In the code below, why can i add elements to locals but not delete them?
>>> def function(argument):
... print locals()
... modify(locals())
... print locals()
...
>>> def modify(locals):
... del locals['argument']
... locals['new_argument'] = 'new_value'
...
>>> function(10)
{'argument': 10}
{'argument': 10, 'new_argument': 'new_value'}