Make an identifier out of a string

gistfile1.py

def makeidentifier(s):
    import string
    s=s.strip().replace(' ','_')
    valid_chars = "_%s%s" % (string.ascii_letters, string.digits)
    out=''.join(c for c in s if c in valid_chars)
    if len(out)==0:
        return '_'
    else:
        return out