descriptors_and_orm.md

Descriptors and ORM concept

Helpful information

Study following links:

• https://docs.python.org/2/howto/descriptor.html
• http://www.ibm.com/developerworks/library/os-pythondescriptors/
• https://speakerdeck.com/lig/your-own-orm-in-python-how-and-why
• https://www.smallsurething.com/python-descriptors-made-simple/

Additional topics which might help you with home tasks:

• https://github.com/coleifer/peewee/blob/master/peewee.py
• https://peewee.readthedocs.org/en/latest/
• https://wiki.python.org/moin/HigherLevelDatabaseProgramming

Subtask 1

Create simple data descriptors like BirthdayField, NameField, PhoneField which can perform simple check of types and values.

Example:

from datetime import datetime
from fields import BirthdayField, NameField, PhoneField


class Person(object):
    name = NameField()
    birthday = BirthdayField()
    phone = PhoneField()

Interactive usage example:

>>> yury = Person()
>>> print(yury.name)
None
>>> yury.name = "Yury Krasouski"
>>> print(yury.name)
Yury Krasouski
>>> yury.birthday = datetime.strptime("1986-04-09", "%Y-%m-%d")
>>> yury.phone = "375 25 9355570"
>>> print(yury.phone)
375 (25) 935-55-70
>>> print(yury.birthday)
1986-04-09 00:00:00
>>> yury.birthday = "April 09"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Birthday must be datetime type
>>> yury.name = None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Name must be str type
>>> yury.phone = "375 (25) 9355570"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Phone must be in format XXX XX XXXXXXX
etc.

Subtask 2

Create non-data descriptor like DocAPI which can provide class-level and instance-level documentation about user methods and attributes.

Example:

from doclib import DocAPI


class Foo(object):
    __doc__ = API()
    def __init__(self, x):
        self.x = x
    def meth(self, y):
        """Multiplies two values self.x and y."""
        return self.x * y

Interactive usage example:

>>> print(Foo.__doc__)
meth : Multiplies two values self.x and y.
>>> foo = Foo(10)
>>> print(foo.__doc__)
x : int
meth : Multiplies two values self.x and y.

Subtask 3

Create simple ORM using data descriptors like in Subtask 1 which stores descriptor values in a database or files. E.g SQLite3.

Example:

from myorm import Model, BirthdayField, NameField, PhoneField


class Person(Model):
   name = NameField()
   birthday = BirthdayField()
   phone = PhoneField()

Interactive usage example:

>>> p = Person()
>>> p.name = "Yury"
>>> p.phone = "375 25 9355570"
>>> p.save()  # All changes saved.

Note: Implementation of method Model.save() is up to you. You can save value within __set__ method of descriptors.

Note: This task requires knowledge of class decorators or metaclasses "magic".