Prints almost matched parens

parens.py

#!/usr/bin/python
from random import random

BE_EVIL = True
MAX_DEPTH = 20
PROB_t = 0.05
PROB_p = 0.4

def production(depth):
    depth += 1
    if depth > MAX_DEPTH or random() < PROB_t:
        return terminal()
    elif random() < PROB_p:
        return '%s%s' % (production(depth),
                         production(depth))
    else:
        return '(%s)' % production(depth)

def terminal():
    global BE_EVIL
    if BE_EVIL:
        BE_EVIL = False
        return '('
    else:
        return '()'

print '(%s)' % production(0)

Why are you printing almost matched parens?