python shadowing variables in inner scopes

nestedFuncs.py

def foo(a):
    print "foo a", a
    def bar(x):
        print "bar reading a", a

    def baz(x):
        a = x
        print "baz wrote to a", a

    def quux(x):
        a += x
        print "a is quux'd", a
        
    return bar, baz, quux

ar, az, uux = foo(100)

ar(1)  # works
az(2)  # also passes
uux(4) # UnboundLocalError: local variable 'a' referenced before assignment