Property set correctly inside an object but not accessible outside

Question

I'm working under python pyramid, with Python3.

I have a model that looks like this:

class OneTimeCode(Base):
    __tablename__ = 'otc_one_time_codes'
    otc_one_time_code_id = Column(Integer, primary_key=True)
    otc_one_time_code = Column(String(32))
    otc_usr_user_id = Column(Integer, ForeignKey('usr_users.usr_user_id'), nullable=True)
    otc_expire_time = Column(DateTime)

    def __init__(self, otc_usr_user_id, otc_expire_time=None):
        self.otc_usr_user_id = otc_usr_user_id
        if otc_expire_time is None:
            self.otc_expire_time = (datetime.now() + timedelta(6*365/12)).isoformat()
        else:
            self.otc_expire_time = otc_expire_time

    @classmethod
    def get_code(self, hlength=6):    
        seed = datetime.now() + timedelta(random.randrange(1,10000))
        tmp_hash = hashlib.md5(seed.strftime("%Y-%m-%d %H:%M:%S.%F").encode('utf-8')).hexdigest()

        if hlength == 32:
            self.otc_one_time_code = tmp_hash
        else:  
            self.otc_one_time_code = tmp_hash[0 : hlength]

        print(self.otc_one_time_code)

The problem is, when I instantiate one of these objects and then explicitly call get_code, the print line at the end prints to the screen the code successfully.

However, in my view, if I explicitly try to print that property, it's 'None'

Here's what my view code looks like:

   otc = OneTimeCode(
        otc_usr_user_id = user.usr_user_id
    )

    otc.get_code()
    pprint.pprint(vars(otc))
    session.add(otc)

And the console output looks like this:

0d097c 
{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x50877d0>,  'otc_expire_time': '2015-02-13T10:56:14.244447',  'otc_usr_user_id': 1} 2014-08-14 22:56:14,245 
INFO  [sqlalchemy.engine.base.Engine][Dummy-2] INSERT INTO otc_one_time_codes (otc_one_time_code, otc_usr_user_id, otc_expire_time) VALUES (%(otc_one_time_code)s, %(otc_usr_user_id)s, %(otc_expire_time)s) RETURNING otc_one_time_codes.otc_one_time_code_id 2014-08-14 22:56:14,245 
INFO  [sqlalchemy.engine.base.Engine][Dummy-2] {'otc_one_time_code': None, 'otc_expire_time': '2015-02-13T10:56:14.244447', 'otc_usr_user_id': 1} 2014-08-14 22:56:14,247 
INFO  [sqlalchemy.engine.base.Engine][Dummy-2] COMMIT

You can see the value inside the model: 0d097c, and also the pprint object, where it doesn't look like the property exists.

Why can't I get access to this property?

Answer 1

Looks like you should be using a @property instead of a OTC , however it also seems like this may be something you DON'T want to calculate each time!

# for all the docstrings, let multi = Multi(2)
class Multi(object):
    def __init__(self, attribute):
        """When instantiated, set self.attribute to attribute"""
        self.attribute = attribute

    @property
    def attribute_times_ten(self):
        """accessed via multi.attribute_times_ten
        and will return 20. Use properties to signify
        a variable that requires some work done to it
        that needs to calculated each time it's called."""
        return attribute_times_ten

    @classmethod
    def times_ten(cls, num):
        """Not the best example, but a @classmethod will
        give the class as its first argument, NOT the
        instance. This is useful in lots of constructor
        settings, e.g. CreateClass.fromstring("attributes")"""
        return num * 5

    def generate_number(self, multiplier):
        """This is just a normal method. This is what I think
        you want, tbh, and you should probably call it in your
        __init__ method since you NEED this to run in your OTC
        for it to work as intended. Methods (like properties)
        are automagically passed the instance as the first
        argument, so we can CHANGE self.attribute with that."""
        self.attribute = self.attribute * multiplier

Docstrings should be self descriptive, but:

multi = Multi(2)
multi.attribute_times_ten # returns 20
Multi.times_ten(8) # returns 80, note the capital M!
multi.generate_number(3) # self.attribute is now 6
multi.attribute_times_ten # returns 60

A real-world case where you might need all of the above:

class _Tile(object):
    def __init__(self, x, y):
        """A naive implementation of Tile that doesn't care
        what its side length is and doesn't have any properties
        to hide its attributes"""
        self.x = x
        self.y = y
    @classmethod
    def tiles_to_pixels(cls, tile):
        return cls(tile._x * tile.side_length, tile._y * tile.side_length)
    @classmethod
    def tiles_to_tiles(cls, tile):
        return cls(tile._x, tile._y)

class Tile(object):
    def __init__(self, x, y, side_length):
        """A tile object in a map"""
        self._x = x # x-coord in tiles
        self._y = y # y-coord in tiles
        self.side_length = side_length # pixels per tile
    @property
    def in_pixels(self):
        """self.in_pixels returns an object whose .x and .y
        correspond to the x and y position IN PIXELS of the
        top-left corner of the tile."""
        _tile = _Tile.tiles_to_pixels(self)
        return _tile
    @property
    def in_tiles(self):
        """self.in_tiles returns an object whose .x and .y
        correspond to the x and y position IN TILES of the
        top-left corner of the tile."""
        _tile = _Tile.tiles_to_tiles(self)
        return _tile
    def change_side_length(self, new_length):
        """Use to change the side length. This can break
        your whole map since it's naive, so be careful."""
        self.side_length = new_length

my_tile = Tile(0,0,32) # 32 pixel tile starting at (0,0)
my_tile.x # NameError, since it's called my_tile._x
my_tile.in_tiles.x # 0
my_tile.in_pixels.y # 0
other_tile = Tile(4,7,32) # 32 pixel tile starting at (4,7)
other_tile.y # NameError, see above
other_tile.in_tiles.y # 7
other_tile.in_pixels.x # 128