sys.modules是一個全局字典，該字典是python啟動后就加載在內存中。每當程序員導入新的模塊，sys.modules都將記錄這些模塊。字典sys.modules對於加載模塊起到了緩沖的作用。當某個模塊第一次導入，字典sys.modules將自動記錄該模塊。當第二次再導入該模塊時，python會直接到字典中查找，從而加快了程序運行的速度。
字典sys.modules具有字典所擁有的一切方法，可以通過這些方法了解當前的環境加載了哪些模塊
import sys
print sys.modules.keys()
print sys.modules.values()
print sys.modules["os"]
import sys print(sys.modules[__name__]) print(sys.modules.values()) print(sys.modules.keys()) print(sys.modules.items())
<module '__main__' from 'D:/python自動化21/untitled/07/上課/序列化.py'>
dict_values([<module 'builtins' (built-in)>, <module 'sys' (built-in)>, <module '_frozen_importlib' (frozen)>, <module '_imp' (built-in)>, <module '_warnings' (built-in)>, <module '_thread' (built-in)>, <module '_weakref' (built-in)>, <module '_frozen_importlib_external' (frozen)>, <module 'io' (built-in)>, <module 'marshal' (built-in)>, <module 'nt' (built-in)>, <module 'winreg' (built-in)>, <module 'zipimport' (built-in)>, <module 'encodings' from 'D:\\python36\\lib\\encodings\\__init__.py'>, <module 'codecs' from 'D:\\python36\\lib\\codecs.py'>, <module '_codecs' (built-in)>, <module 'encodings.aliases' from 'D:\\python36\\lib\\encodings\\aliases.py'>, <module 'encodings.utf_8' from 'D:\\python36\\lib\\encodings\\utf_8.py'>, <module '_signal' (built-in)>, <module '__main__' from 'D:/python自動化21/untitled/07/上課/序列化.py'>, <module 'encodings.latin_1' from 'D:\\python36\\lib\\encodings\\latin_1.py'>, <module 'io' from 'D:\\python36\\lib\\io.py'>, <module 'abc' from 'D:\\python36\\lib\\abc.py'>, <module '_weakrefset' from 'D:\\python36\\lib\\_weakrefset.py'>, <module 'site' from 'D:\\python36\\lib\\site.py'>, <module 'os' from 'D:\\python36\\lib\\os.py'>, <module 'errno' (built-in)>, <module 'stat' from 'D:\\python36\\lib\\stat.py'>, <module '_stat' (built-in)>, <module 'ntpath' from 'D:\\python36\\lib\\ntpath.py'>, <module 'genericpath' from 'D:\\python36\\lib\\genericpath.py'>, <module 'ntpath' from 'D:\\python36\\lib\\ntpath.py'>, <module '_collections_abc' from 'D:\\python36\\lib\\_collections_abc.py'>, <module '_sitebuiltins' from 'D:\\python36\\lib\\_sitebuiltins.py'>, <module 'sysconfig' from 'D:\\python36\\lib\\sysconfig.py'>]) dict_keys(['builtins', 'sys', '_frozen_importlib', '_imp', '_warnings', '_thread', '_weakref', '_frozen_importlib_external', '_io', 'marshal', 'nt', 'winreg', 'zipimport', 'encodings', 'codecs', '_codecs', 'encodings.aliases', 'encodings.utf_8', '_signal', '__main__', 'encodings.latin_1', 'io', 'abc', '_weakrefset', 'site', 'os', 'errno', 'stat', '_stat', 'ntpath', 'genericpath', 'os.path', '_collections_abc', '_sitebuiltins', 'sysconfig']) dict_items([('builtins', <module 'builtins' (built-in)>), ('sys', <module 'sys' (built-in)>), ('_frozen_importlib', <module '_frozen_importlib' (frozen)>), ('_imp', <module '_imp' (built-in)>), ('_warnings', <module '_warnings' (built-in)>), ('_thread', <module '_thread' (built-in)>), ('_weakref', <module '_weakref' (built-in)>), ('_frozen_importlib_external', <module '_frozen_importlib_external' (frozen)>), ('_io', <module 'io' (built-in)>), ('marshal', <module 'marshal' (built-in)>), ('nt', <module 'nt' (built-in)>), ('winreg', <module 'winreg' (built-in)>), ('zipimport', <module 'zipimport' (built-in)>), ('encodings', <module 'encodings' from 'D:\\python36\\lib\\encodings\\__init__.py'>), ('codecs', <module 'codecs' from 'D:\\python36\\lib\\codecs.py'>), ('_codecs', <module '_codecs' (built-in)>), ('encodings.aliases', <module 'encodings.aliases' from 'D:\\python36\\lib\\encodings\\aliases.py'>), ('encodings.utf_8', <module 'encodings.utf_8' from 'D:\\python36\\lib\\encodings\\utf_8.py'>), ('_signal', <module '_signal' (built-in)>), ('__main__', <module '__main__' from 'D:/python自動化21/untitled/07/上課/序列化.py'>), ('encodings.latin_1', <module 'encodings.latin_1' from 'D:\\python36\\lib\\encodings\\latin_1.py'>), ('io', <module 'io' from 'D:\\python36\\lib\\io.py'>), ('abc', <module 'abc' from 'D:\\python36\\lib\\abc.py'>), ('_weakrefset', <module '_weakrefset' from 'D:\\python36\\lib\\_weakrefset.py'>), ('site', <module 'site' from 'D:\\python36\\lib\\site.py'>), ('os', <module 'os' from 'D:\\python36\\lib\\os.py'>), ('errno', <module 'errno' (built-in)>), ('stat', <module 'stat' from 'D:\\python36\\lib\\stat.py'>), ('_stat', <module '_stat' (built-in)>), ('ntpath', <module 'ntpath' from 'D:\\python36\\lib\\ntpath.py'>), ('genericpath', <module 'genericpath' from 'D:\\python36\\lib\\genericpath.py'>), ('os.path', <module 'ntpath' from 'D:\\python36\\lib\\ntpath.py'>), ('_collections_abc', <module '_collections_abc' from 'D:\\python36\\lib\\_collections_abc.py'>), ('_sitebuiltins', <module '_sitebuiltins' from 'D:\\python36\\lib\\_sitebuiltins.py'>), ('sysconfig', <module 'sysconfig' from 'D:\\python36\\lib\\sysconfig.py'>)])