EP 15 Know How Closures Interact with Variable Scope ref: http://qiita.com/giwa/items/ef67c710fd007039c48a

file0.txt

def sort_priority(values, group):
    def helper(x):
        if x in group:
            return (0, x)
        return (1, x)
    values.sort(key=helper)

file1.txt

In [2]: numbers = [8, 3, 1, 2, 4, 5, 7,6]

In [3]: group = {2, 3, 5, 7}

In [4]: sort_priority(numbers, group)

In [5]: print(numbers)
[2, 3, 5, 7, 1, 4, 6, 8]

file2.txt

def sort_priority2(numbers, group):
    found = False # Scope: sort_priority2
    def helper(x):
        if x in group:
            found = True # Scope: helper -- Bad
            return 0, x
        return 1, x
    numbers.sort(key=helper)
    return found

file3.txt

>>> l = [4, 6,2,5,7,9, 0]
>>> found = sort_priority2(l, [2,5,8])
>>> found
False
>>> l
[2, 5, 0, 4, 6, 7, 9]

file4.txt

def sort_priority2(numbers, group):
    found = False # Scope: sort_priority2
    def helper(x):
        nonlocal found
        if x in group:
            found = True # Scope: helper -- Bad
            return 0, x
        return 1, x
    numbers.sort(key=helper)
    return found