Following up on Reason for unintuitive UnboundLocalError behaviour (I will assume you've read it). Consider the following Python script:

def f():
    # a+=1          # 1
    aa=a
    aa+=1

    # b+='b'        # 2
    bb=b
    bb+='b'

    c[0]+='c'       # 3
    c.append('c')
    cc=c
    cc.append('c')  # 4

a=1
b='b'
c=['c']
f()
print a
print b
print c

The result of the script is:

1
b
['cc', 'c', 'c']

The commented out lines (marked 1,2) are lines that would through an UnboundLocalError and the SO question I referenced explains why. However, the line marked 3 works!

By default, lists are copied by reference in Python, therefore it's understandable that c changes when cc changes. But why should Python allow c to change in the first place, if it didn't allow changes to a and b directly from the method's scope?

I don't see how the fact that by default lists are copied by reference in Python should make this design decision inconsistent.

What am I missing folks?