classmethod和staticmethod

假设有这么一个 class

class Date(object):

    def __init__(self, day=0, month=0, year=0):
        self.day = day
        self.month = month
        self.year = year

现在要把一个字符串 11-09-2012 作为变量，方法一：

string_date = '20-16-2017'
day, month, year = map(int, string_date.split('-'))
date1 = Date(day, month, year)
print date1.day

但是每次初始化一个 class 的 instance 都要重复操作一次。所以，方法二：

class Date(object):

    def __init__(self, day=0, month=0, year=0):
        self.day = day
        self.month = month
        self.year = year

    @classmethod
    def from_string(cls, date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        date1 = cls(day, month, year)
        return date1
date2 = Date.from_string('20-06-2017')
print date2.day

staticmethod和classmethod类似，区别在于不接收变量。例如在 class 加入一个判断：

    @staticmethod
    def is_date_valid(date_as_string_for_check):
        day, month, year = map(int, date_as_string_for_check.split('-'))
        return day <= 31 and month <= 12 and year <= 3999

date2 = Date.from_string('20-06-2017')
is_date = Date.is_date_valid('20-06-2017')
print date2.day
print is_date

另外一个使用了 classmethod和staticmethod的例子：

class Employee:
    num_of_emps = 0
    raise_amount = 1.04

    def __init__(self, first, last, pay):

        self.first = first
        self.last = last
        self.pay = pay
        self.email = first + '.' + last + '@company.com'

        Employee.num_of_emps += 1

    def fullname(self):
        return '{} {}'.format(self.first, self.last)

    def apply_raise(self):
        self.pay = int(self.pay * self.raise_amount)

    @classmethod
    def set_raise_amt(cls, amount):
        cls.raise_amount = amount

    @classmethod
    def from_string(cls, emp_str):
        first, last, pay = emp_str.split('-')
        return cls(first, last, pay)

    @staticmethod
    def is_workday(day):
        if day.weekday() == 5 or day.weekday() == 6:
            return False
        return True

emp_1 = Employee('Corey', 'Schafer', 50000)
emp_2 = Employee('Test', 'User', 60000)

import datetime
my_date = datetime.date(2016, 7, 11)

print(Employee.is_workday(my_date))

另外一个例子：

class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern

    def __iter__(self):
        yield from self.pattern

    def __str__(self):
        output = []
        for blip in self:
            if blip == '.':
                output.append('dot')
            else:
                output.append('dash')
        return '-'.join(output)

    @classmethod
    def from_string(cls, pattern_string):
#        new_input_str = []
        pattern_string = pattern_string.split('-')
        for i, _ in enumerate(pattern_string):
            if _ == 'dot':
                pattern_string[i] = '.'
            elif _ == 'dash':
                pattern_string[i] = '_'
        cls = cls(pattern_string)
        return cls
#        for _ in input_str:
#            if _ == 'dot':
#                new_input_str.append('.')
#            elif _ == 'dash':
#                new_input_str.append('_')

class S(Letter):
    def __init__(self):
         pattern = ['.', '.', '.']
         super().__init__(pattern)