README.md

Index

1. classes are objects too
2. how to create a simple metaclass
3. using namespace in metaclasses
4. an awesome example using metaclass
5. how to use metaclasses to create singleton classes in python
6. how django rest framework uses metaclasses
7. how django uses metaclasses to maintain backward compatability

how to use metaclasses to create singleton classes in python

First let's see how call works with normal classes:

>>> class A:
...  def __init__(self,a):
...   self.a = a
...  def __call__(self,a):
...   self.a = a
... 
>>> a = A(1)
>>> a.a
1
>>> a(42)
>>> a.a
42
>>> 

So, the call provides us a way of reinitialising a class. So, when we try to initialse an instance the call method is call. The type class also provides us a call function which gets called every time you initialise a class. (Notice the difference b/w initialising and reinitialising)

>>> class S(type):
...  instance = None
...  def __call__(cls, *args, **kwargs): #cls is the class object A
...   import pdb; pdb.set_trace()
...   if cls.instance is None: 
...    cls.instance = super(S,cls).__call__(*args, **kwargs)
...   return cls.instance
... 
>>> class A(metaclass=S):
...  def __init__(self, a):
...   self.a = a
... 
>>> a = A(42)
> <console>(5)__call__()
(Pdb) cls
<class 'A'>
(Pdb) cls.instance
(Pdb) cls.instance is None 
True
(Pdb) n
> <console>(6)__call__()
(Pdb) cls.instance
(Pdb) n
> <console>(7)__call__()
(Pdb) cls.instance
<A object at 0x106f26240>
(Pdb) cls.instance.a
42
(Pdb) args
cls = <class 'A'>
args = (42,)
kwargs = {}
(Pdb) c
>>> a
<A object at 0x106f26240>
>>> a.a
42
>>> b = A()
> <console>(5)__call__()
(Pdb) cls.instance 
<A object at 0x106f26240>
(Pdb) c
>>> a is b 
True
>>> 

This approach will allow you to define a class in one file and then you can use instances of the class across different files maintaining dynamic global variables.