Python 简明教程

Python - Reflection

在面向对象编程中， reflection 指的是提取正在使用的任何对象信息的能力。你可以了解对象的类型、它是否为其他任何类的子类、它的 attributes ，以及更多内容。Python 的标准库有若干函数可以反映对象的各个属性。 Reflection 有时也称为 introspect 。

In object-oriented programming, reflection refers to the ability to extract information about any object in use. You can get to know the type of object, whether is it a subclass of any other class, what are its attributes, and much more. Python’s standard library has several functions that reflect on different properties of an object. Reflection is also sometimes called introspect.

以下是 Python 中的反射函数列表 −

Following is the list of reflection functions in Python −

  1. type() Function

  2. isinstance() Function

  3. issubclass() Function

  4. callable() Function

  5. getattr() Function

  6. setattr() Function

  7. hasattr() Function

  8. dir() Function

The type() Function

我们已经多次使用该函数。它告诉你对象属于哪个类。

We have used this function many times. It tells you which class an object belongs to.

Example

以下语句打印不同内置数据类型对象的相应类

Following statements print the respective class of different built-in data type objects

print (type(10))
print (type(2.56))
print (type(2+3j))
print (type("Hello World"))
print (type([1,2,3]))
print (type({1:'one', 2:'two'}))

在这里，你会得到以下 output −

Here, you will get the following output −

<class 'int'>
<class 'float'>
<class 'complex'>
<class 'str'>
<class 'list'>
<class 'dict'>

让我们验证一下用户自定义类对象的类型 -

Let us verify the type of an object of a user-defined class −

class test:
   pass

obj = test()
print (type(obj))

它将生成以下 output −

It will produce the following output −

<class '__main__.test'>

The isinstance() Function

这是 Python 中的另一个内置函数，它可以确定对象是否是给定类的实例。

This is another built-in function in Python which ascertains if an object is an instance of the given class.

Syntax

isinstance(obj, class)

此函数总是返回一个 Boolean value ，如果对象确实属于给定类，则为 true；否则为 false。

This function always returns a Boolean value, true if the object is indeed belongs to the given class and false if not.

Example

以下语句返回 True -

Following statements return True −

print (isinstance(10, int))
print (isinstance(2.56, float))
print (isinstance(2+3j, complex))
print (isinstance("Hello World", str))

它将生成以下 output −

It will produce the following output −

True
True
True
True

相反，这些语句打印 False。

In contrast, these statements print False.

print (isinstance([1,2,3], tuple))
print (isinstance({1:'one', 2:'two'}, set))

它将生成以下 output −

It will produce the following output −

False
False

你也可以使用用户自定义类进行检查

You can also perform check with a user defined class

class test:
   pass

obj = test()
print (isinstance(obj, test))

它将生成以下 output −

It will produce the following output −

True

在 Python 中，即使类也是对象。所有类都是 object 类对象。这可以通过以下代码验证 -

In Python, even the classes are objects. All classes are objects of object class. It can be verified by following code −

class test:
   pass

print (isinstance(int, object))
print (isinstance(str, object))
print (isinstance(test, object))

以上所有 print 语句都打印 True。

All the above print statements print True.

The issubclass() Function

此函数检查某个类是否另一个类的子类。它属于类，而不是它们的实例。

This function checks whether a class is a subclass of another class. Pertains to classes, not their instances.

如前所述，所有 Python 类都是 object 类的子类。因此，以下 print 语句的输出对于所有类而言都是 True。

As mentioned earlier, all Python classes are subclassed from object class. Hence, output of following print statements is True for all.

class test:
   pass

print (issubclass(int, object))
print (issubclass(str, object))
print (issubclass(test, object))

它将生成以下 output −

It will produce the following output −

True
True
True

The callable() Function

如果对象调用某个过程，则该对象是可调用的。执行某个过程的 Python function 是可调用对象。因此，callable(function) 返回 True。任何函数、内置函数、用户自定义函数或 method 都可以调用。诸如 int、str 等内置 data types 的对象不可调用。

An object is callable if it invokes a certain process. A Python function, which performs a certain process, is a callable object. Hence callable(function) returns True. Any function, built-in, user-defined, or method is callable. Objects of built-in data types such as int, str, etc., are not callable.

Example

def test():
   pass

print (callable("Hello"))
print (callable(abs))
print (callable(list.clear([1,2])))
print (callable(test))

string 对象不可调用。但 abs 是一个可调用的函数。列表的 pop 方法可调用，但 clear() 实际上是对函数的调用，而不是函数对象，因此不可调用。

A string object is not callable. But abs is a function which is callable. The pop method of list is callable, but clear() is actually call to the function and not a function object, hence not a callable

它将生成以下 output −

It will produce the following output −

False
True
True
False
True

如果类实例具有 call() 方法，则该实例可调用。在下面的示例中，test 类包括 call() 方法。因此，它的对象可以用作我们调用某个函数的情况。因此，具有 call() 函数的类的对象可调用。

A class instance is callable if it has a call() method. In the example below, the test class includes call() method. Hence, its object can be used as if we are calling function. Hence, object of a class with call() function is a callable.

class test:
   def __init__(self):
      pass
   def __call__(self):
      print ("Hello")

obj = test()
obj()
print ("obj is callable?", callable(obj))

它将生成以下 output −

It will produce the following output −

Hello
obj is callable? True

The getattr() Function

getattr() 内置函数会检索对象的命名属性值。

The getattr() built-in function retrieves the value of the named attribute of object.

Example

class test:
   def __init__(self):
      self.name = "Manav"

obj = test()
print (getattr(obj, "name"))

它将生成以下 output −

It will produce the following output −

Manav

The setattr() Function

setattr() 内置函数会向对象添加一个新属性并赋予它一个值。它还可以更改现有属性的值。

The setattr() built-in function adds a new attribute to the object and assigns it a value. It can also change the value of an existing attribute.

在下面的示例中，test 类的对象具有单个属性 − name。我们使用 setattr() 来添加 age 属性并修改 name 属性的值。

In the example below, the object of test class has a single attribute − name. We use setattr() to add age attribute and to modify the value of name attribute.

class test:
   def __init__(self):
      self.name = "Manav"

obj = test()
setattr(obj, "age", 20)
setattr(obj, "name", "Madhav")
print (obj.name, obj.age)

它将生成以下 output −

It will produce the following output −

Madhav 20

The hasattr() Function

此内置函数在给定对象参数可以使用给定属性时返回 True，否则返回 False。我们使用相同的测试类，然后检查它是否有某个属性。

This built-in function returns True if the given attribute is available to the object argument, and false if not. We use the same test class and check if it has a certain attribute or not.

class test:
   def __init__(self):
      self.name = "Manav"

obj = test()
print (hasattr(obj, "age"))
print (hasattr(obj, "name"))

它将生成以下 output −

It will produce the following output −

False
True

The dir() Function

如果此内置函数在没有参数的情况下被调用，则返回当前作用域中的名称。对于任何作为参数的对象，它返回给定对象属性的列表以及从该对象可到达的属性。

If this built-in function is called without an argument, return the names in the current scope. For any object as an argument, it returns a list of the attributes of the given object and attributes reachable from it.

  1. For a module object − the function returns the module’s attributes.

  2. For a class object − the function returns its attributes, and recursively the attributes of its bases.

  3. For any other object − its attributes, its class’s attributes, and recursively the attributes of its class’s base classes.

Example

print ("dir(int):", dir(int))

它将生成以下 output −

It will produce the following output −

dir(int): ['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getnewargs__', '__getstate__', '__gt__', '__hash__', '__index__', '__init__', '__init_subclass__', '__int__', '__invert__', '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'as_integer_ratio', 'bit_count', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

Example

print ("dir(dict):", dir(dict))

它将生成以下 output −

It will produce the following output −

dir(dict): ['__class__', '__class_getitem__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__ior__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__ror__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

Example

class test:
   def __init__(self):
      self.name = "Manav"

obj = test()
print ("dir(obj):", dir(obj))

它将生成以下 output −

It will produce the following output −

dir(obj): ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'name']