The following is the test code.

class C1:
	myurl = []	
	def test(self):
		url = [5,6,7]
		self.myurl.extend(url)		
def testv():
	c = C1()
	c.test()
	print(c.myurl)
	
i = 0
while i<10 :
	testv()
	i = i+1

The output is :

[5, 6, 7]
[5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 
7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 
7, 5, 6, 7]
[5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 7, 5, 6, 
7, 5, 6, 7, 5, 6, 7]

The myurl of class C1 is not set to [] when a new object is created.
All objects use the same memory.