No standard library allowed exercise :S

.gitignore

__pycache__
details.csv
errors.csv
*.pyc

merge.py

"""
WITHOUT using the standard library.

Write a program that reads orders and trades from csv files in ONE PASS ONLY,
never keeping in memory more than ONE ORDER AND TRADE AT A TIME and writes
trades with matching orders to the file details.csv, trades with no matching
order to the file errors.csv.

Orders and trades are sorted by order_id (first item).
"""


def main():
    orders, trades = map(reader, ('orders', 'trades'))

    detail, error = map(writer, ('details', 'errors'))

    doit(orders, trades, detail, error)

def doit(orders, trades, detail, error):
    """
    For testability's sake
    """
    process = inject(detail, error)

    order = orders.next()

    for trade in trades:
        while not process(trade, order):
            try:
                order = orders.next()
            except StopIteration:
                error(*trade)
                break

def inject(detail, error):
    """
    Faster and arguably more readable than a callable class
    """
    def closure(trade, order):
        """
        trade lag order => error and return True
        trade match order => detail return True
        order lag trade => return False
        """

        trade_order_id, trade_id, trade_data = trade
        order_id, order_data = order

        if order_id > trade_order_id:
            error(trade_order_id, trade_id, trade_data)
        elif order_id == trade_order_id:
            detail(order_id, trade_id, order_data, trade_data)
        else:
            return False

        return True

    return closure

def reader(filename):
    """
    Ad hoc csv file reader (iterator)
    """
    with open(csv(filename)) as f:
        for line in f:
            yield line[:-1].replace(' ', '').split(',')

def writer(filename):
    """
    Ad hoc callable csv writer
    """
    file_ = open(csv(filename), 'w')
    def closure(*args):
        print filename, args
        file_.write(','.join(args))
        file_.write('\n')
    return closure

def csv(filename):
    """
    Append .csv to filename
    """
    return '{0}.csv'.format(filename)


if __name__ == '__main__':
    main()

orders.csv

test_merge.py

import merge


def test_basic_match():
    orders = reader(""" 1, order_1 """)
    trades = reader(""" 1, 101, trade_101 """)
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == [('1', '101', 'order_1', 'trade_101')]
    assert error.data == []

def test_basic_error():
    orders = reader(""" 2, order_2 """)
    trades = reader(""" 1, 101, trade_101 """)
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == []
    assert error.data == [('1', '101', 'trade_101')]

def test_zoltan():
    orders = reader(""" 5, order_5 """)
    trades = reader(""" 1, 101, trade_101
                        2, 102, trade_102
                        5, 105, trade_105 """)
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == [('5', '105', 'order_5', 'trade_105')]
    assert error.data == [('1', '101', 'trade_101'),
                          ('2', '102', 'trade_102')]

def test_inner_while():
    orders = reader(""" 1, order_1
                        2, order_2
                        3, order_3
                        4, order_4 """)
    trades = reader(""" 4, 104, trade_104 """)
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == [('4', '104', 'order_4', 'trade_104')]
    assert error.data == []

def test_stop_iteration():
    orders = reader(""" 5, order_5 """)
    trades = reader(""" 7, 107, trade_107 """)
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == []
    assert error.data == [('7', '107', 'trade_107')]

def test_from_file():
    orders = merge.reader('orders')
    trades = merge.reader('trades')
    detail = Writer()
    error = Writer()

    merge.doit(orders, trades, detail, error)

    assert detail.data == [('4', '102', 'order_4', 'trade_102'),
                           ('4', '103', 'order_4', 'trade_103'),
                           ('5', '104', 'order_5', 'trade_104'),
                           ('9', '106', 'order_9', 'trade_106')]
    assert error.data == [('3', '101', 'trade_101'),
                          ('7', '105', 'trade_105')]


class Writer(object):
    """
    Helper writer class (mock)
    """

    def __init__(self):
        self.data = []

    def __call__(self, *data):
        self.data.append(data)

def reader(string):
    """
    Helper reader function
    """
    for line in string.replace(' ', '').split('\n'):
        yield line.split(',')

trades.csv