentioned by bgporter.