This simple script translates a Python program into another, equivalent, Python program using a minimal set of characters.

minimal_Python.py

"""
Usage: python minimal_Python.py infile outfile

Programs output by this translator will run the same way as the original and
use only the characters:

()+1cehrx

The code is for entertainment only. Converting your code base to minimal Python
is not recommended.
"""

import sys


def convert_txt(txt):
    """Convert the text of a Python program to minimal Python"""
    s = "+".join(["chr(%s)" % "+".join(["1"] * ord(c))
                            for c in txt])
    return 'exec(%s)' % s


def convert_file(infile, outfile):
    """Convert a Python file to minimal Python"""
    with open(infile) as inf:
        txt = inf.read()
    with open(outfile, "w") as outf:
        outf.write(convert_txt(txt))


if __name__ == "__main__":
    [infile, outfile] = sys.argv[1:]
    convert_file(infile, outfile)

Nicely done. I was hoping someone could beat 9 characters. That is also the best I've been able to come up with. My solutions:

https://gist.github.com/3325686

found a 7 character solution, but it uses an external file, which may be 'cheating'. Its also at the above gist.