Counter-example to the two (otherwise very good) generalizations about Python that "assignment never makes new values, and it never copies data" and "Lots of Python data structures [e.g. object attributes] hold values, and each of those is a reference. All of the rules here about names apply exactly the same to any of these references." (see htt…

gistfile1.py

from copy import deepcopy

class MagicSetter(object):
    
    def __setattr__(self, name, value):
        super(MagicSetter, self).__setattr__(name, deepcopy(value))
        
m = MagicSetter()

m.a = ["this", "is", "the", "original", "list"]

# an example of simple assignment that does lead to a copy
m.b = m.a

# so even if we change the second list
m.b.insert(2, "not")

# the original list remains unchanged
print " ".join(m.a)
print " ".join(m.b)