def test_errors(self):
class MyMeta(type):
pass
with self.assertRaises(TypeError):
class MyClass(metaclass=MyMeta, otherarg=1):
pass
with self.assertRaises(TypeError):
types.new_class("MyClass", (object,),
dict(metaclass=MyMeta, otherarg=1))
types.prepare_class("MyClass", (object,),
dict(metaclass=MyMeta, otherarg=1))
class MyMeta(type):
def __init__(self, name, bases, namespace, otherarg):
super().__init__(name, bases, namespace)
with self.assertRaises(TypeError):
class MyClass(metaclass=MyMeta, otherarg=1):
pass
class MyMeta(type):
def __new__(cls, name, bases, namespace, otherarg):
return super().__new__(cls, name, bases, namespace)
def __init__(self, name, bases, namespace, otherarg):
super().__init__(name, bases, namespace)
self.otherarg = otherarg
class MyClass(metaclass=MyMeta, otherarg=1):
pass
self.assertEqual(MyClass.otherarg, 1)
def create_class(
name: str,
bases: Iterable = (),
attrs: dict = {},
metaclass: Optional[Callable[..., Type[T]]] = None,
**kwargs: Any
) -> Type[T]:
"""
Creates a new class. This is similar to :func:`types.new_class`, except it
calls :func:`resolve_bases` even in python versions <= 3.7. (And it has a
different interface.)
:param name: The name of the new class
:param bases: An iterable of bases classes
:param attrs: A dict of class attributes
:param metaclass: The metaclass, or ``None``
:param kwargs: Keyword arguments to pass to the metaclass
:return: A new class
"""
if metaclass is not None:
kwargs.setdefault('metaclass', metaclass)
resolved_bases = resolve_bases(bases)
meta, ns, kwds = types.prepare_class(name, resolved_bases, kwargs)
ns.update(attrs)
# Note: In types.new_class this is "is not" rather than "!="
if resolved_bases != bases:
ns['__orig_bases__'] = bases
return meta(name, resolved_bases, ns, **kwds)
def new_class(name, bases=(), kwds=None, exec_body=None):
"""Create a class object dynamically using the appropriate metaclass.
"""
import sys
meta, ns, kwds = prepare_class(name, bases, kwds)
if exec_body is not None:
exec_body(ns)
if sys.version_info >= (3, 0):
return meta(name, bases, ns, **kwds)
else:
return meta(name, bases, ns)
def test_prepare_class(self):
# Basic test of metaclass derivation
expected_ns = {}
class A(type):
def __new__(*args, **kwargs):
return type.__new__(*args, **kwargs)
def __prepare__(*args):
return expected_ns
B = types.new_class("B", (object,))
C = types.new_class("C", (object,), {"metaclass": A})
# The most derived metaclass of D is A rather than type.
meta, ns, kwds = types.prepare_class("D", (B, C), {"metaclass": type})
self.assertIs(meta, A)
self.assertIs(ns, expected_ns)
self.assertEqual(len(kwds), 0)
def test_prepare_class(self):
# Basic test of metaclass derivation
expected_ns = {}
class A(type):
def __new__(*args, **kwargs):
return type.__new__(*args, **kwargs)
def __prepare__(*args):
return expected_ns
B = types.new_class("B", (object,))
C = types.new_class("C", (object,), {"metaclass": A})
# The most derived metaclass of D is A rather than type.
meta, ns, kwds = types.prepare_class("D", (B, C), {"metaclass": type})
self.assertIs(meta, A)
self.assertIs(ns, expected_ns)
self.assertEqual(len(kwds), 0)
def prepare_class():
# Basic test of metaclass derivation
expected_ns = {}
class A(type):
def __new__(*args, **kwargs):
return type.__new__(*args, **kwargs)
def __prepare__(*args):
return expected_ns
B = types.new_class("B", (object, ))
C = types.new_class("C", (object, ), {"metaclass": A})
meta, ns, kwds = types.prepare_class("D", (B, C), {"metaclass": type})
print(meta is A)
print(ns is expected_ns)
print(len(kwds) == 0)
Stock.__module__ = __name__
print(type(Stock)) # <class 'abc.ABCMeta'>
Stock = collections.namedtuple('Stock', ['name', 'shares', 'price'])
print(Stock) # <class '__main__.Stock'>
def named_tuple(classname, fieldnames):
# Populate a dictionary of field property accessors
cls_dict = {
name: property(operator.itemgetter(n))
for n, name in enumerate(fieldnames)
}
# Make a __new__ function and add to the class dict
def __new__(cls, *args):
if len(args) != len(fieldnames):
raise TypeError('Expected {} arguments'.format(len(fieldnames)))
return tuple.__new__(cls, args)
cls_dict['__new__'] = __new__
# Make the class
cls = types.new_class(classname, (tuple, ), {},
lambda ns: ns.update(cls_dict))
# Set the module to that of the caller
cls.__module__ = sys._getframe(1).f_globals['__name__']
return cls
metaclass, kwargs, ns = types.prepare_class('Stock', (), {'metaclass': type})
def update_event(self, inp=-1):
self.set_output_val(
0, types.prepare_class(self.input(0), self.input(1),
self.input(2)))
print(p.x)
# 4
print(p.y)
# 5
# p.x = 2
# AttributeError: can't set attribute
print('%s %s' % p)
# 4 5
"""
这项技术一个很重要的方面是它对于元类的正确使用。你可能像通过直接实例化一
个元类来直接创建一个类：
Stock = type('Stock', (), cls_dict)
这种方法的问题在于它忽略了一些关键步骤，比如对于元类中 prepare () 方法
的调用。通过使用 types.new class() ，你可以保证所有的必要初始化步骤都能得到
执行。比如，types.new class() 第四个参数的回调函数接受 prepare () 方法返回
的映射对象。
如果你仅仅只是想执行准备步骤，可以使用 types.prepare class() 。例如：
"""
import types
metaclass, kwargs, ns = types.prepare_class('Stock', (), {'metaclass':type})
"""
它会查找合适的元类并调用它的 prepare () 方法。然后这个元类保存它的关键
字参数，准备命名空间后被返回。
"""
def __new__(cls, *args):
if len(args) != len(field_names):
raise TypeError(f'Expected {len(field_names)} arguments')
return tuple.__new__(cls, args)
cls_dict['__new__'] = __new__
# Make the class
cls = types.new_class(class_name, (tuple, ), {},
lambda ns: ns.update(cls_dict))
# Set the module to that of the caller
cls.__module__ = sys._getframe(1).f_globals['__name__']
return cls
Point = named_tuple('Point', ['x', 'y'])
print(f'point object: {Point}')
p = Point(5, 8)
print(f'p length = {len(p)}')
print(f'p.x = {p.x}')
print(f'p.y = {p.y}')
# p.x = 3
print(f'p is: {p}')
Course = type('Course', (), cls_dict)
import types
metaclass, kwargs, ns = types.prepare_class('Course', (), {'metaclass': type})
