python import、from import

1, module 的实质

python 中的module 本质上就是一个相对独立的python文件.

当需要重用某些功能时, 将此功能所在的模块import到需要的模块即可.

2, import 的实质

import的过程实际上包括两个主要部分,

1) 类似于shell 的 "source" 命令, 具体说即相当于将被import的module(即python文件)在当前环境下执行一遍.

当多次import同一个module时, python会保证只执行一次. 参考Example1

2) import的第二个部分是使被import的成员(变量名, 函数名, 类....)可见.

为保证不发生命名冲突, 需要以 module.name 的方式访问导入的成员. 参考Example2

3, from *** import 的实质 (import 与 from *** import 的区别)

与import类似, 被导入的module仍然会执行且仅执行一次.

区别是当以 "from *** import " 方式导入module时, python会在当前module 的命名空间中新建相应的命名.

即, "from t2 import var1" 相当于:

import t2

var1= t2.var1

在此过程中有一个隐含的赋值的过程

由于python赋值操作特性(参考python变量名本质),

在当前代码中对"var1"的修改并不能影响到t2.py中"var1"的值. 同时, 在t2.py中对var1的修改也不能反映到当前的代码.

参考Example3

Example1:

t1.py:

import t2

import t3

print "execution of t1.py"

t2.py:

print "execution of t2.py"

t3.py:

import t2

print "execution of t3.py"

(env) D:\test>python t1.py

execution of t2.py

execution of t3.py

execution of t1.py

Example2:

t1.py:

import t2

print "a varialbe in t2 is %s" % t2.var_in_t2

print "now will call a functiong in t2"

t2.func_in_t2

t2.py:

var_in_t2 = 0

def func_in_t2():

print "this is a function in t2.py"

print "execution of t2.py"

(env) D:\test>python t1.py

execution of t2.py

a varialbe in t2 is 0

now will call a functiong in t2

Example3:

t1.py

from t2 import var_in_t2

import t2

print "the initial value in t1.py is %s" % var_in_t2

print "the initial value in t2.py is %s" % t2.var_in_t2

print "now try to change the value in the t1.py"

var_in_t2 = 50

print "now the value in t1.py is %s" % var_in_t2

print "now the value in t2.py is %s" % t2.var_in_t2

print "now try to change the value in the t2.py"

t2.var_in_t2 = 20

print "now the value in t1.py is %s" % var_in_t2

print "now the value in t2.py is %s" % t2.var_in_t2

t2.py

var_in_t2 = 0

def change_var():

global var_in_t2

var_in_t2 = 100

(env) D:\test>python t1.py

the initial value in t1.py is 0

the initial value in t2.py is 0

now try to change the value in the t1.py

now the value in t1.py is 50

now the value in t2.py is 0

now try to change the value in the t2.py

now the value in t1.py is 50

now the value in t2.py is 20