Hi,
I am wondering if someone could help me understand some of the
details of using metaclasses.
I am trying to use metaclasses to auto-generate some attributes
and methods in a class definition. I am attaching a simplified
example of the kind of code I am considering below, and I have
the following questions:
1. Is the code in NamedType.__new __ the "right" way to have a
class which has NamedType as its metaclass inherit from
NamedClass? Are the semantics different from if the class with
NamedType as its metaclass explicitly listed NamedClass as a
superclass in the standard way?
2. Is the try/except clause in NameType.__init __ the "right"
way to prevent KeyErrors for subclasses of classes with
NamedType as its metaclass?
Thanks in advance,
d
---
class NamedClass(obje ct):
def __init__(self, name):
self.name = name
class NamedType(type) :
def __new__(mcl, name, bases, dictionary):
bases = tuple(list(base s)+[NamedClass])
return super(NamedType , mcl).__new__(mc l, name, bases, dictionary)
def __init__(cls, name, bases, dictionary):
try:
name = dictionary['NAME']
except KeyError:
return
def is_a(self):
return self.name == name
setattr(cls, 'is_a_'+name, is_a)
def init(self):
super(cls, self).__init__( name)
setattr(cls, '__init__', init)
class PrintNamedType( NamedType):
def __init__(cls, name, bases, dictionary):
super(PrintName dType, cls).__init__(n ame, bases, dictionary)
def print_name(self ):
print self.name
setattr(cls, 'print_name', print_name)
class NamedFoo:
__metaclass__ = NamedType
NAME = 'foo'
class SubNamedFoo(Nam edFoo):
pass
class NamedBar:
__metaclass__ = PrintNamedType
NAME = 'bar'
I am wondering if someone could help me understand some of the
details of using metaclasses.
I am trying to use metaclasses to auto-generate some attributes
and methods in a class definition. I am attaching a simplified
example of the kind of code I am considering below, and I have
the following questions:
1. Is the code in NamedType.__new __ the "right" way to have a
class which has NamedType as its metaclass inherit from
NamedClass? Are the semantics different from if the class with
NamedType as its metaclass explicitly listed NamedClass as a
superclass in the standard way?
2. Is the try/except clause in NameType.__init __ the "right"
way to prevent KeyErrors for subclasses of classes with
NamedType as its metaclass?
Thanks in advance,
d
---
class NamedClass(obje ct):
def __init__(self, name):
self.name = name
class NamedType(type) :
def __new__(mcl, name, bases, dictionary):
bases = tuple(list(base s)+[NamedClass])
return super(NamedType , mcl).__new__(mc l, name, bases, dictionary)
def __init__(cls, name, bases, dictionary):
try:
name = dictionary['NAME']
except KeyError:
return
def is_a(self):
return self.name == name
setattr(cls, 'is_a_'+name, is_a)
def init(self):
super(cls, self).__init__( name)
setattr(cls, '__init__', init)
class PrintNamedType( NamedType):
def __init__(cls, name, bases, dictionary):
super(PrintName dType, cls).__init__(n ame, bases, dictionary)
def print_name(self ):
print self.name
setattr(cls, 'print_name', print_name)
class NamedFoo:
__metaclass__ = NamedType
NAME = 'foo'
class SubNamedFoo(Nam edFoo):
pass
class NamedBar:
__metaclass__ = PrintNamedType
NAME = 'bar'
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