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To be able to edit a DVD, the user must rst choose the DVD to work on This is just a matter of getting the title since titles are used as keys with the values holding the other data Since the necessary functionality is needed elsewhere (eg, when removing a DVD), we have factored it out into a separate find_dvd() function that we will look at next If the DVD is found we get the user s changes, using the existing values as defaults to speed up the interaction (We have omitted most of the user interface code for this function since it is almost the same as that used when adding a DVD) At the end we store the data just as we did when adding If the title is unchanged this will have the effect of overwriting the associated value, and if the title is different this has the effect of creating a new key value item, in which case we delete the original item
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def find_dvd(db, message): message = "(Start of) title to " + message while True: matches = [] start = Consoleget_string(message, "title") if not start: return None for title in db: if titlelower()startswith(startlower()): matchesappend(title) if len(matches) == 0: print("There are no dvds starting with", start) continue elif len(matches) == 1: return matches[0] elif len(matches) > DISPLAY_LIMIT: print("Too many dvds start with {0}; try entering " "more of the title"format(start)) continue else: matches = sorted(matches, key=strlower) for i, match in enumerate(matches): print("{0}: {1}"format(i + 1, match)) which = Consoleget_integer("Number (or 0 to cancel)", "number", minimum=1, maximum=len(matches)) return matches[which - 1] if which != 0 else None
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To make nding a DVD as quick and easy as possible we require the user to type in only one or the rst few characters of its title Once we have the start of the title we iterate over the DBM and create a list of matches If there is one match we return it, and if there are several matches (but fewer than DISPLAY_LIMIT, an integer set elsewhere in the program) we display them all in case-insensitive order with a number beside each one so that the user
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can choose the title just by entering its number (The Consoleget_integer() function accepts 0 even if the minimum is greater than zero so that 0 can be used as a cancelation value This behavior can be switched off by passing allow_zero=False We can t use Enter, that is, nothing, to mean cancel, since entering nothing means accepting the default)
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def list_dvds(db): start = "" if len(db) > DISPLAY_LIMIT: start = Consoleget_string("List those starting with " "[Enter=all]", "start") print() for title in sorted(db, key=strlower): if not start or titlelower()startswith(startlower()): director, year, duration = db[title] print("{title} ({year}) {duration} minute{0}, by " "{director}"format(Utils(duration), **locals()))
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Listing all the DVDs (or those whose title starts with a particular substring) is simply a matter of iterating over the DBM s items The Utils() function is simply s = lambda x: "" if x == 1 else "s"; so here it returns an s if the duration is not one minute
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def remove_dvd(db): title = find_dvd(db, "remove") if title is None: return ans = Consoleget_bool("Remove {0} "format(title), "no") if ans: del db[title] dbsync()
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Removing a DVD is a matter of nding the one the user wants to remove, asking for con rmation, and if we get it, deleting the item from the DBM We have now seen how to open (or create) a DBM le using the shelve module, and how to add items to it, edit its items, iterate over its items, and remove items Unfortunately, there is a aw in our data design Director names are duplicated, and this could easily lead to inconsistencies; for example, director Danny DeVito might be entered as Danny De Vito for one movie and Danny deVito for another One solution would be to have two DBM les, the main DVD le with title keys and (year, duration, director ID) values, and a director le with director ID (ie, integer) keys and director name values We avoid this aw in the next section s SQL database version of the program by using two tables, one for DVDs and another for directors
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