New York University School of Continuing and Professional Studies Management and Information Technology. Advanced Python. Homework, Session 5

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Colin Hill
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1 New York University School of Continuing and Professional Studies Management and Information Technology Advanced Python Homework, Session design classes Guestbook and Entry. These classes will be designed to model a simple guestbook. guestbook_id = '1' guestbook_title = 'My Guestbook' guestbook = Guestbook(guestbook_id, # retrieves existing Guestbook from guestbook_title) # datastore, or creates new entry = Entry('David', 'I like it!') # populates Entry object attrs #.name,.comment,.timestamp guestbook.add_entry(entry) guestbook.add_entry(entry('george', "I don't!") # adds Entry object to internal list # in its.entries attribute # extra credit for entry in guestbook: # loop through the Guestbook, # producing each Entry object print '{} said {} on {}'.format(entry.author, entry.text, entry.timestamp) class Guestbook models a guestbook (a collection of comments stored sequentially). Each Guestbook object should support the following attributes:.id: an integer id for identifying the guestbook.title: a string title for this guestbook.entries: a list of Entry objects Guestbook should also support the following methods: def init (self, id, title=none): if this Guestbook is new (i.e., if the id can't be found in the datastore), a new Guestbook is created and will be stored; if it is not new, it is retrieved from the datastore (see 'Object persistence', below) def add_entry(self, entry_object): given an Entry object (see below), add the object to the Guestbook's inernal list of entries. def set_title(self, title): allows for the resetting of the Guestbook title Finally, make Guestbook "iterable" by implementing iter () and next (). As the user iterates over the object, it retrieves each entry from its.entries attribute. [continued]

2 class Entry models a single comment to be added to the guestbook. Each Entry object should support the following attributes:.author: a string identifying this entry's author.text: the text of the entry.timestamp: a datetime object (import datetime; datetime.datetime.now()) Entry is expected to have an init constructor: def init (self, author, text): given an author name and text comment, set the object's attributes Object persistence. Choose a method of storing your Guestbook object so that it can be retrieved upon a later run of the program. Then write two functions (these are defined outside the Guestbook and Entry classes): def store_guestbook(guestbook_object): store the object def retrieve_guestbook(guestbook_id): retrieve the object You can choose from one of the following schemes: a. object pickling and local filesystem: this is the simplest of solutions. Each Guestbook object can be stored in a separate.pickle file in a designated folder, and you can list the contents of the folder to find or list the Guestbook objects b. JSON: you can convert the objects to a dict of dicts, then store the structure under the Guestbook id in the local filesystem. Please note that JSON will not open an empty file -- you'll get an error mentioning "could not decode". The easiest way to handle this is if the file doesn't exist, don't create it until you are writing to it; if for some reason the file does get created and is empty, you can handle this by read()ing it into a string a testing string length; if empty, ignore; if not empty, load it with json.loads() (load a string rather than a file). c. Relational database: you can perform an "object-relational" conversion of the objects into entries in a 2-table relational database schema. The CREATE TABLE statements would look like this if applied to SQLite: CREATE TABLE guestbook ( id INTEGER, title TEXT, ); CREATE TABLE entry ( guestbook_id INTEGER, author TEXT, text TEXT, timestamp DATE ); When a new Guestbook is stored, one row in the guestbook table is added.

3 When a new Entry object is stored, one row in the entry table is added, with a guestbook_id value that corresponds to the entry's id value. Next, two functions should be created: store_object() and retrieve_object(). These would read through each object and create an INSERT statement for each one. EXTRA CREDIT 5.2. class PersistDict: remake the PersistDict class so that it inherits from dict and presents the familiar dict interface. As indicated by the filename, the preferred storage format is json. Please attempt to re-implement all dict methods that would change the dictionary -- these should write the changed dictionary to the file. This may be easier to build out than it looks. For most methods that it re-implements, PersistDict can call the parent class methods. For example, PersistDict. setitem simply calls dict. setitem and writes the dict to file. See discussion for details. Program Run 1: mydict['a'] = 5 mydict['b'] = 'WOAH!' exit() Program Run 2: print mydict['a'] # 5 print mydict['b'] # WOAH! exit() Program Run 3 (implementing another dict method, update()): mydict.update({'c': 1, 'd': 2}) print mydict # {'a': 5, 'b': 'WOAH', 'c': 1, 'd': 2}

4 5.3. class AttrType (this assignment has a snag that has stumped many, so beware!): an object that is initialized with a type (int, str, etc. -- any type is possible) and makes sure that attribute values set in the object are of those type(s), raising a TypeError if an incorrect type is attempted to be set. (Use isinstance() to validate type.) x = AttrType(int) x.a = 5 x.a = 10 # allows attribute to be overwritten print x.a # 10 x.b = 'hey' # ValueError: "hey" is not <class 'int'> The init constructor also accepts an optional argument writeonce=true, to indicate if attributes can be set only once. (Use hasattr() to see if the object already holds the attribute.) y = AttrType(str, writeonce=true) y.a = 'hey' y.a = 'hello' # ValueError: can't set attribute more than once init will set two attributes: _type (the initialized type) and _writeonce (indicating if attributes can be set more than once). These attributes start with a single underscore because they are "non-public" attributes -- see variable naming conventions.) The class implements the setattr method to validate attributes before setting. It uses internal instance attributes _type to validate the type and _writeonce to disallow overwriting (if applicable). The method also makes sure that these internal attributes cannot be overwritten. However, beware setting attributes on the object directly! Remember that anytime you set an attribute on self, Python will attempt to call setattr, calling the method again, potentially causing infinite recursion (you would see the error Maximum recursion depth reached). Instead, call the parent class' setattr method: object. setattr (self, attrname, attrval), where attrname and attrval are the attribute name and value. We are also seeing an issue where Python seems to not be able to see an attribute that was set on the object. See discussion. The class also implements two methods: as_list() returns the attribute values as a list, and as_dict() returns the attribute dict (however, make sure the return values don't contain the _type or _writeonce attributes). z = AttrType(float) x.a = 5.5 x.b = 10.0 print x.as_list() # [5.5, 10.0] print x.as_dict() # {'a': 5.5, 'b': 10.0}

5 Extra credit: allow the type arguments 'number' (passed as a string, not an object type) and 'string'. In order to validate these, use isinstance() on the parent type for each: isinstance(var, numbers.number) (imported from the numbers module) or isinstance(var, basestring). This will allow the user to assign any string (including type unicode) or any number (including types float, int or long). z = AttrType('number') x.a = 5.5 x.b = 10 x.c = 'hello' # ValueError: "hello" is not a number z = AttrType('string') x.a = 'hello' x.b = u'this is unicode' x.c = 5 # ValueError: "5" is not a string

Lecture 21. Programming with Subclasses

Lecture 21. Programming with Subclasses Lecture 21 Programming with Subclasses Announcements for This Lecture Assignments Prelim 2 A4 is now graded Mean: 90.4 Median: 93 Std Dev: 10.6 Mean: 9 hrs Median: 8 hrs Std Dev: 4.1 hrs A5 is also graded