Django Manager's methods

In this post i am going to explore how django.db.models.manager.Manager class supports queryset methods.

>> Book.objects.all()           # returns a queryset.
>> Book.object.filter(author='george-orwell' )        # returns all books by author george-orwell.
>> Book.objects.delete()    # deletes all instances.

 Here Book.objects is an instance of Manager class django.db.models.manager.Manager.
Ok, which means all these methods should be defined for this class.  No, all these methods are not defined for manager class but for Queryset class.

Manager class inherits from  django.db.models.manager.BaseManagerFromQuerySet class.
And magic is there is no such class as BaseManagerFromQuerySet.
That class  is dynamically generated by method from_queryset of class BaseManager.
>> BaseManager.from_queryset(QuerySet)

This is where all magic happens.
What it does is it takes all the methods of QuerySet and dynamically attach all these methods to a newly created class BaseManagerFromQuerySet. And this class is used as a base class for Manager class.

And that' s how filter(), delete() etc. can be invoked on manager.



Hope you guys find it useful.

Reference:
manager.py

python: new-style and old-style class

A new-style class inherits from object (or type) where as old-style class does not. In python 3.0 old-style class is removed.

In this post I am going to explore differences between the two.

1: types

Here we can seen that type of obj is not of class but of built-in type instance.  Attribute __class__ of obj gives information about class to which it belongs.
So type and __class__ attributes differ for old-style class.

Lets see what happens with New-style class (python 3.6): 

type and __class__ attributes does not differ for new-style class.


2: MRO (Method Resolution Order)

In addition to above differences, old-style class does not support mro.

whereas new-style class does have that:

Also one can see that new-style class inherits from a object.


3: super
super() does not works with old-style classes.

Works with new-style

the reason being is that super() expect type object for class In case of old-style type(ABC) is classobj not type But in case of new-style type(NewABC) is type.

4: descriptor protocol
New-style classes have implemented descriptor protocol whereas old-style does not.


5: raising an exception
In new style objects cannot be raised as exception unless derived from Exception.

Thanks for reading.

repr and str in python

The purpose of repr() and str() is to display information about object. But there is slight distinction between the two. In this post we will explore that.

By definition repr() method is official representation of object. That means if i want to create an object repr should be descriptive enough to give that information.
Lets take an example of datetime object.

>> from datetime import datetime
>> now_datetime = datetime.now()
>> repr(now_datetime)

prints

>> 'datetime.datetime(2017, 10, 2, 11, 0, 7, 382330)'

so if a want to create a new datetime object repr should be referred for that matter.

new_datetime = datetime.datetime(2017, 11, 12, 15, 56, 9, 343831)

Thats exactly the purpose of repr.

As per doc str() is a 'informal' representation of object. It has be string and you can put whatever you feel like.

>> str(now_datetime)
>> '2017-10-02 11:00:07.382330'


Now the difference
str() it should only return valid string object whereas repr() should either return string or a python expression. 
In case of python expression which can be used to create object.

>> make_this_repr = "x+1"
>> repr(make_this_repr)
>> "'x+1'"
>> x =10
>> eval(make_this_repr)
>> 11


 
Hope you find it useful, thanks for reading.