sys module

 sys module contains a lot of information about pythons import system. First of all , the list of modules currently imported is available through the sys.modules variable. Its a dictionary where the key is the module name and the value is the module object.

>> sys.modules['os']

< module 'os' from /usr/lib/python2.7/os.pyc'>

Standard libraries :

atexit allows you to register functions to call when your program exits.

argparse provides functions for parsing command line arguments.

bisect provides bisection algorithms for sorting lists

calendar provides a number of date-related functions

codecs provides functions for encoding and decoding data

collections provides a variety of useful data structures.

copy provide functions for copying data.

csv provides functions for reading and writing CSV files

datetime provides classes for handling dates and times 

fnmatch provides functions for matching unix-style filename patterns.