if not bicycles increase_stock(bike identity )print("stock movement failed for"bike identitythis snippet opens bike stock file and iterates over all the bicycle records it contains to find the total value (sum of price quantityof the bikes held it then increases the number of "gekkobikes in stock by two and increments the stock held for all bikes whose bike id begins with " uby one all of these actions take place on diskso any other process that reads the bike stock file will always get the most current data although the binaryrecordfile binaryrecordfile works in terms of indexesthe bikestock bikestock class works in terms of bike ids this is managed by the bikestock bikestock instance holding dictionary that relates bike ids to indexes we will begin by looking at the bikestock bike class' class line and initializerthen we will look at few selected bikestock bikestock methodsand finally we will look at the code that provides the bridge between bikestock bike objects and the binary records used to represent them in binaryrecordfile binaryrecordfile (all the code is in the bikestock py file class bikedef __init__(selfidentitynamequantityprice)assert len(identity ("invalid bike identity '{ }'format(identity)self __identity identity self name name self quantity quantity self price price all of bike' attributes are available as properties--the bike id (self __identityas the read-only bike identity property and the others as read/write properties with some assertions for validation in additionthe bike value read-only property returns the quantity multiplied by the price (we have not shown the implementation of the properties since we have seen similar code before the bikestock bikestock class provides its own methods for manipulating bike objectsand they in turn use the writable bike properties class bikestockdef __init__(selffilename)self __file binaryrecordfile binaryrecordfile(filename_bike_struct sizeself __index_from_identity {for index in range(len(self __file))record self __file[indexif record is not none