Question

我试图查看我的对象是否已存在属性的实例。如下所示，如果我的Dog对象具有某个属性，我希望通过do_something_if_has_aged方法执行某些操作。如何检查某个属性是否已被声明？通常你会检查是否存在这样的东西，它返回False：

obj = None
if obj:
    print(True)
else:
    print(False)

这是我的最低可重复性示例：

>>> class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def add_years(self, years):
        self.age += years
        self.has_aged = True
    def do_something_if_has_aged(self):
        if self.has_aged:
            print("The dog has aged and is %d years closer to death" % self.years)
        else:
            print("The dog hasn't aged, apparently.")


>>> dog = Dog('Spot', 3)
>>> dog.age
3
>>> dog.do_something_if_has_aged()
Traceback (most recent call last):
  File "<pyshell#193>", line 1, in <module>
    dog.do_something_if_has_aged()
  File "<pyshell#190>", line 9, in do_something_if_has_aged
    if not self.has_aged:
AttributeError: 'Dog' object has no attribute 'has_aged'
>>> dog.add_years(1)
>>> dog.age
4
>>> dog.do_something_if_has_aged()
The dog hasn't aged, apparently.

很明显，狗已经老了。

如果标题没有反映我下面要传达的内容，我道歉;我是OOP的新手。

Answer 1

看起来您正在寻找hasattr内置函数：

>>> class Dog(object):
...     pass
...
>>> a = Dog()
>>> hasattr(a, 'age')
False
>>> a.age = 7
>>> hasattr(a, 'age')
True

在您的情况下，您可以修改如下：

def do_something_if_has_aged(self):
    if hasattr(self, 'has_aged'):
        pass # do your logic

Answer 2

不是测试属性，而是在类上设置默认值;如果缺少实例属性，Python会查找类属性：

class Dog:
    has_aged = False  # default for all instances
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def add_years(self, years):
        self.age += years
        self.has_aged = True  # sets an instance attribute
    def do_something_if_has_aged(self):
        if self.has_aged:
            print("The dog has aged and is %d years closer to death" % self.years)
        else:
            print("The dog hasn't aged, apparently.")

（请注意，我必须反转您的测试，如果self.has_aged true 您希望进入第一个分支，而不是相反）。

或者您可以在__init__：

中为属性设置默认值

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.has_aged = False
    def add_years(self, years):
        self.age += years
        self.has_aged = True
    def do_something_if_has_aged(self):
        if self.has_aged:
            print("The dog has aged and is %d years closer to death" % self.years)
        else:
            print("The dog hasn't aged, apparently.")

您还可以使用hasattr() function测试属性是否存在：

def do_something_if_has_aged(self):
    if hasattr(self 'has_aged') and self.has_aged:
        print("The dog has aged and is %d years closer to death" % self.years)
    else:
        print("The dog hasn't aged, apparently.")

或使用getattr() function使用默认值：

def do_something_if_has_aged(self):
    if not getattr(self 'has_aged', False):
        print("The dog has aged and is %d years closer to death" % self.years)
    else:
        print("The dog hasn't aged, apparently.")

但是，动态测试属性不应该是您选择的第一个选项;拥有类默认值更清晰。

Answer 3

我会重写__init__方法以包含self.has_aged = False以避免必须进行检查：

class Dog(object):
    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.has_aged = False # Starting value so it is guaranteed to be defined (unless explicitly deleted).

现在，你班上的其他人应该按照书面形式工作。但是，如果要查看是否已在对象上定义属性，可以写下：

class Foo(object):
    def set_bar(self):
        self.bar = True # Define the attribute bar if it is not yet defined.

    def is_bar_set(self):
        return hasattr(self, 'bar')

Answer 4

要检查使用hasattr是否完全正常，但如果您正在寻找代码的快速修复，您可以在事先将变量初始化为false：

class Dog:
  has_aged = False

并修复你的状况，因为我认为它应该被逆转：

def do_something_if_has_aged(self):
  if self.has_aged:    # instead of not self.has_aged
    print("The dog has aged and is %d years closer to death" % self.years)
  else:
    print("The dog hasn't aged, apparently.")

检查方法中是否已存在方法？

4 个答案: