A simple way:
cmd = 'echo %s | tr -d "\n" | pbcopy' % str
os.system(cmd)
A cross-platform way:
https://stackoverflow.com/a/4203897/805627
from Tkinter import Tk
r = Tk()
r.withdraw()
r.clipboard_clear()
r.clipboard_append('i can has clipboardz?')
r.destroy()