Take a predicate and a list and return a pair of lists: those elements that do and do not satisfy the predicate, respectively.

partition.py

def partition(pred, items):
    """
    Take a predicate and a list and return a pair of lists: those elements
    that do and do not satisfy the predicate, respectively.

    >>> partition(str.isalpha, "ab1c")
    (['a', 'b', 'c'], ['1'])

    >>> partition(str.isalpha, [])
    ([], [])
    """
    trues, falses = [], []
    for item in items:
        if pred(item):
            trues.append(item)
        else:
            falses.append(item)
    return trues, falses