class Dict1(dict):
    def __getitem__(self, key):
        print '    __getitem__', repr(key)
        if key == 's':
            return None
        return dict.__getitem__(self, key)

class Dict2(dict):
    def __missing__(self, key):
        print '    __missing__', repr(key)
        return None

source = """if 1:
    print '  1'
    s
    def f():
        print '  2'
        s
    f()"""

print 'Dict1.__getitem__'
try:
    exec(source, Dict1())
except NameError as exc_value:
    print '    %s: %s' % (exc_value.__class__.__name__, exc_value)

print 'Dict2.__missing__'
try:
    exec(source, Dict2())
except NameError as exc_value:
    print '    %s: %s' % (exc_value.__class__.__name__, exc_value)