python中的self详解_Python中returnself的用法详解

在Python中&＃xff0c;有些开源项目中的方法返回结果为self. 对于不熟悉这种用法的读者来说&＃xff0c;这无疑使人困扰&＃xff0c;本文的目的就是给出这种语法的一个解释&＃xff0c;并且给出几个例子。

在Python中&＃xff0c;return self的作用为&＃xff1a;(英语原文&＃xff0c;笔者水平有限&＃xff0c;暂不翻译)

Returning self from a method simply means that your method returns a

reference to the instance object on which it was called. This can

sometimes be seen in use with object oriented APIs that are designed as a

fluent interface that encourages method cascading.

通俗的说法是&＃xff0c; allow chaining(这个是笔者自己的翻译&＃xff1a; 链式调用).

例子&＃xff1a;

class Foo(object):

def __init__(self):

self.myattr &＃61; 0

def bar(self):

self.myattr &＃43;&＃61; 1

return self

f &＃61; Foo()

f.bar().bar().bar()

print(f.myattr)

输出结果为4.

把bar()方法改为返回return None&＃xff0c; 则上述代码会出错。

class Foo(object):

def __init__(self):

self.myattr &＃61; 0

def bar(self):

self.myattr &＃43;&＃61; 1

return None

f &＃61; Foo()

f.bar().bar().bar()

print(f.myattr)

输出结果如下&＃xff1a;

AttributeError: &＃39;NoneType&＃39; object has no attribute &＃39;bar&＃39;

那么return self返回的结果是什么呢&＃xff1f;

class Foo(object):

def __init__(self):

self.myattr &＃61; 0

def bar(self):

self.myattr &＃43;&＃61; 1

#return None

return self

f &＃61; Foo()

print(type(f.bar()))

输出结果为&＃xff1a;

可以发现&＃xff0c;return self返回的是类的实例。

一个真实的例子&＃xff1a;

sklearn模块中很多方法的返回结果为self, 比如大多数模型的fit()方法&＃xff0c;例子如下&＃xff1a;

from sklearn.linear_model import LogisticRegression

X &＃61; [[0,0], [0,1], [1,0], [1,1]]

y &＃61; [0, 1, 1, 0]

clf &＃61; LogisticRegression()

# fit函数返回的结果就是self, 允许链式调用

t &＃61; clf.fit(X,y).predict([[0,2]])

print(t)

输出&＃xff1a;

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