为什么说Python是一门动态语言--Python的魅力

Posted 2021-02-14 ldxsuanfa

动态语言的定义：动态编程语言是高级程序设计语言的一个类别。在计算机科学领域已被广泛应用。它是一类在执行时能够改变其结构的语言：比如新的函数、对象、甚至代码能够被引进。已有的函数能够被删除或是其它结构上的变化。动态语言眼下很具有活力。众所周知的ECMAScript（JavaScript）便是一个动态语言，除此之外如PHP、Ruby、Python等也都属于动态语言，而C、C++等语言则不属于动态语言。

----来自维基百科

你是不是有过给class里面变量赋值却发现程序没达到自己预期结果的遭遇？是不是本来赋值给class.abc却赋给了class.abd？这事实上是动态语言惹的“祸”！【博主曾经玩的是java】我们先来试着玩一玩

>>> class Person():
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age

	    
>>> P = Person("The_Third_Wave", "24")
>>> 

在这里。我们定义了1个类Person。在这个类里。定义了两个初始属性name和age，可是人还有性别啊。假设这个类不是你写的是不是你会尝试訪问性别这个属性呢？

>>> P.sexuality = "male"
>>> P.sexuality
‘male‘
>>> 

这时候就发现问题了，我们定义的类里面没有sexuality这个属性啊！

怎么回事呢？这就是动态语言的魅力和坑！

这里实际上就是动态给实例绑定属性！所以博主“当年”从java转python被“坑”（无知啊）过！我们再看下一个样例

>>> P1 = Person("Wave", "25")
>>> P1.sexuality

Traceback (most recent call last):
  File "<pyshell#21>", line 1, in <module>
    P1.sexuality
AttributeError: Person instance has no attribute ‘sexuality‘
>>> 

我们尝试打印P1.sexuality，发现报错，P1没有sexuality这个属性。----给P这个实例绑定属性对P1这个实例不起作用。

那我们要给全部的Person的实例加上sexuality属性怎么办呢？答案就是直接给Person绑定属性！

>>>> Person.sexuality = None
>>> P1 = Person("Wave", "25")
>>> print P1.sexuality
None
>>> 

我们直接给Person绑定sexuality这个属性，重行实例化P1后。P1就有sexuality这个属性了！

那么function呢？怎么绑定？

>>> class Person():
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age
	def eat(self):
	    print "eat food"

	    
>>> def run(self, speed):
	print "Keeping moving, the speed is %s km/h" %speed

	
>>> P = Person("The_Third_Wave", "24")
>>> 
KeyboardInterrupt
>>> P.run()

Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    P.run()
AttributeError: Person instance has no attribute ‘run‘
>>> P.eat()
eat food
>>> import types
>>> Person.run = types.MethodType(run, None, Person)
>>> P.run(180)
Keeping moving, the speed is 180 km/h
>>> 

绑定我们了解了，可是怎么删除呢？

请看下面样例首先给的是属性的真删：

>>> P.name
‘The_Third_Wave‘
>>> P.sex

Traceback (most recent call last):
  File "<pyshell#32>", line 1, in <module>
    P.sex
AttributeError: Person instance has no attribute ‘sex‘
>>> setattr(P, "sex", "male") # 増
>>> P.sex
‘male‘
>>> delattr(P, "name") # 删
>>> P.name

Traceback (most recent call last):
  File "<pyshell#36>", line 1, in <module>
    P.name
AttributeError: Person instance has no attribute ‘name‘
>>> 

加入方法呢？

>>> class Person():
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age
	def eat(self):
	    print "eat food"

	    
>>> P = Person("The_Third_Wave", "24")
>>> P.eat()
eat food
>>> P.run()

Traceback (most recent call last):
  File "<pyshell#41>", line 1, in <module>
    P.run()
AttributeError: Person instance has no attribute ‘run‘
>>> def run(self, speed):
	print "Keeping moving, the speed is %s" %speed

	
>>> setattr(P, "run", run)
>>> P.run(360)

Traceback (most recent call last):
  File "<pyshell#45>", line 1, in <module>
    P.run(360)
TypeError: run() takes exactly 2 arguments (1 given)
>>> P.run(1, 360)
Keeping moving, the speed is 360
>>> 

删除

>>> delattr(P, "run")
>>> P.run()

Traceback (most recent call last):
  File "<pyshell#48>", line 1, in <module>
    P.run()
AttributeError: Person instance has no attribute ‘run‘
>>> 

通过以上样例能够得出一个结论：相对于动态语言，静态语言具有严谨性！

所以。玩动态语言的时候，小心动态的坑！

那么怎么避免这样的情况呢？请使用__slots__。可是我的是2.7.6版本号，測试是不行的。代码例如以下：

>>> class Person():
	__slots__ = ("location", "run")
	
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age
	    
	def eat(self):
	    print "eat food"

	    
>>> P = Person()
>>> P.sex

Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    P.sex
AttributeError: Person instance has no attribute ‘sex‘
>>> P.sex = "male"
>>> 

详细原因是什么呢，本来是准备请等待更新：ing...的

BUT。我多写了个object就出来了。。。

这可真是个神坑！soga！

>>> class Person(object):
	__slots__ = ("location", "run")
	
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age
	    
	def eat(self):
	    print "eat food"

	    
>>> P = Person()

Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    P = Person()
  File "<pyshell#11>", line 5, in __init__
    self.name = name
AttributeError: ‘Person‘ object has no attribute ‘name‘ # 顺便还发现了个注意事项：要预先定义的属性也要写到tuple里面！

>>> class Person(object):
	__slots__ = ("name", "age", "eat", "location", "run")
	
	def __init__(self, name = None, age = None):
	    self.name = name
	    self.age = age
	    
	def eat(self):
	    print "eat food"

	    
>>> P = Person()
>>> P.sex = "male"

Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    P.sex = "male"
AttributeError: ‘Person‘ object has no attribute ‘sex‘
>>> P.location = "china"
>>> P.location
‘china‘
>>> def run(self, speed):
	print "Keeping moving, the speed is %s km/h" %speed

	
>>> setattr(P, "run", run)
>>> P.run(u"请注意这儿參数和上面有个样例不一样哦", 720)
Keeping moving, the speed is 720 km/h
>>> 

顺便还发现了个注意事项：要预先定义的属性也要写到tuple里面！

临时写到这，不定期更新ing...

关于slots的demo原文：https://docs.python.org/2/reference/datamodel.html?

highlight=__slots__#__slots__

本文由@The_Third_Wave原创。不定期更新。有错误请指正。

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