Cant figure out this Python behaviour

Dacelonid

Am currently doing a course in College that includes a Python comprehension test.
Am not the best at Python, but getting through it for the most part.
Have come across a sample question though, that I can't for the life of me figure out why it is failing and was hoping someone could point out the obvious fault that I am just not seeing.
Here is the code

#!/usr/bin/python
class Employee:
        def __init__(self, emp_name, emp_dependents=[], emp_salary=0):
                self.name = emp_name
                self.dependents = emp_dependents
                self.salary = emp_salary

        def addDependent(self, emp_name):
        # Of course from time to time one adds dependents.
                self.dependents.append(emp_name)

        def dependentNames(self):
                return str(self.dependents)

        def printMe(self):
                print ("Name:\t%s" % self.name)
                print ("Dependents:\t%s" % self.dependentNames())

And here is the test that is failing.

from Employee import Employee

import unittest

class TestEmployeeFunctions(unittest.TestCase):

        def setUp(self):
                self.emp1 = Employee("Dylan Thomas", ["Caitlin MacNamara", "Llewelyn Edouard Thomas"])
                self.emp2 = Employee("Frank O Hara")
                self.emp3 = Employee("Elizabeth Bishop")

        def tearDown(self):
                pass

        def test_dependents(self):
                self.assertEqual(self.emp1.dependents, ["Caitlin MacNamara", "Llewelyn Edouard Thomas"])
                self.emp2.addDependent("Jackson Pollock")
                self.assertEqual(self.emp2.dependents, ["Jackson Pollock"])
                self.assertEqual(self.emp3.dependents, [])

        if __name__ == '__main__':
                unittest.main()

And it is failing the assert on line self.assertEqual(self.emp3.dependents, [])

The dependents of emp3 is not empty, it contains "Jackson Pollock", which is the dependent of emp2. The only way this could be true in my mind is if emp2 and emp3 are the same, but they are not (the employee names are different)

Have put in print statements to help debug it and emp3 has no dependents until
self.emp2.addDependent("Jackson Pollock") which seems to add Jackson Pollock to both emp2 and emp3

Am really stumped here, so any help would be greatly appreciated.

Procasinator

Read this: http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments

Dacelonid

Well **** me.
That is the stupidest thing I have ever seen. I am sure some Python programmer will disagree, but that is ridiculous.
When is a default value not a default value, when someone else, that you might not know about has already called that method!!!!

Having said that Procasinator, I thank you. That has instantly cleared that up for me. Thanks