class attributes & data attributes


J

james_027

hi everyone,

I am now in chapter 5 of Dive Into Python and I have some question
about it. From what I understand in the book is you define class
attributes & data attributes like this in python

class Book:

total # is a class attribute

def __init__(self):
self.title # is a data attributes
self.author # another data attributes

To define class attributes is like defining a function in class, to
define a data attributes is defining a variable inside the __init__
method.

what makes me confuse is this model from Django

from django.db import models

class Person(models.Model):
first_name = models.CharField(maxlength=30)
last_name = models.CharField(maxlength=30)

I believe the first_name and last_name are data attributes? but why it
is they look like a class attributes as being define.

Thanks in advance for explaining

james
 
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D

Diez B. Roggisch

james_027 said:
hi everyone,

I am now in chapter 5 of Dive Into Python and I have some question
about it. From what I understand in the book is you define class
attributes & data attributes like this in python

class Book:

total # is a class attribute

def __init__(self):
self.title # is a data attributes
self.author # another data attributes

To define class attributes is like defining a function in class, to
define a data attributes is defining a variable inside the __init__
method.

what makes me confuse is this model from Django

from django.db import models

class Person(models.Model):
first_name = models.CharField(maxlength=30)
last_name = models.CharField(maxlength=30)

I believe the first_name and last_name are data attributes? but why it
is they look like a class attributes as being define.

First of all, the common term for what you call a data attribute is
is "instance attribute".

Furthermore - you're right and wrong.

The django-code above defines a model class, which has some class-attributes
declaring the fields the database shall have. and the instances as well!

So while the above clearly are class attributes, the ORM runtime of django
will create instances of that class that have instance attributes of the
same name.

Something along these lines (albeit a contrived example):

class Foo(object):
bar = "baz"

def __init__(self):
for name, value in self.__class__.__dict__.items():
if isinstance(value, str):
setattr(self, name, "some other value")

f = Foo()
print f.bar

Which should result in "some other value". But it's untested code above.

Diez
 
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B

Bruno Desthuilliers

james_027 a écrit :
hi everyone,

I am now in chapter 5 of Dive Into Python and I have some question
about it. From what I understand in the book is you define class
attributes & data attributes like this in python
s/data/instance/

class Book:

total # is a class attribute

def __init__(self):
self.title # is a data attributes
self.author # another data attributes

To define class attributes is like defining a function in class, to
define a data attributes is defining a variable inside the __init__
method.

what makes me confuse is this model from Django

from django.db import models

class Person(models.Model):
first_name = models.CharField(maxlength=30)
last_name = models.CharField(maxlength=30)

I believe the first_name and last_name are data attributes? but why it
is they look like a class attributes as being define.

first_name and last_name are actually class attributes. AFAICT, they are
descriptors[1] controlling access to the resultset returned by the db query.

[1] cf the doc for the descriptor protocol on python.org. This is the
feature that - amongst other things - allow Python to have a support for
'computed attributes' (aka properties).



HTH
 

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