python 语句执行顺序_一个针对 Python 语句执行顺序的练习

摘自 Fluent Python

evalsupport.py

print(' evalsupport module start')

def deco_alpha(cls):

print(' deco_alpha')

def inner_1(self):

print(' deco_alpha:inner_1')

cls.method_y = inner_1

return cls

class MetaAleph(type):

print(' MetaAleph body')

def __init__(cls, name, bases, dic):

print(' MetaAleph.__init__')

def inner_2(self):

print(' MetaAleph.__init__:inner_2')

cls.method_z = inner_2

print(' evalsupport module end')

evaltime.py

from evalsupport import deco_alpha

print(' evaltime module start')

class ClassOne():

print(' ClassOne body')

def __init__(self):

print(' ClassOne.__init__')

def __del__(self):

print(' ClassOne.__del__')

def method_x(self):

print(' ClassOne.method_x')

class ClassTwo(object):

print(' ClassTwo body')

@deco_alpha

class ClassThree():

print(' ClassThree body')

def method_y(self):

print(' ClassThree.method_y')

class ClassFour(ClassThree):

print(' ClassFour body')

if __name__ == '__main__':

print(' ClassOne tests', 30 * '.')

one = ClassOne()

one.method_x()

print(' ClassThree tests', 30 * '.')

three = ClassThree()

three.method_y()

print(' ClassFour tests', 30 * '.')

four = ClassFour()

four.method_y()

print(' evaltime module end')

evaltime_meta.py

from evalsupport import deco_alpha

from evalsupport import MetaAleph

print(' evaltime_meta module start')

@deco_alpha

class ClassThree():

print(' ClassThree body')

def method_y(self):

print(' ClassThree.method_y')

class ClassFour(ClassThree):

print(' ClassFour body')

def method_y(self):

print(' ClassFour.method_y')

class ClassFive(metaclass=MetaAleph):

print(' ClassFive body')

def __init__(self):

print(' ClassFive.__init__')

def method_z(self):

print(' ClassFive.method_y')

class ClassSix(ClassFive):

print(' ClassSix body')

def method_z(self):

print(' ClassSix.method_y')

if __name__ == '__main__':

print(' ClassThree tests', 30 * '.')

three = ClassThree()

three.method_y()

print(' ClassFour tests', 30 * '.')

four = ClassFour()

four.method_y()

print(' ClassFive tests', 30 * '.')

five = ClassFive()

five.method_z()

print(' ClassSix tests', 30 * '.')

six = ClassSix()

six.method_z()

print(' evaltime_meta module end')

>>> import evaltime

evalsupport module start

MetaAleph body # 说明类的 body 在导入时就执行

evalsupport module end

evaltime module start

ClassOne body

ClassTwo body # 类的 body 中有其他类, 也会被执行

ClassThree body # 先执行类的body, 然后将类作为类装饰器的参数

deco_alpha

ClassFour body # 类继承并不会继承类装饰器

evaltime module end

python evaltime.py

evalsupport module start

MetaAleph body

evalsupport module end

evaltime module start

ClassOne body

ClassTwo body

ClassThree body

deco_alpha

ClassFour body

ClassOne tests ..............................

ClassOne.__init__

ClassOne.method_x

ClassThree tests ..............................

deco_alpha:inner_1 # 装饰器修改了类属性

ClassFour tests ..............................

deco_alpha:inner_1 # ClassFour 继承的是经由装饰器修改后的ClassThree

evaltime module end

ClassOne.__del__ # 退出时垃圾回收

>>> import evaltime_meta

evalsupport module start

MetaAleph body

evalsupport module end

evaltime_meta module start

ClassThree body

deco_alpha

ClassFour body

ClassFive body

MetaAleph.__init__ # 先 ClassFour 定义, 然后交给其元类对他加工

ClassSix body

MetaAleph.__init__ # 先 ClassFour 定义, 然后交给其元类对他加工, 由于继承自 ClassFive, 其元类同上(注意区分基类与元类)

evaltime_meta module end

python3 evaltime_meta.py

evalsupport module start

MetaAleph body

evalsupport module end

evaltime_meta module start

ClassThree body

deco_alpha

ClassFour body

ClassFive body

MetaAleph.__init__

ClassSix body

MetaAleph.__init__

ClassThree tests ..............................

deco_alpha:inner_1

ClassFour tests ..............................

ClassFour.method_y

ClassFive tests ..............................

ClassFive.__init__

MetaAleph.__init__:inner_2

ClassSix tests ..............................

ClassFive.__init__

MetaAleph.__init__:inner_2

evaltime_meta module end