How to run python command as default opener for particular file extension

This simple program will get you started.

#!/bin/python

import os
file_ext_dic = {
    'pdf' : 'evince %s ',
    'c'   : 'gvim %s ',
    'py'  : 'python %s '
    }

if __name__ == "__main__":
    argv     = os.sys.argv
    argc     = len(argv)
    filename = ""
    extension= ""

    if (argc == 2):
        filename = argv[1]
        fileparts=filename.rsplit(".")
        key=fileparts[-1] #file extension
        if file_ext_dic.has_key(key):
                cmd_to_open=(file_ext_dic[key]%filename)
                os.system(cmd_to_open)
        else:
                print 'key "{0}" not found'.format(key)
    else:
        print 'no filenames passed'

It doesn't include all validations and you could add more functionalists to extend this further as per the need. The one useful extension would be to add additional settings to the command associated with the file extension.