Python中@classmethod 和@staticmethod用法和区别

@classsmethod 类装饰器：当用此装饰器定义方法时，将类而不是类的实例作为第一个参数，这意味着可以在此方法中直接使用类的属性，而不是特定的实例的属性，因此不必进行硬编码。

@staticmethod 静态装饰器：当用此装饰器定义方法时，不会传递类或实例作为它的参数，这意味着可以在类中放置一个函数。静态方法就是普通的函数，只是碰巧在类的定义体中，而不是在模块层定义。

在《流畅的Python》中，作者对这两个装饰器的评价：classmethod 装饰器非常有用，但是我从未见过不得不用 staticmethod 的情况。如果想定义不需要与类交互的函数，那么在模块中定义就好了。有时，函数虽然从不处理类，但是函数的功能与类紧密相关，因此想把它放在近处。即便如此，在同一模块中的类前面或后面定义函数也就行了。

class Date(object):

<span class="token keyword">def</span> <span class="token function">__init__</span><span
        class="token punctuation">(</span>self<span class="token punctuation">,</span> day<span
        class="token operator">=</span><span class="token number">0</span><span
        class="token punctuation">,</span> month<span class="token operator">=</span><span
        class="token number">0</span><span
        class="token punctuation">,</span> year<span class="token operator">=</span><span
        class="token number">0</span><span class="token punctuation">)</span><span
        class="token punctuation">:</span>
    self<span class="token punctuation">.</span>day <span class="token operator">=</span> day
    self<span class="token punctuation">.</span>month <span class="token operator">=</span> month
    self<span class="token punctuation">.</span>year <span class="token operator">=</span> year

@<span class="token builtin">classmethod</span>
<span class="token keyword">def</span> <span class="token function">from_string</span><span
        class="token punctuation">(</span>cls<span class="token punctuation">,</span> date_as_string<span
        class="token punctuation">)</span><span class="token punctuation">:</span>
    day<span class="token punctuation">,</span> month<span class="token punctuation">,</span> year <span
        class="token operator">=</span> <span class="token builtin">map</span><span
        class="token punctuation">(</span><span class="token builtin">int</span><span
        class="token punctuation">,</span> date_as_string<span
        class="token punctuation">.</span>split<span class="token punctuation">(</span><span
        class="token string">'-'</span><span class="token punctuation">)</span><span
        class="token punctuation">)</span>
    date1 <span class="token operator">=</span> cls<span class="token punctuation">(</span>day<span
        class="token punctuation">,</span> month<span class="token punctuation">,</span> year<span
        class="token punctuation">)</span>
    <span class="token keyword">return</span> date1

@<span class="token builtin">staticmethod</span>
<span class="token keyword">def</span> <span class="token function">is_date_valid</span><span
        class="token punctuation">(</span>date_as_string<span class="token punctuation">)</span><span
        class="token punctuation">:</span>
    day<span class="token punctuation">,</span> month<span class="token punctuation">,</span> year <span
        class="token operator">=</span> <span class="token builtin">map</span><span
        class="token punctuation">(</span><span class="token builtin">int</span><span
        class="token punctuation">,</span> date_as_string<span
        class="token punctuation">.</span>split<span class="token punctuation">(</span><span
        class="token string">'-'</span><span class="token punctuation">)</span><span
        class="token punctuation">)</span>
    <span class="token keyword">return</span> day <span class="token operator">&lt;=</span> <span
        class="token number">31</span> <span class="token keyword">and</span> month <span
        class="token operator">&lt;=</span> <span class="token number">12</span> <span
        class="token keyword">and</span> year <span class="token operator">&lt;=</span> <span
        class="token number">3999</span>

date2 = Date.from_string(‘11-09-2012’)
is_date = Date.is_date_valid(‘11-09-2012’)

使用静态方法写基类，注意静态方法使用了实例的硬编码。

class Date:
  def __init__(self, month, day, year):
    self.month = month
    self.day   = day
    self.year  = year

  def display(self):
    return “{0}-{1}-{2}”.format(self.month, self.day, self.year)
  @staticmethod
  def millenium(month, day):
    return Date(month, day, 2000)
new_year = Date(1, 1, 2013)               # Creates a new Date object
millenium_new_year = Date.millenium(1, 1) # also creates a Date object. 
# Proof:
new_year.display()           # “1-1-2013”
millenium_new_year.display() # “1-1-2000”
isinstance(new_year, Date) # True
isinstance(millenium_new_year, Date) # True

继承之后，millenium方法就用不了了。

class DateTime(Date):
  def display(self):
      return "{0}-{1}-{2} - 00:00:00PM".format(self.month, self.day, self.year)

datetime1 = DateTime(10, 10, 1990)
datetime2 = DateTime.millenium(10, 10)
isinstance(datetime1, DateTime) # True
isinstance(datetime2, DateTime) # False
datetime1.display() # returns “10-10-1990 - 00:00:00PM”
datetime2.display() # returns “10-10-2000” because it’s not a DateTime object but a Date object.

改成类方法，

@classmethod
def millenium(cls, month, day):
    return cls(month, day, 2000)

就正确了

datetime1 = DateTime(10, 10, 1990)
datetime2 = DateTime.millenium(10, 10)

isinstance(datetime1, DateTime) # True
isinstance(datetime2, DateTime) # True
datetime1.display() # “10-10-1990 - 00:00:00PM”
datetime2.display() # “10-10-2000 - 00:00:00PM”