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python编程之hook import

hook python import (让我们知道import发生了什么)

在import的第一个阶段，主要是完成了查找要引入模块的功能，这个查找的过程如下：

1. 检查 sys.modules (保存了之前import的类库的缓存），如果module被找到，则⾛到第二步。
2. 检查 sys.meta_path。meta_path 是一个 list，⾥面保存着一些 finder 对象，如果找到该module的话，就会返回一个finder对象。
3. 检查⼀些隐式的finder对象，不同的python实现有不同的隐式finder，但是都会有 sys.path_hooks, sys.path_importer_cache 以及sys.path。
4. 抛出 ImportError。

上面说的sys.meta_path在开始时其实是空的，我们可以自己向里面加入finder对象，如果在sys.modules中找不到，就会调用sys.meta_path中的finder对象找，用这种方法，我们可以看到程序找module的过程。

× 比如我们加入一个finder对象到meta_path:

import sys
class watcher(object):
    @classmethod
    def find_module(cls, name, path, target=None):
        print("Import -> ", name, path, target)
        return None
sys.meta_path.insert(0, watcher)
import web

然后就可以看到import web到底干了什么了:

>>> import sys
>>> class watcher(object):
...     @classmethod
...     def find_module(cls,name,path,target=None):
...             print "import -- ",name,path,target
...             return None
...
>>> sys.meta_path.insert(0,watcher)
>>> import web
import --  web None None
import --  web.__future__ ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  __future__ None None
import --  web.utils ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.re ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.sys ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.time ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  time None None
import --  web.threading ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  threading None None
import --  thread None None
import --  collections None None
import --  _collections None None
import --  keyword None None
import --  heapq None None
import --  itertools None None
import --  _heapq None None
import --  web.itertools ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.traceback ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.os ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None
import --  web.subprocess ['C:\\python27\\lib\\site-packages\\web.py-0.37-py2.7.egg\\web'] None

参考 ython 中 import 的机制与实现

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