Load modules conditionally Python

I'm wrote a main python module that need load a file parser to work, initially I was a only one text parser module, but I need add more parsers for different cases.
parser_class1.py
parser_class2.py
parser_class3.py

Only one is required for every running instance, then I'm thinking load it by command line:

mmain.py -p parser_class1

With this purpose I wrote this code in order to select the parser to load when the main module will be called:

#!/usr/bin/env python

import argparse
aparser = argparse.ArgumentParser()
aparser.add_argument('-p',
            action='store',
            dest='module',
            help='-i module to import')
results = aparser.parse_args()

if not results.module:
    aparser.error('Error! no module')
try:
    exec("import %s" %(results.module))
    print '%s imported done!'%(results.module)
except ImportError, e:
    print e

But, I was reading that this way is dangerous, maybe no stardard..

Then is this approach ok? or I must find another way to do it?
Why?
Thanks, any comment are welcome.