[英]How to instantiate many different classes from lists of names and classes in Python
[英]How to instantiate objects from different classes from a list of dictionaries?
從外部Web服務,我收到一個如下所示的JSON響應(為方便起見,下面已經對它進行了反序列化):
alist = [
{
'type': 'type1',
'name': 'dummy',
'oid': 'some_id'
},
{
'type': 'type2',
'name': 'bigdummy',
'anumber': 10
}
]
對於每種type
都有一個類。 我想要的是實例化各個類的對象,而無需使用一長串if-elif
。
我嘗試如下:
使用from_dict
classmethod
定義類A
(對於type1
)和類B
(對於type2
):
class A():
def __init__(self, name, oid, optional_stuff=None):
self.name = name
self.oid = oid
self.optional_stuff = optional_stuff
@classmethod
def from_dict(cls, d):
name = d['name']
oid = d['oid']
optional_stuff = d.get('optional_stuff')
return cls(name, oid, optional_stuff)
def foo(self):
print('i am class A')
class B():
def __init__(self, name, anumber):
self.name = name
self.number = anumber
@classmethod
def from_dict(cls, d):
name = d['name']
anumber = d['anumber']
return cls(name, anumber)
然后定義一個映射字典:
string_class_map = {
'type1': A,
'type2': B
}
最后轉換alist
到東西from_dict
功能可以輕松地使用:
alist2 = [
{
di['type']: {k: v for k, v in di.items() if k != 'type'}
}
for di in alist
]
[{'type1': {'name': 'dummy', 'oid': 'some_id'}},
{'type2': {'name': 'bigdummy', 'anumber': 10}}]
object_list = [
string_class_map[k].from_dict(v) for d in alist2 for k, v in d.items()
]
那給了我想要的輸出; 當我做:
a = object_list[0]
a.name
確實會打印'dummy'
。
問題是從alist
(此輸入我無法更改)到object_list
是否有更好的方法。
只要參數名稱完全匹配,就不需要from_dict
類方法-盡管您可能仍希望將它們作為添加額外錯誤處理的地方進行研究。 我們要做的就是使用參數解壓縮。
首先,我將總結創建單個對象的過程。 也就是說,單個“ from_dict” -y方法應處理類型的確定,准備其他參數的dict並調用類工廠。
從工廠創建這些類的基類似乎很有用-畢竟它們至少有一個共同點,就是可以用這種方式創建; 您可以在該級別添加調試內容; 這是工廠邏輯本身的便利位置。
您可以使用裝飾器或元類來完成查找映射的創建,從而避免維護單獨的數據塊。
綜上所述,我得到:
class JsonLoadable:
_factory = {}
def __str__(self):
return f'{self.__class__.__name__}(**{{{self.__dict__}}})'
@staticmethod # this is our decorator.
def register(cls):
# We use the class' __name__ attribute to get the lookup key.
# So as long as the classes are named to match the JSON, this
# automatically builds the correct mapping.
JsonLoadable._factory[cls.__name__] = cls
return cls
@staticmethod
def from_dict(d):
d = d.copy()
cls = JsonLoadable._factory[d.pop('type')]
# this is the magic that lets us avoid class-specific logic.
return cls(**d)
# I'm pretty sure there's a way to streamline this further with metaclasses,
# but I'm not up to figuring it out at the moment...
@JsonLoadable.register
class A(JsonLoadable):
def __init__(self, name, oid, optional_stuff=None):
self.name = name
self.oid = oid
self.optional_stuff = optional_stuff
@JsonLoadable.register
class B(JsonLoadable):
def __init__(self, name, anumber):
self.name = name
self.number = anumber
# And now our usage is simple:
objects = [JsonLoadable.from_dict(d) for d in alist]
與其為在alist
中可能遇到的每個'type'
編寫自定義類, alist
使用允許您訪問其屬性的通用類(使用示例類所做的一切)會更簡單。
為此,下面的代碼定義了內置dict
類的子類,該類將允許訪問它們中的值,就好像它們是實例屬性一樣。
這就是我的意思:
from collections import Iterable, Mapping
from pprint import pprint, pformat
class AttrDict(dict):
def __init__(self, d):
for k, v in d.items():
if isinstance(v, Mapping):
d[k] = AttrDict(v)
elif isinstance(v, Iterable) and not isinstance(v, str):
d[k] = [AttrDict(x) if isinstance(x, Mapping) else x
for x in v]
self.__dict__.update(d)
def __repr__(self):
return 'AttrDict({})'.format(repr(self.__dict__))
alist = [
{
'type': 'type1',
'name': 'dummy',
'oid': 'some_id'
},
{
'type': 'type2',
'name': 'bigdummy',
'anumber': 10
}
]
object_list = [AttrDict(obj) for obj in alist]
pprint(object_list)
print() # -> [AttrDict({'type': 'type1', 'name': 'dummy', 'oid': 'some_id'}),
# -> AttrDict({'type': 'type2', 'name': 'bigdummy', 'anumber': 10})]
a = object_list[0]
print(repr(a.name)) # -> 'dummy'
根據您對我的第一個答案的反饋,這是另一個-完全不同的-一個使您可以輕松創建任意多或少獨立類的方法。 我認為它比@Karl Knechtel的回答有所改進,因為不需要裝飾器並使用它來“注冊”每個子類-通過從公共基類派生每個子類來有效地自動進行。
基本上,這只是我在回答問題時使用的模式的改編:
class Base:
class Unknown(Exception): pass
@classmethod
def _get_all_subclasses(cls):
""" Recursive generator of all class' subclasses. """
for subclass in cls.__subclasses__():
yield subclass
for subclass in subclass._get_all_subclasses():
yield subclass
def __new__(cls, d):
""" Create instance of appropriate subclass using type id. """
type_id = d['type']
for subclass in cls._get_all_subclasses():
if subclass.type_id == type_id:
# Using "object" base class method avoids recursion here.
return object.__new__(subclass)
else: # No subclass with matching type_id found.
raise Base.Unknown(f'type: {type_id!r}')
def __repr__(self):
return f'<{self.__class__.__name__} instance>'
class A(Base):
type_id = 'type1'
def __init__(self, d):
self.name = d['name']
self.oid = d['oid']
self.optional_stuff = d.get('optional_stuff')
def foo(self):
print('I am class A')
class B(Base):
type_id = 'type2'
def __init__(self, d):
self.name = d['name']
self.anumber = d['anumber']
alist = [
{
'type': 'type1',
'name': 'dummy',
'oid': 'some_id'
},
{
'type': 'type2',
'name': 'bigdummy',
'anumber': 10
}
]
object_list = [Base(obj) for obj in alist]
print(f'object_list: {object_list}') # -> [<A instance>, <B instance>]
a = object_list[0]
print(repr(a.name)) # -> 'dummy'
b = object_list[1]
print(repr(b.name)) # -> 'bigdummy'
如果您使用的是Python 3.6+,則可以使用該版本中添加的object.__init_subclass__()
類方法來實現更簡潔的實現:
class Base:
_registry = {}
@classmethod
def __init_subclass__(cls, **kwargs):
type_id = kwargs.pop('type_id', None)
super().__init_subclass__(**kwargs)
if type_id is not None:
cls._registry[type_id] = cls
def __new__(cls, d):
""" Create instance of appropriate subclass. """
type_id = d['type']
subclass = Base._registry[type_id]
return object.__new__(subclass)
def __repr__(self):
return f'<{self.__class__.__name__} instance>'
class A(Base, type_id='type1'):
def __init__(self, d):
self.name = d['name']
self.oid = d['oid']
self.optional_stuff = d.get('optional_stuff')
def foo(self):
print('I am class A')
class B(Base, type_id='type2'):
def __init__(self, d):
self.name = d['name']
self.anumber = d['anumber']
alist = [
{
'type': 'type1',
'name': 'dummy',
'oid': 'some_id'
},
{
'type': 'type2',
'name': 'bigdummy',
'anumber': 10
}
]
object_list = [Base(obj) for obj in alist]
print(f'object_list: {object_list}') # -> [<A instance>, <B instance>]
a = object_list[0]
print(repr(a.name)) # -> 'dummy'
b = object_list[1]
print(repr(b.name)) # -> 'bigdummy'
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