Python Program – Create a class with Metaclass
A metaclass in Python is a class of a class that defines how a class behaves. A class is itself an instance of a metaclass. A class in Python defines how the instance of the class will behave. In order to understand metaclasses well, one needs to have prior experience working with Python classes.
Metaclasses are the ‘stuff’ that creates classes.
if we have a variable having an integer value then its type is int. You can get the type of anything using the type() function.
The metaclass is responsible for the generation of classes, we need to write our custom metaclasses to modify the way classes are generated by performing extra actions or injecting code. In another way, we do not need custom metaclasses but sometimes it’s necessary.
class M(type):
pass
class A(object):
pass
class B(object):
__metaclass__ = M
a = A()
print(type(a))
b = B()
print(type(b))
class Meta(type):
def __init__(self, *args, **kwargs):
print('Meta.__init__')
print(self) # <class '__main__.C'>
print(args) # ('C', (<type 'object'>,),
# {'__module__': '__main__',
# '__metaclass__': <class '__main__.Meta'>})
print(kwargs) # {}
class C(object):
__metaclass__ = Meta
c = C()
print(type(c))
class MyABC(type):
def __init__(self, *args):
print('Meta.__init__')
print(args) # ('C', (<type 'object'>,),
# {'__module__': '__main__',
# '__metaclass__': <class '__main__.Meta'>})
class Base(object):
__metaclass__ = MyABC
# Create a new-style class
class A(object):
pass
print(type(A)) # <type 'type'>
a = A()
print(type(a)) # <class '__main__.A'>
B = type('B', (), {})
print(type(B)) # <type 'type'>
b = B()
print(type(b)) # <class '__main__.B'>
# old style
class C():
pass
print(type(C)) # <type 'classobj'>
c = C()
print(type(c)) # <type 'instance'>
# Have attributes in the class
class AA(object):
name = 'Foo'
print(AA.name) # Foo
aa = AA()
print(aa.name) # Foo
BB = type('BB', (), {'name' : 'Bar'})
print(BB.name) # Bar
bb = BB()
print(bb.name) # Bar
# Intherit from a class
class AAA(AA):
pass
print(AAA.name) # Foo
aaa = AAA()
print(aaa.name) # Foo
BBB = type('BBB', (BB,), {})
print(BB.name) # Bar
bbb = BBB()
print(bbb.name) # Bar
def f(self):
print(self.name)
class AAAA(object):
name = 'AAAA-Foo'
def show(self):
print(self.name)
aaaa = AAAA()
aaaa.show() # AAAA-Foo
BBBB = type('BBBB', (), { 'name': 'BBBB-Bar', 'show' :
f})
bbbb = BBBB()
bbbb.show() # BBBB-Bar
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