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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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