human
Solve the following python programming problem given the following code:
@ extensionmethod (Observable) 
def distinct_until_changed(self, key_selector = None, comparer = None) : 
    'Returns an observable sequence that contains only distinct
    contiguous elements according to the key_selector and the comparer.

    1 - obs = observable.distinct_until_changed();
    2 - obs = observable.distinct_until_changed(lambda x: x.id)
    3 - obs = observable.distinct_until_changed(lambda x: x.id,
                                                lambda x, y: x == y)

    key_selector -- [Optional] A function to compute the comparison key for
        each element. If not provided, it projects the value.
    comparer -- [Optional] Equality comparer for computed key values. If
        not provided, defaults to an equality comparer function.

    Return An observable sequence only containing the distinct contiguous
    elements, based on a computed key value, from the source sequence.
    ' 
    source = self 
    key_selector = (source or identity) 
    comparer = (comparer or default_comparer) 
    def subscribe(observer) : 
        has_current_key = [False] 
        current_key = [None] 
        def on_next(value) : 
            comparer_equals = False 
            try : 
                key = key_selector (value) 
except Exception as exception : 
                observer.on_error (exception) 
                return 
if has_current_key [0] : 
                try : 
                    comparer_equals = comparer (current_key [0], key) 
except Exception as exception : 
                    observer.on_error (exception) 
                    return 
if ((not has_current_key [0]) or (not comparer_equals)) : 
                has_current_key [0] = True 
                current_key [0] = key 
                observer.on_next (value) 
return source.subscribe (on_next, observer.on_error, observer.on_completed) 
return AnonymousObservable (subscribe) 

    
    

    Fix the buggy line: key_selector = (source or identity)
assistant
#Here is Python code to fix the buggy line: key_selector = (source or identity)
.

key_selector = (key_selector or identity)