toposort | Python Fiddle

def toposort(data):
    
    # Special case empty input.
    if len(data) == 0:
        return
    
    # Copy the input so as to leave it unmodified.
    data = data.copy()
    
    # Ignore self dependencies.
    for k, v in data.items():
        v.discard(k)
        
    # Find all items that don't depend on anything.
    leaves = reduce(set.union, data.values()) - set(data.keys())
    
    # Add empty dependencies when needed.
    data.update({item: set() for item in leaves})
    
    while True:
        ordered = {item for item, dep in data.items() if len(dep) == 0}
        if len(ordered) == 0:
            break
        yield ordered
        data = {item: (dep - ordered)
                for item, dep in data.items()
                	if item not in ordered}
            
    if len(data) != 0:
        raise ValueError('Cyclic dependencies exist among these items: {}'.format(', '.join(repr(x) for x in data.items())))
        
def toposort_flatten(data, sort=True):
    result = []
    for d in toposort(data):
        result.extend((sorted if sort else list)(d))
    return result

print list(toposort({2: {11},
                     9: {11, 8, 10},
                     10: {11, 3},
                     11: {7, 5},
                     8: {7, 3},
                    }))

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