python中from module import * 的一个坑
但还有另外一个问题 - 你以为你修改了某个变量，其实，被from module import *后的那个并没有被更新，非常危险，因为程序有可能还可以正常运行， 只不过结果错了，到了production才被发现就比较惨了。
举个例子：
你定义了一些变量在base模块中：
# reference data type
class Demo:
def __init__(self, name):
self.name = name
demo = Demo('Demo')
# primitive type
foo = 1
然后在一个模块中用from module import 的方式读它：
from base import *
def read():
print 'reference data id: ' + str(id(demo))
print 'reference data value : ' + demo.name
print 'primitive data id: ' + str(id(foo))
print 'primitive data value: ' + str(foo)
在另外一个模块中写它：
import base
def write():
print "\nOriginal:"
print "Original reference data id: " + str(id(base.demo))
base.demo.name = "Updated Demo" # this will reflect that change
#base.demo = base.Demo("Updated Demo") # this won't relfect the change
print "Original data id: " + str(id(base.foo))
base.foo = 1000
print "Original data id after assignment: " + str(id(base.foo))
然后先写，后读，看写的内容是否有效：
import read import write print "before write" read.read() write.write() print "\nafter write" read.read()
结论是没有，原因是：
当你用from module import时，其实是copy了一份reference或者pointer，指向一份内存，var和module.var都指向同一份内存
当你修改module.var时，其实你是让它指向了另外一份内存，此时var和module.var指向的是不同的内存
所以，虽然module.var的值变了，var还是指向原来那份内存，原来的值
这个对于object，比较容易理解，你可以直接修改object里的值，这个是有效的，但是当你指向另外一个object时就无效了。 对于primitive类型来讲，其实也是一个道理，因为每次赋值，都是让其指向一个不同的内存地址，而不是inplace修改已有的那份内存 - 这个很容易验证：
In [1]: a = 10 In [2]: id(a) Out[2]: 20429204 In [3]: a = 100 In [4]: id(a) Out[4]: 20430108
所以，建议是除非是一个quick and dirty的脚本，否则不要使用from module import *！
例子： https://github.com/baiyanhuang/blog/tree/master/arena/python/from_module_import
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