def f(s):
    return 'a' in s
 
def satisfiesF(k):
    """
    Assumes L is a list of strings
    Assume function f is already defined for you and it maps a string to a Boolean
    Mutates L such that it contains all of the strings, s, originally in L such
            that f(s) returns True, and no other elements. Remaining elements in L
            should be in the same order.
    Returns the length of L after mutation
    """
    res = []
    for a in k:
        if f(a):
            res.append(a)
    global L
    L = res
    return len(res)
 
L = ['a', 'b', 'a']
print(satisfiesF(L))
print(L)

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