main.py

def filter_custom(l, f):
    '''Filter a list using a function

    Return a new list that contains all the elements e of l
    for which f(e) is True

    :param l: a list
    :param f: a function that takes one argument and returns either
        True or False
    '''
    filtered_list = [e for e in l if f(e)]
    return filtered_list

def map_custom(l, f):
    '''Map a list using a function

    Return a new list that applies f(e) for every element e in l

    :param l: a list
    :param f: a function that takes one argument and returns a value'''

#    new_list = []
#    for e in l:
#        new_list.append(f(e))
    new_list = [f(e) for e in l]
    return new_list

def reduce_custom(l, f, starting_value):
    '''Reduce a list using a reducer function and a starting value

    Return a single value that applies f(v, e) for every
    e in l from left to right.  The initial value for v
    should be starting_value, and subsequent values should be the previously
    calculated value from f(v, e)

    :param l: a list
    :param f: a function that takes one argument and returns a value
    :param starting_value: the beginning value for the reducer function
        computation
    '''

    reduced_list = []
    v = starting_value

    for e in l:
        reduced_list.append(f(v,e))
        v = f(v, e)
    return reduced_list

if __name__ == '__main__':
    l = [7,8,9]
#    f = lambda x: x * 10
#    print(map_custom(l,f))

#    def f(e):
#        if e + 1 > 8:
#            return True
#        else:
#            return False
#    print(filter_custom(l, f))

    starting_value = 500

    def f(v, e):
        return v + e
    print(reduce_custom(l, f, starting_value))