《Python3程序开发指南（第二版）》第4章 控制结构与函数 练习

1. 编写一个交互式程序，用于对文件中的字符串列表进行维护。

import osYES = frozenset({"y", "Y", "yes", "Yes", "YES"})def main():    dirty = False    items = []    filename, items = choose_file()    if not filename:        print("Cancelled")        return    while True:        print("\nList Keeper\n")        print_list(items)        choice = get_choice(items, dirty)        if choice in "Aa":            dirty = add_item(items, dirty)        elif choice in "Dd":            dirty = delete_item(items, dirty)        elif choice in "Ss":            dirty = save_list(filename, items)        elif choice in "Qq":            if (dirty and (get_string("Save unsaved changes (y/n)",                                      "yes/no", "y") in YES)):                save_list(filename, items, True)            breakdef choose_file():    enter_filename = False    print("\nList Keeper\n")    files = [x for x in os.listdir(".") if x.endswith(".lst")]    if not files:        enter_filename = True    if not enter_filename:        print_list(files)        index = get_integer("Specify file's number (or 0 to create "                            "a new one)", "number", maximum=len(files),                            allow_zero=True)        if index == 0:            enter_filename = True        else:            filename = files[index - 1]            items = load_list(filename)    if enter_filename:        filename = get_string("Choose filename", "filename")        if not filename.endswith(".lst"):            filename += ".lst"        items = []    return filename, itemsdef print_list(items):    if not items:        print("-- no items are in the list --")    else:        width = 1 if len(items) < 10 else 2 if len(items) < 100 else 3        for i, item in enumerate(items):            print("{0:{width}}: {item}".format(i + 1, **locals()))    print()def get_choice(items, dirty):    while True:        if items:            if dirty:                menu = "[A]dd  [D]elete  [S]ave  [Q]uit"                valid_choices = "AaDdSsQq"            else:                menu = "[A]dd  [D]elete  [Q]uit"                valid_choices = "AaDdQq"        else:            menu = "[A]dd  [Q]uit"            valid_choices = "AaQq"        choice = get_string(menu, "choice", "a")        if choice not in valid_choices:            print("ERROR: invalid choice--enter one of '{0}'".format(                  valid_choices))            input("Press Enter to continue...")        else:            return choicedef add_item(items, dirty):    item = get_string("Add item", "item")    if item:        items.append(item)        items.sort(key=str.lower)        return True    return dirtydef delete_item(items, dirty):    index = get_integer("Delete item number (or 0 to cancel)",                        "number", maximum=len(items),                        allow_zero=True)    if index != 0:        del items[index - 1]        return True    return dirtydef load_list(filename):    items = []    fh = None    try:        for line in open(filename, encoding="utf8"):            items.append(line.rstrip())    except EnvironmentError as err:        print("ERROR: failed to load {0}: {1}".format(filename, err))        return []    finally:        if fh is not None:            fh.close()    return itemsdef save_list(filename, items, terminating=False):    fh = None    try:        fh = open(filename, "w", encoding="utf8")        fh.write("\n".join(items))        fh.write("\n")    except EnvironmentError as err:        print("ERROR: failed to save {0}: {1}".format(filename, err))        return True    else:        print("Saved {0} item{1} to {2}".format(len(items),              ("s" if len(items) != 1 else ""), filename))        if not terminating:            input("Press Enter to continue...")        return False    finally:        if fh is not None:            fh.close()def get_string(message, name="string", default=None,               minimum_length=0, maximum_length=80):    message += ": " if default is None else " [{0}]: ".format(default)    while True:        try:            line = input(message)            if not line:                if default is not None:                    return default                if minimum_length == 0:                    return ""                else:                    raise ValueError("{0} may not be empty".format(                                     name))            if not (minimum_length <= len(line) <= maximum_length):                raise ValueError("{name} must have at least "                        "{minimum_length} and at most "                        "{maximum_length} characters".format(                        **locals()))            return line        except ValueError as err:            print("ERROR", err)def get_integer(message, name="integer", default=None, minimum=0,                maximum=100, allow_zero=True):    class RangeError(Exception): pass    message += ": " if default is None else " [{0}]: ".format(default)    while True:        try:            line = input(message)            if not line and default is not None:                return default            i = int(line)            if i == 0:                if allow_zero:                    return i                else:                    raise RangeError("{0} may not be 0".format(name))            if not (minimum <= i <= maximum):                raise RangeError("{name} must be between {minimum} "                        "and {maximum} inclusive{0}".format(                        " (or 0)" if allow_zero else "", **locals()))            return i        except RangeError as err:            print("ERROR", err)        except ValueError as err:            print("ERROR {0} must be an integer".format(name))main()