让我说我上了这个课:
class Person:
def __init__(self, name):
self.name = name
如果我要实例化Person
,我可以这样做:
me = Person("António")
但是如果Person
的类型为name
时我只想实例化str
怎么办?
我尝试过:
class Person:
def __init__(self, name):
if type(name) == str:
self.name = name
但是当我这样做时:
me = Person("António")
print(me.name)
you = Person(1)
print(you.name)
我明白了:
所有发生的事情是:
name
是str
,则实例具有.name
方法name
不是str
,则实例没有.name
方法但是我真正想要的是,如果name不是str
,则停止全部实例化。
换句话说,我希望不可能通过Person
类创建一个非str
name
对象。
我该怎么做?
答案 0 :(得分:1)
您可以使用检查参数的工厂,并在一切正常的情况下返回Person
对象,或者引发错误:
也许是这样:
class PersonNameError(Exception):
pass
class Person:
def __init__(self):
self.name = None
def person_from_name(name: str) -> Person:
"""Person factory that checks if the parameter name is valid
returns a Person object if it is, or raises an error without
creating an instance of Person if not.
"""
if isinstance(name, str):
p = Person()
p.name = name
return p
raise PersonNameError('a name must be a string')
p = person_from_name('Antonio')
位置:
p = person_from_name(123) # <-- parameter name is not a string
引发异常:
PersonNameError Traceback (most recent call last)
<ipython-input-41-a23e22774881> in <module>
14
15 p = person_from_name('Antonio')
---> 16 p = person_from_name(123)
<ipython-input-41-a23e22774881> in person_from_name(name)
11 p.name = name
12 return p
---> 13 raise PersonNameError('a name must be a string')
14
15 p = person_from_name('Antonio')
PersonNameError: a name must be a string
答案 1 :(得分:0)
怎么样:
set.seed(16)
x.test<-data.frame(runif(100,0,2))
eps.test<-rnorm(100,sd=0.1)
y.test<-sin(x.test)+eps.test
linear_prediction<-predict(model, x.test, interval="prediction")
答案 2 :(得分:0)
您应该使用factory design pattern
。您可以详细了解here。简单来说:
创建将检查条件的类/方法,并仅在满足这些条件时才返回新的类实例。
答案 3 :(得分:0)
如果你想修改实例化行为, 您可以创建一个构造函数, 使用类方法。
class Person:
def __init__(self, name):
self.name = name
print("ok")
@classmethod
def create(cls, name):
if not isinstance(name, str):
raise ValueError(f"Expected name to be a string, got {type(name)}")
return cls(name)
me = Person.create("António")
print(me.name)
you = Person.create(1)
print(you.name)
OK 打印一次,只证明一次实例化
ok
António
Traceback (most recent call last):
File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 31, in <module>
start(fakepyfile,mainpyfile) File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 30, in start
exec(open(mainpyfile).read(), __main__.__dict__)
File "<string>", line 17, in <module>
File "<string>", line 11, in create
ValueError: Expected name to be a string, got <class 'int'>
[Program finished]
这里,这是一个正在完成的显式测试。 很少需要覆盖 new 并且对于日常正常课程,我认为应该避免它。这样做可以使类实现变得简单。
class Test(object):
print("ok")
def __new__(cls, x):
if isinstance(x, str) :
print(x)
else:
raise ValueError(f"Expected name to be a string, got {type(x)}")
obj1 = Test("António")
obj2 = Test(1)
ok
António
Traceback (most recent call last):
File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 31, in <module>
start(fakepyfile,mainpyfile) File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 30, in start
exec(open(mainpyfile).read(), __main__.__dict__)
File "<string>", line 14, in <module>
File "<string>", line 10, in __new__
ValueError: Expected name to be a string, got <class 'int'>
[Program finished]